Answer from Joe (Technician)
A solid state relay also known as SSR, is an electrical switching device. It switches off and on when a small amount of voltage is passed across its control terminals. It can be used to load either Alternating Current (AC) or Direct Current (DC) and serves the same functions as the Electromechanical Relay except for a few differences.
Packaged solid state relays can switch current up to about a hundred amperes (100A) and they do this by the help of power semiconductors such as Thyristors and Transistors. Compared to electromechanical relays, they have faster switching speed and generally last longer because they do not have contact parts to wear out. However, solid state relays have a lower capacity to withstand momentary overload and also have a higher level of resistance to the “ON” state, in contrast to the electromechanical relays. Also, unlike the electromechanical relays, SSR devices provide limited switching arrangements.
Obviously, there are sharp contrasts between Solid State Relays and the Electromechanical Relays. Solid state relays have a few advantages over the electromechanical relay devices. Below are a number of these advantages:
- They have no moving or contact parts that can wear out or build up carbon, thus they have the tendency to last longer than electromechanical relays.
- It has a totally silent and bounceless operation.
- They are generally smaller in size than similar electromechanical relay specifications with slimmer profiles which allow for tighter packing.
- The amount of time an optical solid state relay device takes to switch ON/OFF is usually between a few microseconds to milliseconds. Thus they have a faster switching speed range than electromechanical relays.
- They are safer, especially in explosion prone areas or environments because they do not create any sparks while switching ON/OFF.
- No matter the amount of usage, the output resistance of SSR devices stays the same.
- Compared to electromechanical devices, they have lower sensitivity to environmental and physical conditions such as humidity, external magnetic or electromagnetic force fields, mechanical shock, vibration, etc.
Despite this number of advantages, solid state relays have their fair share of disadvantages. These include:
- Due to their faster switching speed, they have the tendency to switch spuriously because of sudden but short-lived changes in voltage. Also, due to the presence of body diodes, they take longer to recover from these short-lived voltage changes. This amount of time is called Transient Reverse Recovery (Trr) time.
- While electromechanical relay devices have the tendency to fail open, solid state relays generally fail shorted on their outputs.
- For gate charge circuits, solid state relays require an isolated bias supply.
- Unlike electromechanical relays, some SSR devices are affected by polarity.
- When closed or OFF, solid state relays have a higher resistance which generates more heat and produce more electrical noise.
- When open or ON, they have lower resistance and a reverse leakage of current which can cause distortions in waveforms.
Answer from James (Technician)
A solid state relay device is a switching device. It is very similar to the electromechanical relay and it can load either AC or DC.
All solid state relay by square D here.