Types of switch
Any device which could be used to distrupt/interrupt electrons circulating within a circuit is known as electrical switch. Essentially, switches are binary devices: they are either entirely on (“closed”) or totally off (“open”). There are a lot of different kinds of switches and in this chapter we will discuss some of those forms.
Although it may seem odd to discuss this basic electrical subject at such a late stage in this book series, I do so because the following chapters address an older field of digital technology focused on mechanical switch contacts rather than solid-state gate circuits, and the undertaking needs a thorough understanding of switch types. Understanding the function of switch-based circuits while also mastering solid-state logic gates helps make both topics easier to understand and it sets the stage for an improved learning experience in Boolean algebra, the digital logic circuit mathematics.
The simplest type of switch is referred to as contact block, where the action of an actuating mechanism brings two electrical conductors into contact with one another. Some switches are more complicated, comprising electronic circuits that are capable of turning on or off based on some perceived physical stimuli (such as magnetic field or light). In any scenario, the final output of any kind of switch will be (at least) a pair of wire-connection terminals which are either bound by the internal communication mechanism (“closed”) of the switch, or not linked together (“open”).
Any switch intended to be operated by a human is generally referred to as a hand switch, and it is made in several varieties:
A lever which is angled in one of two or more positions actuates the toggle switches. A toggle switch is one example of a popular light switch which is used in household wiring. Most toggle switches are designed come to rest at any of their lever positions, and some will have an internal spring mechanism which will return the lever to some kind of normal position allowing what is termed “momentary” operation.
The push-button switches are two-position devices that are controlled with a push and release button. For momentary action, majority of push-button switches have a spring mechanism internally which returns the button to its “unpressed,” or “out,” position. With every press of the button, several push-button switches will turn on or off alternately. Some push-button switches will remain in their “pressed” or “in” position till the button is pulled back out. Normally, this type of push-button switches does have a mushroom-shaped button for quick push-pull operation.
This type of switches are actuated with some sort of rotary knob or lever to select one of two positions or more. Compared to the toggle switch, selector switches can either stop at any of their positions or have spring-return mechanisms for momentary activity.
A joystick switch is controlled by a lever which can move in more than one direction. One or more of the mechanisms for switch contact are actuated based on how far and which way the lever is pushed. On the switch symbol is the circle-and-dot notation which shows the direction of the joystick lever motion needed to actuate the contact. Joystick hand switches are widely used for operating cranes and robots.
Many switches are designed specifically to be controlled by a machine’s action, rather than by a human operator’s hand. Such motion controlled switches are commonly referred to as limit switches, as they are often used to restrict a machine’s motion by shutting off the actuating control to a device if it goes too far. Limit switches are of several types, as with hand switches:
Lever actuator limit switch
These limit switches closely resemble selector hand or rugged toggle switches equipped with a lever driven by the system component. The levers are often tipped with a small roller bearing which prevents the lever from getting worn off by repeated contact with the part of the machine.
Proximity switches detect either a magnetic or high frequency electromagnetic field approaching a metallic system component. Simple proximity switches use a permanent magnet to control a sealed switch mechanism while closing the system component (usually 1 inch or less). More sophisticated proximity switches operate like a metal detector, energizing a wire coil with a high-frequency current, and measuring the amplitude of that current electronically. If a metallic (not inherently magnetic) part gets close enough to the wire, the current increases, and the control circuit goes off. The proximity switch symbol shown here is of the electronic type, as shown by the diamond-shaped box circling the switch. A non-electronic limit switch and the lever-actuated limit switch have the same symbol.
The optical switch, containing a photocell and a light source, is another type of proximity switch. The position of the machine is sensed either by a light beam interruption or by reflection. Often, optical switches are effective in protection systems, where light beams can be used to identify workers approaching a dangerous area.
The monitoring of various physical quantities with switches is necessary in many industrial processes. These switches may be used to sound warnings, signaling that a process variable has surpassed standard thresholds, or they may be used to shut down systems or facilities when those variables have reached unsafe or harmful rates. There are different types of process switches:
These switches detect a shaft’s rotational speed either through a centrifugal weight device installed on the shaft, or through some kind of non-contact sensing of shaft motion such as magnetic or optical.
Liquid pressure or Pressure switch Gas may be used to actuate a switch function if that pressure is applied to a bellow, diaphragm, or piston, which transforms pressure to mechanical force.
The “bimetallic strip” is an inexpensive temperature sensing mechanism: a thin strip of two metals, joined back to back, with each metal having a different thermal expansion rate. Differing levels of thermal expansion between the two metals allow it to bend when the strip heats or cools. Then, a switch contact mechanism can then be actuated by the bending of the strip. Many temperature controls use a brass bulb packed with either a gas or liquid, with a tiny tube linking the bulb to a switch that detects heat. The gas or liquid expands as the filament is heated, creating an increase in pressure which then stimulates the switching mechanism.
Liquid level switch
A floating body may be used to actuate a switch mechanism when the volume of liquid in a tank increases past a certain point. If the liquid is electrically conductive, the liquid itself may be used as a conductor for bridging at the appropriate depth between two metal probes embedded in the tank. The conductivity strategy is typically applied through the conductive material, with a specific relay configuration caused by a small amount of current. In most situations, moving the circuit’s full load current through a liquid is inefficient and risky.
The amount of solid materials such as animal feed, coal, wood chips or grain in a hopper, bin, or a storage silo can also be detected by the use of specially designed level switches. A typical design for this usage is a small paddle wheel, placed at the desired height into the bin which is turned gradually by a small electric motor. The material stops the paddle wheel from spinning once the solid material fills the bin to this height. The small engine’s torque response then trips the switch mechanism. One other design utilizes a metal prong shaped “tuning fork,” placed into the bin at the desired height from the outside. The fork is vibrated by an electronic circuit and the magnet / electromagnet coil assembly at its resonant frequency. When the bin fills up to that height, the solid material dampens the fork force, the vibration intensity and/or frequency shift that the electronic circuit senses.
Liquid flow switch
Inserted in a pipe, a flow switch will detect any liquid or gas flow rate that exceeds a certain level, usually with a small vane or paddle that is pushed by the flow. Other flow switches are designed as differential pressure switches, measuring the pressure drop over a pipe-built restriction.
Another kind of level switch is the nuclear switch, ideal for detecting solid or liquid materials. Composed of a radiation detector and a radioactive source material, both are mounted for either liquid or solid material over the diameter of a storage vessel. Any material height above the level of the detector/source arrangement will reduce the radiation strength that reaches the detector. This decrease in detector radiation may be used to activate a relay mechanism to provide a switch contact for vessel level measurement, alarm point, or even control.
Both source and detector are outside the vessel, and there is no intrusion except for the radiation flux itself. The radioactive sources used do not pose any immediate threat to maintenance or operations staff as they are fairly weak.
Usually, there is more than a way of implementing the switch to serve as an operator control or monitor a physical process. For any application, there is usually no single “perfect” switch, although some obviously show some advantages over others. Switches have to be intelligently adapted to the task for reliable and efficient operation.
• The switch is an electrical device. It is usually electromechanical and is used to control the continuity between two points.
• Hand switches are powered with human touch.
• Limit switches are machine movement driven.
• Process switches are activated by some physical process changes (temperature, flow, level, etc.).
Switch contact design
A switch may be built with any mechanism putting two conductors into contact with each other in a regulated manner. This can be as easy as having two copper wires to meet each other through the action of a lever or by moving two metal strips into direct contact. Nevertheless, a proper switch design must be reliable and rugged, and avoid the possibility of electrical shock posed to the user. Industrial switch designs therefore are never that basic.
The conductive components used for making and breaking the electrical connection in a switch are called contacts. Contacts are made primarily from silver or silver cadmium alloy, the conductive properties of which are not significantly impaired by oxidation or surface corrosion. Gold contacts have the greatest resistance to corrosion, but are reduced in current-carrying capacity and may be “cold welded” when combined with high mechanical force. The switch contacts are driven by a system that guarantees even and square contact for optimum reliability and minimal resistance, whatever the metal preference.
Contacts like these can be built to accommodate extremely large quantities of electric current, in some cases up to thousands of volts. The limiting factors for the ampacity of the switch contact are:
• Sparking triggered by opening or closing contacts;
• Voltage over open switch contacts (potential of jumping current across the gap).
• Heat produced by the current from metal contacts (while closed).
The exposure of the contacts in the standard switch contacts to the surrounding atmosphere is a major disadvantage. This is usually not a problem in a good, tidy, control-room setting. But most industrial conditions aren’t so innocuous. The existence of corrosive chemicals in the air will cause the contacts to prematurely deteriorate and fail. The risk of regular contact sparking which causes flammable or explosive chemicals to ignite is even more worrying.
When there are such environmental concerns, certain forms of contacts for small switches may be considered. These other contact types are sealed from outside air contact and therefore do not experience the same exposure problems as standard contacts do.
The mercury switch is one common type of sealed-contact switch. Mercury is a metallic element and it is liquid at room-temperature. Being a metal, it has outstanding conductive characteristics. It can be put into contact within a sealed chamber with metal probes (to close a circuit) as it a liquid simply by inclining the chamber so that the probes are on the bottom Most commercial switches use small mercury-containing glass tubes that are tilted one way to open the contact, and tilted another way to close it. Such systems are an excellent alternative to open-air transfer connections anywhere environmental exposure issues are concerned, apart from the problems of tube breakage and spilled mercury (which however is a toxic material), and sensitivity to vibration.
There, in the open position, a mercury switch (often referred to as tilt switch) is shown where the mercury is seen to be out of contact with the two metal contacts and also at the other end of the glass bulb: the same switch is shown in the closed position. Gravity also holds the liquid mercury in contact with the two metal contacts, ensuring electrical stability from one to the other: Mercury switch contacts are inefficient to create in large sizes, so you will typically find these contacts rated at no more than 120 volts and no more than a few amps. Of course, there are variations but these are some common limits.
Another type of sealed-contact switch is the magnetic reed switch. As with the mercury switch, the contacts of a reed switch are found within a sealed tube. Unlike the mercury switch that utilizes liquid metal as the contact medium, the reed switch is essentially a pair of very small, magnetic, metal strips (hence the term “reed”) that are brought into contact with each other by adding a strong magnetic field outside the sealed loop. The magnetic field source in this type of switch is typically a permanent magnet, which is pushed by the actuating mechanism closer or further away from the tube. This form of contact is usually measured at lower voltages and currents than the normal mercury switch, due to the small size of the reeds. Usually, reed switches tolerate vibration better than mercury contacts though, because there is no liquid inside the conduit to splash about.
General-purpose switch contact voltage and current levels are commonly found to be higher on any specified switch or relay if the electrical power being transferred is AC instead of DC. The explanation for this is the tendency of an alternating-current arc over an air gap to self-extinguish. Due to the fact that 60 Hz power line current ceases and reverses direction 120 times per second, there are many ways for an arc’s ionized air to lose adequate temperature to avoid conducting current, to the extent where the arc will not restart at the next voltage level. However, DC is a steady, constant movement of electrons that tends to keep an arc even smoother over an air gap. Therefore, by switching a specified value of the direct current, switch contacts of any kind cause more wear than for the same amount of alternating current. When the load has a significant amount of inductance, the issue of switching DC is amplified, as there will be very strong voltages produced through the contacts of the switch when the circuit is opened (the inducer will do its best to maintain circuit current at the same magnitude as when the switch is closed).
Contact arcing can be reduced with both AC and DC by inserting a “snubber” circuit (a resistor and capacitor wired in series) in parallel with the contact, such as this: a sudden rise in voltage across the switch contact induced by the contact opening will be tempered by the charging operation of the capacitor (the capacitor countering the rise in voltage through drawing current). The amount of current discharged through the contact whenever it closes again is limited by the resistor. If the resistor was not present, the capacitor could actually make the arcing worse during contact closure than the arcing during contact opening without a capacitor! While this extension to the circuit tends to reduce contact arcing, it is not without disadvantage: a prime factor is the probability of a broken (shortened) resistor / capacitor combination supplying electrons with a route to pass through the circuit at all times, even if the contact is open and the current is not needed. The threat of this malfunction, and the extent of the resulting consequences, must be weighed against the enhanced contact wear without the snubber circuit (and eventual failure of the contact).
One thing which is not new is the use of snubbers in DC switch circuits: this has been practiced by automobile manufacturers on engine ignition systems for years, reducing arcing across the switch contact “points” in the supplier with a tiny capacitor called a condenser. As any technician will tell you: the service life of the “points” of the distributor directly depends on the operation of the condenser.
With all this talk about minimizing switch contact arcing, one might assume that for a mechanical switch, less current is always better. This isn’t always so, however. A small amount of intermittent arcing will work out great for the switch contacts, because it protects the contact faces from corrosion and small amounts of debris. The contacts may tend to accumulate undue resistance if a mechanical switch contact is worked with too little current and may fail prematurely! This minimum amount of electrical current that is required to sustain mechanical switch contact in good health is called the wetting current.
In a properly designed system, the wetting current rating of a switch is normally well below its maximum current rating, and also well below its normal operating current load. Nonetheless, there are situations where the contact a mechanical switch may be needed to manage currents regularly below usual wetting current limits (for example, when a mechanical selector switch has to open or close an analog electronic or a digital logic circuit where there is an extremely small current value). Gold-plated switch contacts are highly recommended to be listed in those applications. Gold will not corrode like other metals as it is a “noble” metal. As a consequence, such contacts have extremely low current wetting criteria. Regular contacts with the silver or copper alloy will not provide effective operation when used in this low-current service!
• Reed switches are another form of sealed-contact system, the contact being created by two thin metal “reeds” inside a glass tube, coupled by an external magnetic field effect.
• Mercury switches use the liquid mercury metal slug as a moving contact. Shielded in a glass tube, the spark of the mercury contact is shielded from the outside world, rendering this form of switch ideally suited for atmospheres that host potentially explosive vapors.
• The parts of the switch responsible for making and breaking electrical synchronization are considered the “contacts.” Usually made of corrosion-resistant metal alloy, contacts are designed to meet each other by a process that helps to maintain proper alignment and spacing.
• Switch contacts suffer from DC switching more than AC. This is due primarily to the self-extinguishing nature of an AC arc.
• To reduce contact arcing, a resistor-capacitor network called a “snubber” may be connected in parallel to a switch contact.
• The minimum amount of electric current needed to carry a switch contact to be self-cleaned is called wetting current. This value is normally far below the maximum current rating for the switch
The make/break sequence and “normal” state of contact
Any kind of switch contact can be configured such that the contacts “close” (establish continuity) when actuated, or “open” (interrupt continuity) when actuated. In switches that have a spring-return function in them, the spring returns it to a direction with no applied force. This direction is known as the normal position Then contacts open in this position are normally called open, and contacts closed in this position are normally called closed.
The normal position, or condition, for process switches is what the switch is in when there is no process control over it. One easy way to figure out a process switch’s usual state is to find the switch status as it lies uninstalled on a storage shelf. Definitions of “normal” process switch situations are as follows:
• Pressure switch: When the pressure applied is zero
• Speed switch: shaft not spinning
• Level switch: empty bin or tank
• Flow switch: Zero flow of liquid
• Temperature switch: ambient temperature
One important thing to do is to distinguish between the “normal” state of the switch and its “normal” use in the operational process Consider the example of a liquid flow switch that acts as a low-flow alarm in a system of cooling water. The cooling water system’s natural, or properly functioning state is to have a fairly constant coolant flow through this pipe. If we want to close the contact of the flow switch in case of a lack of coolant flow (for example, to complete an electrical circuit that triggers an alarm siren), we would like to rather than use a flow switch with normally-open contacts use one with normally-closed contacts. The contacts of the switch are forced open when there is sufficient flow through the pipe; when the flow rate decreases to an abnormally low point the contacts revert to their usual (closed) state. Thinking of “normal” as the process’s usual state could be confusing, so be careful to always think of the “normal” state of a switch as that in which it is as it is sitting on a shelf.
The symbol for switches varies depending on the intent and actuation of the switch. A normally open switch contain contact is designed in such a way that it indicates an open connection ready to shut when it is actuated. In comparison, a switch that is normally closed is designed as a closed connection that when actuated, opens. Remember the following symbols: Each switch contact also has a common symbol, representing the contact points within a switch with a pair of vertical lines. Normally open contacts are marked by non-touching lines while normally closed contacts are defined with a bridging diagonal line between the two lines. Compare the two:
When actuated, the switch seen on the left will close and open while in the normal state (unactuated). Once actuated, the switch on the right opens and is closed in the normal (non-actuated) position. When switches are described with these common symbols, the type of switch should typically be indicated in text next to the symbol immediately Please note the icon on the left should not be mistaken with a capacitor’s. If a capacitor requires to be depicted in a diagram of control logic, it will be shown as follows:
In regular electronic symbology, the above figure is intended for polarity-sensitive capacitors. The capacitor symbol is used for every type of capacitor in control logic symbology, even when the capacitor is not prone to polarity, so as to clearly differentiate it from a normal-open switch contact
A further design aspect must be addressed for multiple-position selector switches: that is, the process of breaking old connections and making new connections as the switch is pushed from position to position, the moving contact reaching numerous stationary contacts in sequence
The selector switch shown above moves the common contact lever to one out of five different positions, to connect wires numbered 1 to 5. The most popular design of a multi-position switch like this is one such that contact is interrupted with one place before contact is made with the next position. This design is known as break-before-make. For example, if the switch was placed at position number 3 and turned slowly in the clockwise direction, the contact lever would shift off the position number 3, getting the circuit to open, going to a position between numbers 3 and 4 (both circuit paths open), and then touching position number 4, and getting the circuit to close.
There are situations where full opening of the circuit attached to the common wire at any point in time is unacceptable. A make-before-breakswitch mechanism can be designed for such an operation, in which the movable contact lever essentially bridges between two contact positions (between number 3 and number 4, in the scenario above) as it moves between positions. The concession here is that, as the selector knob is rotated from position to position, the circuit must be able to manage switch closures between neighboring position contacts (1 and 2, 2 and 3, 3, and 4, 4 and 5). Here is such switch:
If movable contacts with stationary contacts can be put into one of many positions, such positions are sometimes referred to as throws. Often, the number of movable contacts is called poles. The selector switches seen above would be defined as “single-pole, five-throw” switches with five stationary contacts and one moving contact.
If two identical five-throw, single-pole switches were mechanically coupled in such a way that they were operated by the same mechanism, the whole assembly will be called a “double-pole, five-throw” switch:
With process switches, this is the uninstalled state it is in while sitting on a shelf.
Some common configurations of switch and their abbreviated descriptions:
- An open unactuated switch is called normally-open switch. An unactuated closed switch is referred to as normally-closed. The terms “normally-closed” and “normally-open” are sometimes abbreviated as N.C and N.O., respectively.
- The general symbol for N.C. and N.O.
- Multi-position switches can be either make-before-break or the very common one, break-before-make.
- The switch “poles” refers to the number of moving contacts whereas the switch “throws” refers to the amount of stationary contacts per moving contact.
In a single, clean instant, when a switch is triggered and contacts touch each other under the actuating force they are expected to maintain continuity. However, though, switches do not achieve exactly this aim. Because of any inherent elasticity in the mechanism and/or contact materials, and mass of the moving contact, contacts will “bounce” for a period of milliseconds upon closure before coming to full rest and providing unbroken contact. Switch bounce is of no concern in many applications: it matters little if a switch regulating an incandescent lamp “bounces” for a few cycles whenever it is actuated. Since the warm-up time of the lamp exceeds considerably the bounce duration, there will be no irregularity in the activity of the lamp.
Nevertheless, if the switch is used to send a signal with a quick response time to an electronic amplifier or to some other circuit, contact bounce may cause very visible and undesired effects:
A closer look at the oscilloscope monitor shows a rather hideous set of makes and breaks when the switch is triggered one time:
When, for example, this switch is used to transmit a “clock” signal to a digital counter circuit, so that each pushbutton switch is intended to raise the counter by a value of 1, what will happen then is that the counter increments (by several counts) each time the switch is actuated. Since mechanical switches also interface in modern systems with digital electronic circuits, switch contact bounce is a frequent feature of the architecture. Somehow, it is necessary to eliminate the “chattering” created by bouncing contacts, so that the receiving circuit has a smooth, crisp off / on transition:
Switch contacts can be debounced in several different ways. The most straightforward method, is to address the problem at its core: the switch itself. Here are some ideas for the design of minimal bounce switch mechanisms:
- Reducing the kinetic energy of the moving contact. This reduces the impact force when it comes to rest on the stationary contact, thus reducing the bounce.
- Use the stationary contact(s) with “buffer springs” so that they are able to rebound and softly absorb the impact force from the moving contact
- Build the “wipe” or “slide” contact switch rather than the direct impact. Sliding contacts are used in “knife” switch designs.
- Dampen the action of the switch mechanism using the shock absorber” device for air or oil.
- Using sets of contacts in parallel, each slightly different in mass or contact gap such that when one is rebounding off the stationary contact at least one of the others is still in firm contact.
- Get the contacts “wet” in a sealed environment with liquid mercury. When initial contact is made, mercury “surface tension should maintain continuity of the circuit although the moving contact may bounce off the stationary contact multiple times.
Each of these solutions compromises some element of switch efficiency for limited bounce, and designing all switches with limited contact bounce in mind is unpractical. Attempts made to minimize the contact’s kinetic energy may result in a small open-contact distance or a slow-moving contact, which reduces the voltage that the switch can accommodate and the amount of current it can interrupt Although non-bouncing, sliding contacts often create “noise” (abnormal current induced by irregular contact resistance during movement) and suffer from more mechanical damage than normal contacts.
Multiple parallel contacts provide fewer bounce, but only more cost and switch complexity. The use of mercury to “wet” the contacts is a very effective means of reducing the bounce, but it is sadly restricted to low-ampacity switch contacts. Also in mounting position mercury-wetted contacts are usually limited, because gravity may trigger the contacts to bridge inadvertently if they are positioned the wrong way.
If re-designing the switch system is not a solution, mechanical switch contacts can be externally debounced, utilizing supplementary circuit components to modify the signal. For example, a low-pass filter circuit connected to the switch output can minimize the voltage / current fluctuations produced by the contact bounce:
Switch contacts can be electronically debounced using hysteretic transistor circuits (high or low-state “latch” circuits) with built-in time delays (known as “one-shot” circuits), or two inputs that are controlled by a double-throw switch.