All about Generator termination Box Line

Electricity importance in this modern industrial era is like blood in the human body. Its availability is vital for optimum productivity levels and to keep employees out of risk. Is there any possible way to stay out of trouble and safely provide your business with electric power? Yes, there is you can supply a continuous power effectively and safely to business by portable generator systems with the help of generator termination box line and load principles.

We provide quality products with custom manufacturing by considering precisely the need of clients. We also help in providing the most reliable electrical solutions which help or customer in leading the industry. Learn more about termination box systems with our experts.

You can put your trust, in us while thinking of procuring termination box systems or power solutions we will provide you the best quality product. Check our product guide for more offerings that best matches your need.

What Is a Generator Termination Box?

Generator termination box functionality is to provide you the safest way to use the backup generator by making the connection permanent. In case of emergency, they provide you with a reliable solution of backing the power then unsafe method of swindling with cords.

Difference between a tap box and generator termination box:

The difference is in the practical use of tools. Portable generators typically use the generator termination box while the robust business and stationary use the tap box generators.

Definition of Line & Load in Electrical:

It can take hours if we go in detail of line and load; we try to explain it briefly for your understanding.

Line- is the electricity is coming into the breaker panel of the business from the electrical utility company.

Load- is the electricity distribution within the business facility to machines and equipment, etc.

Line and load termination boxes essentially to two things:

1. The vital part is that they safely and quickly break off the connection between utility and current line and load connection. It avoids overloading and reduces the potential risk to workers’ life and equipment.

2. It set up a connection between line and load and the backup generator you are going to use.

It is the generator termination box that establishes a stable and secure connection between line and load connection through the generator to the appliances.

If you still find it difficult to understand, see this example that mud water is coming in the filter bottle and going out as clean after filtration. The same is the line the power (mud water) that is coming into the device (Filter), and load is power (clean water) that is coming out of the filter which is safe and can be of your use.

Procurement of Generator Termination Boxes:

If you are finding a reliable source, who can meet your every demand related to electric and power supply you are in the right place. We are the reliable source whom you can trust to procure the generator termination boxes for your business.

User Guide for Eaton Magnum Manual Transfer Switch

Eaton Magnum Manual Transfer Switch User Guide

A transfer switch is a device which controls the flow of electric power between utility and backup source. Today we are going to discuss a manual transfer switch manufactured by Eaton. Magnum Manual- made by Eaton, can safely switch the power between utility and backup source if used correctly. In this guide, we will describe the basics of its installation, working, and maintenance.

Handling & Storage

Some electrical devices are very sensitive and extra precautions must be taken while handling and installing. Listed below are few precautionary measures to be taken while handling and storage of Electrical equipment, in this case specifically Eaton transfer switch:

·         Do not remove the protective packing until ready for installation

·         Keep safe from getting affected by any source

·         Do a thorough inspection for damage before installation

·         Do not stack with another switch on top or anything else

·         Store in dry and moistureless place

·         Store with intact packaging

Eaton Magnum Manual Transfer Switch Installation

Before installing the switch, ensure that the surface is rigid and dust-free. No moisture should be present around. There is a special NEMA enclosure available in the market for transfer switches. Also, look out for concealed cables and conduits while selecting the location so, that any potential hazards can be avoided. Make sure that the openings have enough clearance for incoming and outgoing wires.

After finalizing the location place the enclosure to the firm around the wall and fix it with screws. Now, place the switch inside the enclosure and bolt it firmly. Double-check the enclosure and switch to ensure it is attached firmly and remove any leftover packing material.

Keeping the transfer switch parts safe, make the openings for incoming and outgoing cables as much as needed. File the openings and make them smooth. Inspect all the cables and make sure their compliance with standard electrical codes and voltage ratings as per agreement.

Run a continuity, insulation and short circuit test for cables before installing. After that prepare the cables for termination, use proper terminating kit and components for it. Now, take out the wirings diagram supplied with switch and install the wires as per given instructions.

Testing & Troubleshooting

After installing the switch, testing should be done to identify any problems either mechanical or electrical that may be present at an early stage. After erasing all the faults that may be present at an early stage, performs the tests as per the generator set.

Troubleshooting a generator varies model to model, but basic tests are usually the same for the generator sets. Normally you need to start with voltage ratings, frequency, general lookup for burned parts or components, continuing to check cables and terminal voltages between equipment to locate the problematic area.

Eaton Magnum Manual Transfer Switch Maintenance

All electrical devices and equipment need to be properly maintained. A transfer switch is a low-maintenance device, however it is also important to periodically maintain the device for its proper function. Here we will share Eaton’s maintenance procedure:

·         Make sure to disconnect all sources before inspection and maintenance.

·         Lookup for any physical damage to the switch

·         Clean the surface from dust or any other pollutant using a cloth or soft brush

·         Erase the sources of pollution

·         Check for any mechanical issues

·         Replace the contacts if contacts are worn out

·         Workout the switching device for easy operation

·         Inspect and replace enclosure filters in required

·         Replace all the equipment taken off after completing the maintenance


A solid state relay makes it possible to manipulate an electrical load without locomotion.

We already know that an electromechanical relay cannot work without a moving part. On the other hand, a solid state relay will function optimally without a spring, coil, or any other mechanical component. To carry out its operation, a solid state relay makes use of the peculiar electrical qualities of a semiconductor.

A solid state relay makes it possible to distinguish between input contact and output contact. A very common example is a switch. Furthermore, a solid state relay can provide control over an alternating current and a direct current. They do not make use of the conventional NO contacts found in electromechanical relays; instead they make use of a switching transistor.

Although the operating principle of a solid state relay is similar to that of an electromechanical relay, the electromechanical relay has several limitations. One of such limitations is the slow pace of switching between input and output voltage. The size and lifecycle of an electromechanical relay is also a problem.  The solid state relay does not have any of these drawbacks.

A solid state relay is not subject to wear and tear due to the absence of a moving component. This means it is possible to switch between voltages at a much faster pace. Unlike an electromechanical relay, a solid state relay does not produce any sound.

Although solid state relays are readily available. They are quite expensive and this is the comparative advantage an electromechanical relay has over it. A solid state relay of any capacity, size, or calibration can be bought from a standard electrical store.

With a minute input voltage, a solid state relay can manipulate an output voltage with a high value.

Solid State Relay Input

A solid state relay has a component that makes it easy to distinguish between input voltage and output voltage. This component is known as an optocoupler or opto-isolator. The opto-isolator consists of a diode that constantly emits light and supplies illumination.

This diode is linked to the input drive section of the solid state relay such that when an electrical current flows through it, it lightens up. The opto-isolator is also used for transmitting signals whose frequency value is not high. It is also used for the transmission of direct currents.

The circuit structure of a solid state relay is such that the opto-isolar and a resistor are connected in series.

The minimum permissible voltage value of a solid state relay is 3V DC. Therefore, before it can be powered on an input current greater than this value must be passed through its input terminals. A logic gate and switch are good sources of direct current signals.

Solid State Relay DC Input Circuit

A direct current voltage is not the only way to switch on a solid state relay. We can also employ a sinusoidal waveform to achieve it.

The input voltage is only permitted to be equal to the solid state relay starting voltage on one condition and that condition is that the activating signal must come from a mechanical contact or relay contact.

Solid State Relay output

The measurement of the output of a solid state relay is similar to its input value. This means it can either be an alternating current or direct current. The configuration and circuit structure of a solid state relay output is such that only a form of switching operation can be executed. Power transistors are the most popular switching devices used for direct current solid state relays

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Relay Switch Circuit: What is it?


A relay switch circuit is used to control ON and OFF switching in a relay device.

What then is a relay device? A relay, also known as electromagnetic relay is a device that controls the opening and closing of contacts in a system, with the help of an electromagnetic or an electromagnetic field. A great benefit of relay devices is that they do not require large amounts of electrical energy or power in the form of electrical current or voltage. They can be used to control electronic devices and equipment such as lamps, transformers, motors, etc. They also come in different shapes, forms and sizes and can be used in a wide range of electronic circuits and systems.

As stated earlier, relay switch circuits are used to control switching in relay devices. Transistors and MOSFET devices are used for this purpose, especially when the relay in question is a smaller one. There are so many types and configurations of relay switch circuits. In this article, we would attempt to learn and understand the most common relay switch circuits and how they work. They include:

NPN Relay Switch Circuit

An NPN transistor switch is the most commonly used relay switch circuit. As seen in the figure above, current enters into it through the Base (B) and gets to the relay through the Collector (C). The state of the transistor is highly dependent on the amount of voltage available. If the base voltage is zero or negative, no current will flow through the collector to the relay and thus, the relay will be inactive and the contact switch will be open.

When a positive current of sufficient magnitude enters the base, it would saturate the transistor and send current to the emitter. The current from the relay coil which flows from the collector to the emitter, usually has a very high magnitude. The base to emitter current is used to check this relay coil current. In many transistors, the relay coil current could have a magnitude about 50 to 800 times greater than the magnitude of current needed to concentrate or saturate the transistor.

As current flows through the relay coil, some of it is collected and stored in its magnetic field. This happens as a result of the inductive nature of the coil that provides a DC resistance and thus causes large amounts of current to flow through the coil.

Some of this current stays in the coil’s magnetic field even after current has ceased to flow from the transistor to the relay and the field itself crashes down. In order to sustain and control the amount of current in the relay, the energy stored up in the magnetic field has to be let out and this causes a backward voltage to be produced. This would result in a sudden surge in the magnitude of voltage in the coil, and if this voltage is allowed to continue increasing, it might have a serious negative effect on the transistor.

In most arrangements such as this, a Flywheel Diode is connected across the relay coil to forestall any damage that might have been done to the system. Flywheel diodes work by reducing the backward voltage in the coil, thus dispersing the current or energy stored up in the coil. Flywheel diodes are capable of reducing the voltage to about 0.7V.

However, flywheel diodes cannot be used when the supply of voltage is DC. In cases where AC current is used, an RC Snubber circuit is used instead.

NPN Darlington Relay Switch Circuit

Darlington transistors are used to deal with currents of large magnitude, unlike the NPN relay switch circuit which is used primarily for relays with smaller loads such as lamps, bulbs, etc.

In this configuration, one single NPN transistor is substituted with a darlington pair of transistors. This would not only increase the sensitivity of the relay but also increase the amount of load it can carry or control.

An NPN darlington relay switch circuit can be made simply by connecting two individual bipolar NPN transistors, as shown in the diagram below.

As shown above, the base of the second transistor, TR2 is connected to the emitter of the first transistor, TR2. Thus, the emitter of TR1 becomes the base current for TR2. In this case, TR2 is the switching transistor. When sufficient current passes through TR1, TR2 is automatically turned on and activated.

A small resistor, of about 100 to 1000 ohms, is normally placed between the emitter of TR1 and the base of TR2. This is to make sure that TR2 turns off properly. As in the standard NPN relay switch circuit, a flywheel diode is connected to the relay to protect the darlington transistor from the backward voltage produced by the energy stored up in the magnetic field of the relay coil after its current supply has been cut off.

PNP Relay Switch Circuit

The working principle of the PNP relay switch circuit is similar and yet very polar to that of the NPN relay switch circuit. A very clear example is the reverse position of the emitter and collector. Unlike in the NPN transistor switch, both the base current and emitter current have to be less than the emitter current for current to flow from the emitter to the collector.

This means that, the PNP transistor as well as the relay coil, is turned off when the input voltage is high. Inversely, when the input voltage is low and less than the voltage in the emitter, the transistor is turned on. The collector current powers up the relay coil and it is dependent on the base current. The base current, itself, is dependent on the value of the base resistor.

PNP Collector Relay Switch Circuit

This relay switch circuit has the same mode of operation as the PNP relay switch circuit. Here, the relay coil is connected to the collector of the transistor of the PNP transistor. When the input voltage is low, the transistor is turned on and the relay coil is turned on too. Inversely, when the magnitude of the input voltage is high, the transistor is turned off and the relay coil is de-energised in the process.

Differences Between the NPN Transistor Switch and the PNP Transistor Switch

  1. In an NPN transistor switch circuit, the transistor is switched on when the input voltage or base voltage is high or greater than the emitter voltage, while in a PNP transistor switch circuit, the transistor is switched on when the input voltage is low or less than the emitter voltage.
  2. In an NPN transistor, the transistor turns off when the input voltage is low or less than the emitter voltage, while in a PNP transistor, the transistor turns off when the input voltage is high or greater than the emitter voltage.
  3. Current flows from the collector to an emitter in an NPN transistor switch, while in a PNP transistor switch, current flows from the emitter to the collector.

Emitter Follower Relay Switch Circuit

This arrangement is also known as the Common Collector Configuration. Here, the relay coil is connected to the emitter of the transistor and this arrangement forms the Emitter Follower Circuit. The input voltage flows directly to the base of the transistor and the output is gotten from the emitter.

In systems where the value of the input impedance is very high, especially when compared to the low output impedance, this emitter follower circuit arrangement is very important. As with the NPN transistor switch circuit, a relatively high voltage must be transferred to the base of the transistor before it would switch on and then activate the relay coil.

Emitter Darlington Relay Switch Circuit

The diagram above depicts the Darlington transistor form of the emitter follower relay switch circuit. When a positive current of low magnitude is applied to the base of TR1, a current of higher magnitude flows through the collector to TR2. This is as a result of the propagation of the two beta values.

A very important benefit of the emitter darlington transistor is its suitability for large relay coils. This is due to its high input impedance and low output impedance. It also promotes an increase in current and power.

MOSFET Relay Switch Circuits

MOSFET relay switch circuits work in almost the same way as the entire bipolar junction transistor operated switch circuits discussed above. In fact, a MOSFET can be configured into any of the circuits above. Despite their similarities, there are some crucial disparities between the two, especially where their operation is concerned.

Their operation is dependent on the voltage in the system. The MOSFET gate is separated electrically from the drain-source. Their input impedance is high and thus, the gate current is zero. This means that a base resistor is not required.

The transistor is off when the MOSFET’s conductive channel is closed. As more voltage is passed through the gate, the channel widens and increases. Thus, the working condition of the transistor is controlled by the enhancement or improvement of the conductive channel which is dependent on a steady increase in the gate voltage. This is why it is known as an Enhancement MOSFET or E-MOSFET.

There are several types of MOSFET devices but the most common is the N-Channel MOSFET or N-Channel Enhancement MOSFET (NMOS).

N-Channel E-MOSFET Relay Switching Device

The N-channel E-MOSFET relay switching device is the most preferred MOSFET switching device. This is because of the straightforward way in which it operates.

When the voltage at the gate (VGS) is zero or negative, the conductive channel stays closed and thus the transistor remains off. When there is an increase in the value of voltage at the gate, the channel begins to open up. Once it exceeds the lower limit value voltage value of the MOSFET (VT), the channel opens up completely and current flows freely, switching on the transistor which in turn, activates the relay coil.

The E-MOSFETs are the best for switching and controlling small relays. This is because they have a temperate ON resistance and a very high OFF resistance.

P-Channel E-MOSFET Relay Switching Device

The structure of the P-channel E-MOSFET relay switch is very similar to that of the N-channel E-MOSFET relay switch, except for one small but very significant difference – the voltage at the ate terminal has to be negative. This simply means that for the transistor to be switched on the value of the voltage at the gate (VGS) has to be negative or less than the limit voltage value set by the MOSFET. As such, when a high voltage is passed through the gate, the transistor will be turned off.

As seen in the figure above, the Source is linked to the +Vdd while the Drain is linked through the relay coil to the ground.

When the E-MOSFET is turned off, the channel experiences a very high resistance to current. When a low voltage is passed through the gate, it reduces the resistance and opens up the channel. The transistor will now be turned on and current flows freely to the relay coil, thus activating it.

The P-Channel and N-Channel E-MOSFET devices are top-class voltage relay switching circuits, especially when the voltage is low. They can also be configured into a wide range of logic and microprocessor controlled relay switching devices.

Micro-Controller Relay Switching Device

The figure above illustrates how an E-MOSFET relay switching device can be controlled with the help of micro-controllers or PICs.

Logic Controlled Relay Switching Device

This figure shows that E-MOSFETs can be controlled using digital logic gates. This configuration allows E-MOSFET devices to devices and loads with high power.


In this article, we have considered the importance of relay switch circuits. We have also looked at several types and forms of relay switch circuits including the Bipolar Junction Transistors (NPN and PNP relay switching devices) and the E-MOSFET relay switching devices (the P-channel and N-channel E-MOSFETs). In all the relay switching devices listed and discussed above, flywheel diodes were connected to the relay coil to prevent damage from the backward voltage caused by the stored up current in the electromagnetic field of the relay coil.

Generally, E-MOSFET relay switch circuits are preferable to the BJT relay switching devices because they are voltage-reliant and do not require any electrical current to be operated. They can also be configured to carry high power loads. They can switch speedily, do not consume electrical current and power and have a high input impedance.

We’ve looked at how the BJTs can be configured as in the NPN darlington relay switch circuit and the PNP darlington relay switch circuit. The E-MOSFETs can also be interfaced with digital devices and applications such as PICs, micro-processors and logic gates to enhance their features and capabilities.

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Linear Solenoid Actuator


To properly understand the concept of linear solenoid actuators, we must understand the basics first.

A linear solenoid is an electromagnetic actuator. An electromagnetic actuator is a device that converts electrical energy into a magnetic field, creating movement or motion in the process.

In the simplest terms, a linear solenoid also known as a Linear Electromechanical Actuator (LEMA) is an electromagnetic device that is capable of producing mechanical energy or motion from electrical energy. Its working principle is very similar to that of the electromechanical relay device.

Basically, a linear solenoid is made up of a tube with a solenoid coil wrapped around it. Inside the tube is an iron-core or plunger with enough space for it to slide in and out of the solenoid coil.

Solenoids are very useful in the creation of mechanical energy or motion. They are used in the operation of robots, opening/closing of valves, doors, etc.

Apart from linear solenoids, there are several other types of solenoids. The rotary solenoid is a very common solenoid form. 

Magnetic fields are formed when electrical current is passed through a conductive material, and the flow of current within the coil determines the direction of the magnetic field.

A linear solenoid is made up of a coil of wire (solenoid core) around an iron core (electrical conductor), thus when current is passed through it, a magnetic field is formed and the coil becomes an electromagnet having permanent North and South poles.

The strength of the magnetic field formed is dependent on two things:

  • The magnitude of the current flowing through the coil and
  • The number of loops on the coil

As current passes through the coil, the coil begins to behave more like an electromagnet and a magnetic flux is created which attracts the plunger to the middle of the body of the coil. This movement of the plunger causes the small spring attached to it to get squeezed and compacted. The magnitude of the magnetic flux created influences the force that acts on the plunger, making it move and the speed with which it moves towards the coil.

Once the flow of current to the coil is cut off, the electromagnetic field produced completely breaks down and the coil loses its electromagnetic properties. The plunger which was formerly drawn to the coil in its electromagnetic state is pulled back to its initial position by the energy stored in the body of the compacted spring. As the plunger moves to and fro the coil, solenoid strokes are formed. This is basically the utmost distance the plunger can move, it is calculated in millimetres (mm).

Structure of the Linear Solenoid

The linear solenoid is so called because it causes the plunger attracted to it to move and act in a straight line. Based on the direction in which the plunger moves, there are two types or forms of linear solenoids.

  • Pull Type – Here, when there is a flow of current, the electromagnetic force of the coil works to bring the object closer to itself.
  • Push Type – When there is a flow of current, the electromagnetic force of the coil acts on the object, taking it further away from itself in the opposite direction.

What makes the difference between the two types of linear solenoids is the position of the spring (return spring) attached to it and the makeup of the plunger.

Pull-type Linear Solenoid

When a “push” or “pull”, “in” and “out” or “open” and “close” movement is required, linear solenoids come in very handy. They form the principal mechanism of robotic motion, door locks, irrigation valves, self-propelled engines, etc.

They come in different forms sealed tubular type, and open and closed frames.

Rotary Solenoid

Another common type of electromagnetic actuators is the Rotary Solenoid. While the linear solid is concerned with movement in a straight line, rotary solenoids effect rotational movement around a fixed point. This movement could be clock-wise, anti-clock-wise, or move in both directions.

Where the rotational movement needed is small, DC motors and stepper motors can be substituted with rotary solenoids. In determining the angular orientation, you consider where the movement started from, and the point at which it ends.

A rotary solenoid can move 25, 35, 45, 60 and 90 degrees. It can also move through a particular angle in several ways:

  • 2-position Self-restoring movement also known as Return to Zero rotation, e.g. 0˚ to 45˚ to 0˚.
  • 3-position Self-restoring Rotation e.g. 0˚ to 60˚ to 0˚ to -60˚
  • 2-position latching

Just like in the case of the linear solenoid, a rotary solenoid produces movement, rotational movement, when current is passed through it. It also produces movement when there is a change in the position of the electromagnetic coil. This is due to changes in the polarity of the electromagnetic field.

The structure of the rotary solenoid is made up of a coil of wire wrapped around a steel object. Just above the coil, there’s a magnetic disc linked to the output shaft. When current flows through the coil, the electromagnetic field created produces several North and South poles which repulse the standard north and south poles of the magnetic disc. This causes a rotational movement of the disc. The direction it turns is determined by the mechanical structure of the solenoid.


  1. The rotary solenoid is an integral part of valve controls, cameras, vending machines, etc.
  2. Some rotary solenoids have low power but a sufficient torque magnitude. They are used in automotive machines and applications, dot matrix printers, etc.

Using AC Current in Solenoids

As already mentioned earlier, linear solenoid energize their system by allowing the flow of DC voltage through the coil. However, AC current can also be used. This can be achieved by changing the supply with the use of full-wave bridge rectifiers. Smaller solenoids that make use of DC current can be operated using MOSFET switches or transistors.

As an inductive device, the solenoid coil generates large amounts of electromotive force (emf). To forestall any damage caused by high emf voltages, the Fly-wheel Diode is usually used.

How to Reduce Energy Leakage in Solenoids

As mentioned above, most solenoid coils, including the linear solenoid, are inductive devices. This means that they generate heat from some of the electrical energy in the coil. This is because of the wire’s resistance.

This means that when connected to a source of electrical energy for some time, its temperature increases and keeps on increasing as long as it is connected to the source. Also, the increase in temperature has an effect on the electrical resistance of the wire which in turn increases the flow of electrical current, thus increasing its temperature further.

If the coil receives a constant supply of current, the temperature will rise steadily, especially if the power supply never stops. As this steady rise in temperature would eventually damage the solenoid, it is important to check the amount of current flowing through the coil. This can be achieved by controlling the supply of voltage to the solenoid. The illustration below shows to do this simply by linking the solenoid in series with an appropriate resistor:

In the diagram above, the NC contacts are closed. This allows the current to flow freely into the coil by stopping the free flow of resistance in the system. Once a sufficient amount of current has entered into the system, the contacts automatically open up and a series connection is formed between the solenoid coil and the holding resistor.

With the help of the technique above, the coil can receive a constant flow of supply voltage. This is because the consumption of energy and the generation of heat have been cut down drastically.

However, it must be noted that the holding resistor would also produce some amount of heat. This is due to the effect of Ohm’s Law.

An even better way to cut down the loss of energy and production of heat is the Solenoid’s Duty Cycle also known as the Intermittent Duty Cycle. An intermittent duty cycle involves continuously switching the coil ON and OFF at appropriately fixed times. This would help to ensure that just enough current gets into the system enough to keep the plunger working even when it’s been switched off without letting in current steadily.

The ratio of the time the solenoid switch is ON to the absolute total of the ON and OFF time for one operation cycle is what is called the Duty Cycle of the Solenoid. Simply put, the total duty cycle time is a cumulation of the OFF time and ON time.

 The illustration below further explains the concept of the duty cycle. Thus, if the total ON time is 25 seconds and the total HOT time is 75 seconds, it means the total cycle time is 100 seconds. The duty cycle of the solenoid coil is 25%. As per:

(25/100)*100 = 25%

Using this formula, the total ON time of the coil can be calculated, as far as the values of the OFF time and the total cycle time.

As an example, consider a coil with a switch off time of 35 seconds, and total cycle time of 60 seconds. Thus, the switch on time of the coil is 25 seconds.

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A relay is a switch that is operated by electric current. This Switch may contain many types of contact forms which we will look into further in to the article.

Low level controls or signals switch the relays and contractors to a higher voltage or current supply using different arrangements on contact.

For now we have seen a collection of input devices that be used to detect different physical variables and signals. So they are known as sensors. Already, Actuators can control or operate some external physical processes and are classed as output devices. Actuators convert an electrical signal into a corresponding physical quantity such as sound. It changes one type of quantity for another and is commonly used by a low voltage command signal hence it is classified as a transducer. Depending on the number of stable forms the output has, actuators can be classified into either binary or continuous.

The most common types of actuators are electrical relays and motors. Since a relay has two stable forms we can then say it’s an actuator, either energized and latched or de-energized and unlatched. m\a motor however can rotate 3600 motion.

Solenoids can be used to open doors, close valves and open latches electrically through robotic applications. A relay is used so that a solenoid plunger can operate in one or more types of ways, the relay can be used in an infinite number of ways.

The Electro-Mechanical Relay

As previously stated, a relay is known as an electrical switch that provides one with a connection (electrical) between two points or more in response to the application of any control signal. The most common and used kind of electrical relay is the electromechanical relay.

The ability to turn an equipment “on” or “off” is the most fundamental function of the relay. Switches are the easiest way to control the power supply. The ability to manually turn them “off” and “on” is its biggest disadvantage. They are large and slow and posses small current.

On the other hand, electrical relays are electrically operated switches that come differently according to their types of applications. Contractors are single or multiple contracts within a single package with the larger power relays power relays used for high current switching applications.

The fundamental operating principles of “light duty” electro-mechanical relays that can be used in motor or robotic circuits are our main concern in the talk of electrical relays. Relays are usually directly set up on PCB boards and/or connected free standing. Load currents are usually fractions of ampere up to 20 + amperes; they are usually used in general electrical and electronic control circuits.

An electro-mechanical relay converts magnetic flux generated by the application of a low voltage control signal (either DC or AC) across the relay terminals, into useful mechanical energy. The mechanical force that is generated can be used for the control and manipulation of electrical contacts build in the relay. The “primary circuit” is the most common form of electro-mechanical relay consists of an energizing coil tied around a permeable iron core.

The magnetic field circuit is completed by a fixed iron core potion called the yoke and a move-able spring-loaded part called the armature and closing the air gap between them. The armature is pivoted. This allows it to move freely within the generated magnetic field while the electrical contacts are closed. Springs are joined between the armature and the yoke for the return stroke to “reset” the contacts back to their initial rest position when the relay coil is in the “de-energized” condition, A.K.A. turning it off.

Electro-mechanical Relay Construction

In any relay, what can be noted is the presence of two sets of electrically conductive contacts. Relays are known to be “Normally Closed”, or “Normally Open”. The NO labeled relay initiates contact. On the other hand, the NC labeled relay breaks contacts. The contacts are closed in the normally open position if only the field current is “ON”.

The contacts are permanently closed in the normally closed position when the field current is “OFF” as the switch contacts return to its resting position. Note that these terms, i.e. Normally Open and Normally Closed or Make and Break Contacts discuss the state of the electrical contacts when the relay coil is off. Contact elements may be of double or single make or break designs. An example of this arrangement is given below.

The relay contacts are a set of metals that are in contact in order to prevent a short-circuit and permit the flow of current electricity. An example of a device that uses this mechanism is a switch. Hence, they are said to be electrically conductive devices. The resistance between the contacts is very high in the Mega-Ohms when the contacts are open, producing no circuit current flow and an open circuit condition.

 The contact resistance should be zero when the contacts are closed, a short circuit, but it is not always the case. When closed all relay contacts have a certain amount of “contact resistance” and which is called the “On-Resistance”.

This ON-resistance will be very small with a new relay and contacts generally less than 0.2Ωbecause the tips are new and clean, but over time the tip resistance should increase.

For example, if the contacts are passing a load current of say 20A, then the voltage drop across the contacts using Ohms Law is 0.2 x 20 = 4 volts. If we assume that the value of the supply voltage is 16 volts, this means that the value of the load voltage will be 20V. As the contact tips begin to wear, they will start to show signs of arcing damage. This is because the circuit current still wants to flow. As a gap begins to emerge between the contacts, the relay coil loses energy due to excess load. The contact resistance of the tips to increase further as the contact tips becomes damaged if they are not adequately shielded from loads with high inductance and capacitance values. The contact tips may become so burnt and damaged to the point that they are physically closed but do not pass any or very little current If allowed to continue.

As the gap between the contacts begin to reduce, a short-circuit condition arises. This can also lead to destruction of the circuit if the arcing becomes persistent. The volt drop across the contacts for the same load current increases to 1 x 20 = 20 volts DC If the contact resistance has increased due to arcing to say 1Ω. The faulty relay will have to be replaced if the drop in voltage value is massive

 Modern contact tips are made of, or coated with, a variety of silver based alloys to extend their life span as given in the following table to reduce the effects of contact arcing and high “On-resistances”.

Electrical Relay Contact Tip Materials

Ag (fine silver)

1. Electrical and thermal conductivity are the higher than all the other metals.

2. It shows low contact resistance, it is also inexpensive and popularly in use.

3. Contacts tarnish easily through sulphurisation influence.

Silver Cadmium Oxide (AgCdO)

1. Very little possibility to arc and weld, good wear resistance and arc extinguishing properties.

Silver Tungsten (AgW)

1. Hardness and melting point are high, arc resistance is good.

2. It is not a precious metal.

3. High contact pressure is needed to reduce resistance.

4. Contact resistance is a little high, and resistance to corrosion is poor.

Silver Nickel (AgNi)

1. Equivalent to the electrical conductivity of silver, excellent arc resistance.

Platinum, Gold and Silver Alloys

1. Excellent corrosion resistance, and used mainly for low-current circuits.

Data sheets display information about the rating of all load types whether DC or AC. Some form of arc suppression or filtering is required across the relay contacts in order to achieve long life and consistency when load with high inductance and capacitance rating is placed on it.

To improve the longevity of a relay tip, it is a good idea to connect it in parallel with an RC network. This reduces the rate of arcing. The voltage peak, which occurs at the instant the contacts open, will be safely short circuited by the RC network, hence suppressing any arc generated at the contact tips.

Electrical Relay Contact Types

Relay contacts can be classed by their actions as well as the usual Normally Closed, (NC) and Normally Open, (NO) (which basically describes the way relays contacts are connected). These relays can be made up of one or more original switch contacts. Each of these contacts is known as a “Pole”. A relay coil is used to connect the poles and these contact types are usually shown as:

SPST – Single Pole Single Throw

SPDT – Single Pole Double Throw

DPST – Double Pole Single Throw

DPDT – Double Pole Double Throw

“Break” (B) or “Make” (M) would be the action sdiscussed above. An example of a simple relay with one set of contacts would be seen as:

“Single Pole Single Throw – (Break before Make)”, or SPST – (B-M)

There are many examples with more common diagrams used to explain and showcase the electrical relays.

Electromechanical relays are also denoted by the combinations of their contacts or switching elements and the number of contacts combined within a single relay. For instance, a contact which is normally open in the de-energized position “OFF” of the relay is called a “Form A contact” or make contact. Whereas a contact which is normally closed in the de-energized position of the relay is called a “Form B contact” or break contact.

The set of contacts are referred to as “Form C contacts” or change-over contacts when both a make and break set of contact elements are present at the same time so that the two contacts are electrically connected to produce a common point (identified by three connections).

Connecting relay contacts in parallel so these contacts handle higher load currents is not seen as a very smart move. For example, one should never try to provide a 20A load with two relay contacts in parallel that have 10A contact ratings each, as the mechanically operated relay contacts don’t ever open or close at the exact same time. The end result is usually that one of the contacts will overload even for a briefly resulting in premature relay failure over time. This is a final and important point about electrical relays.

Different load voltages should not be mixed with contacts from a sister relay or  from the same relay. Though, separate relays are utilized for safety while electrical relays can also be used to allow low power electronic type circuits to switch high currents or voltages both “Off” and/or “On”.

The most significant part of any electrical relay is the “Coil“. The coil refers to the part which converts all electrical current to electromagnetic flux that is used to mechanically operate all relay contacts. The main problem relay coils have is the possession of highly inductive loads made from wire coils. Any coil of wire has an impedance value made up of Inductance (L) and Resistance (R) in series (i.e. LR Series Circuit).

The magnetic field is generated around the current flows through the coil which is self induced. When magnetic flux of the coil drops, a back emf with high value is generated.  This induced reverse voltage value could be very high in comparison to the changing voltage, and may damage any semiconductor device.  By connecting the reverse biased diode across the coil of the relay, we can secure the protection of the transistor and semiconductor device.  

An induced back E.M.F is generated as the magnetic flux collapses in the coil when the current flowing through the coil is switched “OFF”.

The diode, responsible for the dissipation of energy is triggered by the reverse electromotive force. This protects the semiconductor transistor from developing a fault.

When the method of application of the diode is defined, a free-wheeling diode or fly-back diode can be used. Other types of inductive loads which require a flywheel diode for protection are solenoids, inductive coils and motors.

Other devices used for protection include a combined resistor-capacitor network and Zener Diodes. Semiconductor components can also be secured through the use of a flywheel Diodes.

The Solid State Relay

One of the major shortfalls of an electromechanical relay is that it is a “mechanical device”, i.e.  it has moving parts so their switching speed due to physical movement of the metal contacts using a magnetic field is slow even though the electro-mechanical relay (EMR) are inexpensive, easy to use and permit a load circuit to be switched by an input signal.

The longevity and efficiency of the relay may be affected by erosion and continuous arcing. Also, they are electrically noisy with the contacts suffering from contact bounce could affect any electronic circuits to which they are connected.

 Another type of relay called a Solid State Relay or (SSR) for short. It is an electronic relay that does not require physical contact.  

The solid state relay is only electronic and therefore has no moving parts within its design as thyristors, and power transistors have taken the place of mechanical contacts.  

The opto-coupler type Light Sensor helps us achieve the gap between the output voltage and the input control signal. Together with a much faster and almost instant response time, as compared to the conventional electromechanical relay the Solid State Relay is more reliable and durable. Also, there is no interference from an external signal or arcing.

To prevent overheating of the output semiconductor, it can be strategically placed on an heatsink. The input control power requirements of the solid state relay are generally low enough to make them compatible with most IC logic families without the need for additional buffers, drivers or amplifiers.

Solid State Relay

This solid state relay performs several functions. It prevents overcurrent and prevents arcing that may be caused by load with high inductive and capacitive value.  

Resistor-Capacitor (RC) snubber network is generally required across the output terminals of the SSR. It shields the semiconductor from noise from an external source. It also helps to prevent destruction when a voltage spike takes place.

This RC snubber network is built as standard into the relay itself reducing the need for additional external components is used In most modern SSR’s .

The regulation of the brightness of light in pubs and club houses is done by using the detection ability of an SSR.

The voltage drop across the output terminals of an SSR when “ON” is much higher than that of the electromechanical relay, typically 1.5 – 2.0 volts. For switching large currents for long periods of time an additional heat sink will be required because the output device is a semiconductor.

Input/output Interface Modules

Input/output modules are another type of solid state relay specifically created for computer interface and as micro-controllers to “real world” loads and switches. Currently, there are different types of I/O modules:

  1.  AC or DC Input voltage
  2.  TTL or CMOS logic level output
  3.  AC or DC Output voltage

Each module contains all the necessary circuitry requirements to provide a complete interface and isolation within one small device. They are available as individual solid state modules or integrated into 4, 8 or 16 channel devices.

Modular Input/output Interface System Solid state relays are usually more expensive than electromechanical relays. Also, it is difficult for a solid state relay to switch currents emanating from small load.

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In this tutorial, the different kinds of devices which are tagged input devices are discussed. For the purpose of this text, they are specifically termed “Sensors”. However, we will focus our attention on sensors that helps us keep track of position and displacement. These types of sensors are called position sensors.

As indicated by their name, position sensors identify the position of things, which means they are referenced either from or to a fixed position or point. Such kinds of sensors deliver a “Positional” response.

A way of ascertaining a position is to use either “rotation” (angular movement) or “distance”, which could be the distance between two positions like the distance moved or travelled from a particular spot. For instance, the rotation of a robot’s wheel to ascertain the distance it has covered on the ground. All the same, Position Sensors can identify an object’s movement in a straight line through the use of Linear Sensors or by its angular movement via Rotational Sensors.

The Potentiometer

The Potentiometer is the most widely used of all “Position Sensors” due to the fact that it is inexpensive and easy to use. It features a wiper contact connected to a mechanical shaft that can either be linear (slider type) or angular (rotational) in its movement, and which alters the resistance value between the slider/wiper and the double end connections, sending an electrical signal output that has a proportional relationship between the real wiper position on the resistive track and its resistance value. Essentially, resistance is proportional to position.

Potentiometers are available in a vast array of sizes and designs like the generally available round rotational kind or the flat and lengthier linear slide kinds. When applied as a position sensor, the mobile object is linked directly to the slider or rotational shaft of the Potentiometer.

A DC reference voltage is applied across the two outer fixed connections making the resistive element. The sliding contact’s wiper terminal produces the signal.

The result of this configuration is a potentiometer circuit output that is proportional to the position of the shaft. Subsequently, for instance, if you apply a voltage of about 10v across the resistive element of the potentiometer, the highest output voltage would be the same as the supply voltage at 10 volts, with the least output voltage being 0volts. Thus, the potentiometer wiper will adjust the output signal from 0 to 10 volts, with 5 volts showing that the slider or wiper is at its mid-way or centre point.

The output signal (Volt) from the potentiometer is gotten from the centre wiper connection as it progresses along the resistive track, and is proportional to the angular position of the shaft.

Although resistive potentiometers have several advantages – simple to use, inexpensive, low tech etc., they also have numerous downsides such as poor repeatability, poor accuracy rating, quick wear as a result of moving parts and restricted frequency feedback.

However, there is a major downside to using the potentiometer for sensing position. The scope of movement of its slider or wiper (and consequently the output signal gotten) is restricted to the physical size of the potentiometer being utilized.

For instance, a single turn rotational potentiometer usually just has a fixed mechanical rotation of between 0o and about 240 to 330o maximum. Still, one can get multi-turn pots of up to 3600o (10 x 360o) of mechanical rotation.

Most kinds of potentiometers utilize carbon film for their resistive track but these kinds are electrically raucous (the hiss on a radio volume control) and have a brief mechanical life as well.

Rheostats or Wire-wound pots (as they are also called) in the form of a wound coil or straight resistive wire can be utilised as well. However, wire wound pots experience resolution issues as their wiper moves from one wire segment to the next, making a logarithmic (LOG) output, causing errors in the output signal. These also experience electrical noise.

For high accuracy low noise applications, conductive plastic resistance element type polymer film or cermet type potentiometers can now be gotten. They are smooth and they do not produce much noise. They are also highly durable and have high resolution values. They can also be gotten as both single-turn and multi-turn devices. Steering wheels, computer game joysticks, industrial and robot applications are some applications utilizing this sort of high precision position sensor.

Inductive Position Sensors

Linear Variable Differential Transformer

This type of position sensor does not experience mechanical wear issues. This inductive type position sensor operates on the same principle, as the AC transformer that is utilised in determining movement. It is an incredible device for gauging linear displacement and whose output is proportional to the position of its transferrable core.

It essentially comprises of three coils looped on a hollow tube former, one making the primary coil and the other two coils making matching secondaries electrically linked in series but 180º out of phase either part of the primary coil.

A moveable soft iron ferromagnetic core (occasionally referred to as an “armature”) which is linked to the object being evaluated, glides, or moves up and down within the tubular body of the LVDT.

A little Alternate Current voltage is applied to the main winding. This AC voltage is called the excitation signal. The application of this voltage causes an electromotive force signal in the next two secondary windings (transformer principles).

 The armature made from soft iron is precisely placed in a null position between the windings and tube so that they are 180 degrees out of phase. By this arrangement, the positions of the secondary windings nullify each other. Consequently, the core is moved a bit to one side or the other from this zero or null point, the caused voltage in one of the secondary’s will get higher than the other secondary and an output will be generated.

The direction of the moving core and its displacement determine how polar the output signal is. The output signal increases with the increase in motion of the soft iron core as it moves away from its null position at the center. The output is a differential voltage which has a linear variation from the position of the core. Hence, this kind of position sensor has an output signal with these dual characteristics; a polarity showing the movements direction and amplitude which is as a result of the displacement of the core.

The output signal phase can be likened to the phase in which the primary coil is excited. This helps electronic circuits like the AD592 Linear Variable differential transformer sensor amplifier to determine the part of the coil housing the magnetic core and thus determine the travel direction.

When there is a movement of the armature via the center position from an end to another, there is a change in the voltage of the output from the highest to zero and right back to the highest again.

However, the phase angle is changed by 180º in the process. For this reason, the LVDT is enabled to give out an output AC signal with a magnitude that is representative of how much movement there is from the center position. Also, its phase angle is representative of the core’s direction of motion.

A good example of where a linear variable differential transformer (LVDT) sensor can be used is a pressure transducer. Here, a force is produced by the pushing of the measured pressure against a diaphragm. The sensor then converts the force into a voltage signal that can be read.

In comparison with a resistive potentiometer, some of the advantages of the linear variable differential transformer (LVDT) are that it has excellent voltage output to displacement or linearity, highly accurate, has a great resolution, is highly sensitive and operates without friction. In addition, they can be used harsh environment.

Inductive proximity sensors

The inductive proximity sensors which may also be referred to as an Eddy Current Sensor is another kind of inductive position sensor that is commonly used. Although, these sensors do not measure angular rotation or displacement, their main use is to detect an object in close range with them. This is why they are called proximity sensors. These sensors are position sensors that do not require contact but make use of the magnetic field to detect objects using the reed switch which is the most basic magnetic sensor.

An inductive sensor has a coil wound around an iron core inside a field of electromagnetism to give rise to an inductive loop.

Placing a ferromagnetic object inside an Eddy current field that is produced around the inductive sensor like a ferromagnetic metal plate or screw significantly changes the coil’s inductance. The change generating an output voltage is detected by the detection circuit of the proximity sensors. Hence, the electrical principle of Faraday’s law of Inductance is what fuels the inductive proximity sensors.

There are 4 main components of an inductive proximity sensor namely

  • The oscillator producing the electromagnetic field.
  • The coil producing the magnetic field
  • The detection circuit that picks any field changes when there is encroachment by an object.
  • The output circuit generating the output signal using normally open (NO) contacts or normally closed (NC) contacts.

With inductive proximity sensors, metallic objects in front of the head of the sensor can be detected while making contact with the detected object physically. Therefore, they are excellent for usage in wet or dirty environments. Proximity sensors have very small “sensing” range of about 0.1mm to 12mm.

In addition to industrial use, inductive proximity sensors are used normally to control traffic flow by changing traffic lights at cross roads and junctions. Inductive wire loops that are rectangular in shape are sunken deep into the road surface of the tarmac.

On passing over the inductive loop, cars or other road vehicles have their loop inductance changed by the vehicle’s metallic body and the sensor is activated. This signals the traffic light controller that there is a waiting vehicle.

A major disadvantage of these position sensor types is that they sense metallic objects from all directions otherwise said to be “Omni-directional”.

In addition, non-metallic objects are not detected by these sensors. But there are ultrasonic and capacitive sensors. Other magnetic positional sensors available are hall effect sensors, variable reluctance sensors and reed switches.

Rotary Encoders

Rotary Encoders are a kind of position sensor that look like the potentiometers discussed before. However, these encoders are non-contact optical devices utilised in changing the angular point of a rotating shaft into digital or analogue data code. That is to say, they change mechanical movement into electrical signal (digital preferability)

Every optical encoder operates on the same fundamental principle. I move light from an infrared of LED light source through a rotating high resolution encoded disk that has the needed code patterns, either grey code, BCD or binary. The disk is scanned by photodetectors as it rotates and the information is processed by an electronic circuit into a digital form as released binary output pulses are emptied into controllers or counters which ascertain the correct angular point of the shaft.

The two fundamental kinds of rotary optical encoder are Incremental Encoders and Absolute Position Encoders.

Incremental Encoder

It is also called quadrature encoder. Some people also refer to it as a relative rotary encoder. Of all position sensors, it is the easiest to manipulate. The output they generate is a series of square wave pulses made by a photocell arrangement like the codes disk, with properly spaced dark and transparent lines termed segments in its surface, rotate or moves beyond the light source. The encoder induces a release of square wave pulses, which when numbered, shows the angular point of the rotating shaft

Incremental encoders possess two individual outputs known as “quadrature outputs”. Both outputs are displaced at 90 degrees out of phase from each other and the direction of the shaft’s rotation is measured from the sequence of the output.

The amount of dark and transparent sections on the disk shows the device’s resolution and raising the amount of lines in the pattern raises the resolution per degree rotation. The usual encoded discs have a resolution of close to 256 pulses or 8-bits per rotation.

The easiest incremental encoder is known as a tachometer. It possesses a solitary square wave output and is typically utilised in unidirectional applications where just basic speed or position information is needed. The “Sine Wave” or “Quadrature” encoder is generally used and comes with two output square waves. These waves are usually known as channel A and channel B. The device makes use of double photo detectors, a bit displaced from themselves by 90o thus making two individual cosine and sine output signals.

Simple Incremental Encoder

Via the Arc Tangent mathematical function, the angle of the shaft can be measured in radians. Typically, the optical disk utilised in rotary position encoders is circular in nature. Thus, the output’s resolution will be stated as θ = 360/n, where n is the amount of sections on coded disk.

Consequently, for instance, the amount of sections needed to provide an incremental encoder with a resolution of 1o will be 1o = 360/n, thus n = 360 windows, etc. In addition, the rotation’s direction is ascertained by taking note of which channel first delivers output, either channel B or A providing two rotation directions. B leads A or A leads B.

A major downside to using incremental encoders as a position sensor is that they need external counters to ascertain the complete angle of the shaft inside a particular rotation. If the encoder skips a pulse because of a dirty disc or nose or if power temporarily goes off, the subsequent angular information will give an error. A way of surmounting this downside is to utilize absolute position encoders.

Absolute Position Encoder

Absolute Position Encoders are more complicated than quadrature encoders. They offer an inimitable output code for each position of rotation showing both direction and position. Their coded disk comprises of numerous concentric “tracks” of dark and light sections. Each track is self-sufficient with its individual photo detector to concurrently read an exclusive coded position value for every movement angle. The amount of tracks on the disk matches the binary “bit” resolution of the encoder so a 12-bit absolute encoder would possess 12 tracks and the same coded value shows up just once in each revolution.

4-bit Binary Coded Disc

A major benefit of an absolute encoder is its non-volatile memory which recalls the precise position of the encoder without having to go back to a “home” point if power crashes.  Majority of rotary encoders are identified as “single-turn” devices. However, multi-turn devices that get responses over multiple revolutions by including extra code disks, can be gotten.

The usual application of absolute position encoders is in computer hard drives and CD/DVD drives where the absolute position of the drives write/read heads are observed or in plotters/printers to correctly place the printing heads on top of the paper. We have discussed multiple examples of sensors that can be utilised in ascertaining the presence or position of objects in this Position Sensors tutorial. In the next one, we will be dealing with sensors used to gauge temperature like thermostats, thermocouples, thermistors, which are typically termed Temperature Sensors.

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Basic electric circuits that operate independently can be programmed to intermittently pop a light on. A musical note can also be played. However, to be able to carry out any valuable task, an electronic circuit must be capable of communicating with real things, be it interpreting an input signal sent by an ON/OFF toggle or triggering an output device of sorts to make one light come on.

This is to say that there must be something an electronic circuit or system is capable of doing and the best components for this are sensors and transducers.

Transducer is a word used to collectively refer to sensors that are useful in picking a wide array of various forms of energy like radiant energy, movement, thermal energy, electrical signals or magnetic energy and so on and Actuators which can be utilized in switching currents or voltages.

Various kinds of sensors and transducers exist in both analog and digital forms and have input and output options to pick from. The kind of transducer to be made use of is actually dependent on the signal type or process being picked up or controlled. However, a sensor and transducer can be defined as devices that can change a physical quantity from one form to another.

Normally, Sensors refer to devices that receive input. They are so called because they pick the signal when there is a physical change in any feature that is prone to change when there is excitation. An example is force or heat and change it to an electrical signal. Actuators are devices that are so named for output purposes with the purpose of being in control of an external device. An example is motion or sound.

Electrical transducers are used to transform a kind of energy to another, for instance, a microphone which is an input device transforms sound waves to electrical signals which an amplifier would amplify (and this is the process) after which these electrical signals would be changed back to sound waves by a loud speaker (which is an output device). See an example of this kind of input/output system below.

In the market, a large number of various kinds of sensors and transducers are obtainable and choosing which one to make use of is really dependent on the measured or controlled quantity. Check the table below to see the common ones.

Sensors or transducers that are for input give off a signal output or voltage that is equal to the change in measured quantity (that is the stimulus). The kind of sensor in use determines how much of the signal given by the output or the kind. However, sensors of all types fall into one of two categories; Active sensors or Passive sensors.

In general, an external source of power known as an ‘excitation signal’ is needed by active sensors to function. The sensor uses this signal to create the output signal. Active sensors are devices that produce their own signals because their features are transformed as a result of an external effect, take for instance, an output with a voltage of 1 to 10v DC or an output with a current of 4 to 20mA DC. A signal can also be amplified by active sensors.

An LVDT sensor also known as a strain gauge is a great example of an active sensor.  These are external-based networks that are resistant to pressure in a way that an output voltage is produced in accordance with the amount of strain and/or force the sensor receives.

Contrary to what an active sensor does, a passive sensor does not require extra source of power or excitation. Rather, it produces an output signal when there is an outer stimulus. Take for instance, a thermocouple that produces its own output voltage when heat is imposed on it. Hence, passive sensors are sensors acting directly which convert their physical characteristics like inductance, resistance or capacitance and so on.

Just like analogue sensors, a hidden output is created by a digital sensor which represents a binary number or figure like a logic level “0” or “1”.

Analogue and Digital Sensors

Analogue sensors

These sensors give off a voltage or output signal continuously which is equal to the measured quantity. Analog quantities include physical quantities like pressure, temperature, strain, speed, displacement etc. this is because they have the tendency of naturally being continuous. For instance, a thermometer or a thermocouple can be used to measure a liquid’s temperature. The thermometer will continue to respond to the variations in temperature with heating up or cooling of the liquid.

Smooth-changing output signals which continuously change with the passing of time tend to be produced by analog sensors. The signals usually have quite small values from as little as some mico-volts (uV) to many milli-volts (mV) hence, there is need for a kind of amplification.

Also, circuits measuring signals from analog sensors tend to respond slowly and with reduced accuracy. In addition, signals from analog sensors can be changed easily to digital signals to be used in micro-controller systems using ADCs or Analog to digital converters.

Digital sensors

As it is so named, digital sensors produce hidden digital voltages or output signals which represent the measured quantity digitally. Digital sensors give off output signals in Binary form in a logic “1” or “0” form (that is ON or OFF). The implication is that only discrete values that are not continuous are produced by digital signals and these values may be sent out as single “bit” output (which is called a serial transmission) or a combination of the bits may occur to create one “byte” output. This is called parallel transmission.

In our example above, the rate at which the shaft runs can be measured with equipment known as opto-detector sensor. Usually, some slots are incorporated in the design of the disc. The rotation of the disc causes all slots to generate an output pulse which denotes specific levels and logics.

These pulses are sent to a counter and finally to an output display to show the speed or number of revolutions of the shaft. An increase in the number of slots within the disk can lead to an increase in the production of output pulses for each revolution that the shaft makes. An advantage of this is every little change in resolution can be read and measured. This is due to an increase in accuracy and resolution. This type of arrangement makes it easier to manipulate positions since a reference point has been provided.

The accuracy of a digital signal is greater than that of an analog signal. Furthermore, it is possible to measure and sample a digital signal. Sampling is the process of reducing the wavelength of a signal. This reduction is done at a very high speed. How accurate the digital signals would be depends on the number of bits used to represent the measured quantity. For instance, an accuracy of 0.390% can be achieved by making use of a processor that has a rating of 8 bits. On the other hand, we can reach an accuracy value of 0.0015% by making use of 16 bits rated processor. The fact that it is easier to manipulate digital signals than analog quantities makes it possible for the accuracy to be preserved. 

In most cases, analogue sensors can only be powered by external means. Also, in order to produce a good electrical signal, this signal needs to be filtered and amplified. A good electrical signal is one whose value can be detected with a measuring instrument.  We can also achieve this by making use of an operational amplifier.

Signal Conditioning of Sensors

As discussed ab initio, an operational amplifier makes it possible for us to amplify or enhance signal value. 

Therefore, to provide any useful signal, a sensor’s output signal has to be amplified such that it has a voltage gain of up to 10,000 and a current gain of up to 1,000,000 with the amplification being such that the output signal is an exact reproduction of the input, just with a change in amplitude.

Signal conditioning entails amplification of signals. In order to use an analog sensor, matching of impedance, signal amplification, and chasm between the output and input are required. Without these, it is impossible to use this signal.

When trying to get the value of a physical change in an output signal, foreign signals may interfere and cause inaccuracy and discrepancy. These foreign signals are known as noise. To reduce the noise, we can make use of various filtering methods. The interference can also be curtailed by conditioning signal. This is discussed in our Active Filter tutorial.

By using either a Low Pass, or a High Pass or even Band Pass filter, the “bandwidth” of the noise can be reduced to leave just the output signal required. If the interference is at a particular frequency, we can make use of a notch filter to generate frequency-selective signals

Typical Op-amp Filters

Where some random noise remains after filtering, it may be necessary to take several samples and then find their average to give the final value thereby. This helps ensure there is less noise and more signal. Either way, the importance of filtering and amplifying signals cannot be downplayed. They help sensors and transducers function very well especially in real world applications. 

In subsequent texts, we will discuss positional sensors. Positional sensors give us the accurate value of the position of a body across different axis, angle, and positions.

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Percentage Differential Relay, what is it?


A percentage differential relay device is a more sophisticated form of the Differential Protection Relay and it is the preferred differential relay device. It carries out the same function which is basically to protect the system from damage by monitoring the phase difference of two or more alike electrical quantities within an electrical system.

The only major difference between the Percentage Differential Relay and the Differential Protection Relay is the presence of the restraining coil. The restraining coil is sometimes known as the bias coil. This is because of its ability to generate extra flux for the system. Thus, the percentage differential relay can also be called the Biased Differential Relay.

As shown in the diagram below, the percentage differential relay is made up of the restraining coil, an operating coil, the trip circuit and the protected element.

The restraining coil is connected to the pilot wires and the current induced at the two current transformers, found at both ends of the protected element, flows through the pilot wire to it. The connection of the operating coil is done in the middle of the restraining coil.

The most important function of the restraining coil is to regulate the sensitivity of the relay to changes in current. It controls the way the trip circuit responds to the fault current and helps to create a balance in the current ratio.

How the Percentage Differential Relay Works

 When the system is working optimally, the torque produced by the restraining coil is much more than the torque produced by the operating coil. The torque produced by the restraining coil is responsible for keeping the contacts of the trip circuits open while the torque of the operating coil works to close the contacts. Since the torque of the restraining coil is greater, the trip circuits stay open and thus the relay remains inactive.

When there is an overload of current or the occurrence of any kind of fault in the system, the torque of the operating coil becomes greater than that of the restraining coil. When this happens, it becomes possible for it to close the contacts of the trip circuit and alert the system of danger. This sends signals to shut down the circuit breaker and thus, the relay is activated.

The differential or additional current needed to trip the circuit and activate the relay is not a fixed value. This is because the torque in the restraining coil can be adjusted. The magnitude of the differential current in the operating coil can be found by calculating the difference between the initial (or normal) current and the tripping current, i.e. I1 – I2. The current flows to the restraining coil at mid-point, thus the differential current in the restraining coil is given to be (I1 – I2)/2.

 When there is a fault from external forces or causes, both the initial and tripping current increase, and thus the torque of the restraining coil is increased which helps it function better in controlling the sensitivity of the relay and other faults.

Operating Characteristic

According to the figure above, the ratio of the magnitude of the operating current to the magnitude of the restraining current is constant.

Types of Percentage Differential Relay

Basically, there are two main types of percentage differential relay. They are:

  • Induction-type Biased Differential Relay
  • Three Terminal System Application

Induction-type Biased Differential Relay

This relay is made up of a pivoted disc, a copper ring and two electromagnets which form the restraining and operating elements. The torque possessed by both elements act on the disc and causes it to rotate between the two electromagnets.

When the effect of the restraining torque on the disc is zero, the ring would occupy the same position on the two elements, as in the diagram above. However, if the torque exerted by the restraining torque is more than that exerted by the operating torque, it would shift the portion of the ring on the restraining element further towards the iron core.

Three Terminal System Application

This is a high-speed relay. In this relay, the restraining and operating elements have three terminals each. Each of the coils produces a different torque magnitude which can be added mathematically.

As current flows through the restraining coils, the operating characteristic of the relay changes.


Percentage differential relays are used widely to prevent damage in transmission lines, generators, feeders, etc.

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A generator is a piece of apparatus or equipment that converts energy from the chemical form to electrical form. Generators are subjected to electrical stress on the machine insulation, mechanical forces working on different variable components of the machine and rise in temperature. A machine in good condition doesn’t only keep up a trifling performance for a couple of years, it will also stand up to far more than machine overload. With this in mind, preventive measures should be undertaken to keep off overloads and other abnormal conditions concerning the machine so it serves to perfection. All things must be taken into account, for example construction, economic style, operation, sound etc nothing should be eliminated.

In an electrical generator if there are two faults to look out for: Internal fault and External fault. The electrical generator can be made subject to either or both faults. These generators are usually linked to a power system (or systems) therefore any fault occurring in the power grid should be cleared from the generator quickly as potential otherwise he could develop into a permanent injury in the generator.

There are many schemes designed to protect the generator as the faults are numerous and large in size. Generator protection can be broken down into parts. Care should be taken in synchronising systems used and by extension, the settings obtained so all sensitive, discriminative and selective generator protection themes are achieved. The three types of generator protection are:

1. Protective relays for fault detection within generator

2. Protective relays for fault detection outside the generator.

Apart from these relays, there are oil flow devices, lightning arresters, strator coil winding shaft bearing devices for temperature, overspeed safeguards, electrical devices, oil and winding etc. Some of these protection devices are units of non-trip or stop this means these devices only generate alarm throughout all abnormalities.

Protecting skins basically operate the master tripping relay of the generator. Note that protective relays do not delay fault, they only indicate and minimize its time period of fault to prevent extreme temperature rise within generators else permanent injury will be seen. It is important to avoid unnecessary knots within the generator and to this end, one must put in surge capacitance or a combination of surge capacitance and surge diverter. This helps to reduce the consequences of alternative and lightening surgeries in the machine. Below are some protection schemes usually applicable in generators: Stator Earth Fault Protection, Protection against Insulation failure, Rotor failure Fault Protection, Protection against Stator Overheating, Low Vacm Protection, Protection from unbalanced stator, Protection against Loss of boiler firing, Protection against Prime Mover failure, Protection against Lubrication failure, Overspeed protection, Protection against Rotor Distortion, Protection against vibration, Protection against difference in expansion between rotating and stationary parts and Back up Protection of Generators.

1. Stator Earth Fault Protection

When the device neutral line is past, a resistance in current electrical device is set within the neutral to earth affiliations. When relay operation time is inversely proportional to the fault current it is known as Inverse Time Relay. This phenomenon is applied across the CT secondary section once the generator is connected on to the bus bar; in this case power is fed from the generator via delta star electrical device and instant relay is applied for constant purposes. In the former situation, the planet faults relay is needed to be sequentially arranged with various fault relays within the system. This is possibly the explanation as to why Inverse Time Relay is used in this situation. In the latter situation, the planet fault loop is limited to the mechanical device winding and the first winding of the electrical device therefore there is no segregation with different earth fault relays in the system. For this reason, Inverse Relay is preferred in this case.

2. Protection against Insulation Failure

Longitudinal differential protection is the paramount protection within the winding of a generator.

Repose Flip Fault Protection is another protection for the winding of a generator. In previous days, the protection type was thought to be unfounded as a result of insulation breakdown between points in the same winding section.

Ideally, a generator should provide high voltage as compared to its output and that involves an outsized variety of conductors per slot. As generator size and voltage increases, this protection type is becoming terribly essential for massive units of generation.

3. Rotor Earth Fault Protection

A single Earth fault doesn’t produce any serious disadvantage in the generator but if the second Earth fault occurs, a part of the winding sector can short-circuit and ensure therefore unbalancing magnetic flux in the system and consequently there could be a serious mechanical injury to the generator bearings. There are three working strategies offered in fault groups in the rotor. These are:

a. AC injection method

b. DC injection method

c. Potentiometer method

4. Protection against Stator Overheating

Overloading will cause slow and steady warming in the stator coil winding of the generator. Apart from overloading, insulation and cooling system failures in stator oil laminations also cause overheating of the stator coil winding. The heating is discovered by deeply lodged temperature detectors at different points in the stator coil winding. Usually, temperature detector coils are designed to be highly resistant. They serve as an arm of the bridge circuit. Smaller generators do not usually have these temperature coils. Instead, they have a thermal relay which tracks the flow of current in the stator coil.  By this arrangement, only detection of overheating is possible, there is no protection from overheating (caused by cooling system failure and or short-circuiting stator oil laminations).

5. Low vacuum Protection

Now, this type of protection is within a kind of regulator that compares the vacuum against gas pressure. As a general rule, it is fitted to the generator and its power set higher than thirty (30) MW. The more appealing thing is for the regulator to discharge the set via the secondary governor until the usual vacuum conditions are restored.

6. Unbalanced Stator Loading Protection

An imbalance in load produces negative currents in the starter coil circuit. The negativity of this current produces a reaction field which rotates at double synchronized speed with its rotor and induces a higher magnitude of current in the rotor. This is a giant current which causes heating in the rotor circuit, specifically inside the generator Also, imbalance that occurs as a result of external faults or unbalanced loading in the system can be resolved by not observing said issue. If it continues, these faults can be cleared by putting in a negative section sequence relay with traits to match with the curve of the machine.

7. Protection against Loss of Boiling firing

Two techniques are double in investigating the loss of boiler firing. In the first technique non-distinguishable opened contact area unit given the fan motors that can trip the generator of two motors fail. The second technique uses boiler pressure contacts that unburden the generator if the pressure in the boiler sinks below ninety (90).

8. Protection against the failure of a Prime Mover

It is still possible for the generator to rotate even if energy is not a consequence of the first cause. Driving mode means it takes current from the system rather than send it to the system. In a rotary engine set, the steam acts as a fluid maintaining oil turbine blades at an unyielding temperature. Availability failure can lead to heating due to friction with distorted rotary engine blades. The steam failure of our could cause critical mechanical damage plus enforcing an impressive driving load on the generator. Reverse power relay is used for this reason. Now because the generator starts to rotate in driving mode (look up the meaning above) the reverse power relay could possibly trip the generator.

9. Protection against Lubrication failure

This protection type is not thought of as necessary since lubrication oil is often gained from an identical pump as the failure of governor or oil and we create a stop to shut the valve.

10. Overspeed Protection

Developing mechanical overspeed devices on every Steam and hydro rotary engine produced that operate on the steam throttle or main step valve is the general way of things. Though not the norm to back up these devices by degree overspeed relay on steam driven sets, it is seen as smart and sensible to lead on electricity units and because of the response of the governor, it is sort of slow and so is set liable to over-speed. Once fixed perfectly, the relay is at times provided from the static magnet generator used for governor management.

11. Protection against Rotor Distortion

Following termination, the cooling rates at highest and lowest of the rotary engine casing certain measures (examples: Square) are totally different and this irregular temperature distribution causes destruction of the rotor and thus reduces the is also commonly applied to show the rotor at a low speed though its cooling down within the study of the forces concerned with the massive trendy rotor. It is currently at a requirement to apply shaft eccentricity detectors.

12. Protection against vibration

Vibration detectors are at times set up as pedestals. These detectors are made up of a coil set on springs and sandwiched between some permanent magnets. The voltage output from the coil is proportional to the degree of vibration and is passed from the coil to group action circuits and into interval indicating instrument.

13. Protection against expansive difference between stationary parts and rotating parts of the generator. 

Due to a difference in mass, the rate at which the rotor gets heated is different from that of the casing. The result of this is the expansion of the rotor at a significantly to the casing and it is mandatory that the irregular growth is beat. Propositions are created on the bigger machine for freelance which makes provision for steam to be set to bind joints on the casing. In this, a strategy is produced for axial growth measure to assist in feeding him to all right points and jointly to produce signals of any risky or perilous growth. Shaft axle growth detector is mainly the exact same as portrayed or rooted rotor distortion except that the detector magnets are set to the rotary engine casing.

14. Back up Protection of Generators

Backup protection is good for strongly rated machines like synchronized generators. If faults were to occur and perchance had not been taken care of by the adequate protection theme then identical protection relays should be made and used to erase the fault. Overcurrent relays are mostly used for this reason. As the end product of the synchronous electrical phenomenon of modern machines is a lot larger than hundred percent (100%), the maintained fault current fed from the machine into external fault is invariably below the usual full load current. the typical idmt relays would not prove satisfying as a result of their current settings. The overcurrent relay would furthermore probably work for loss of field on the machine disconnecting it’s finally the only way to beat this is to allow it to be mandatory to associate over current relay together.

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