A solid-state relay (SSR) is an electronic switching device in which a small control signal controls a larger load current or voltage. It consists of a sensor which responds to an appropriate input (control signal), a solid-state electronic switching device which switches power to the load circuitry, and some coupling mechanism to enable the control signal to activate this switch without mechanical parts. The relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts.
The control signal must be coupled to the controlled circuit in a way which isolates the two circuits electrically.
Many SSRs use optical coupling. The control voltage energizes an LED which illuminates and switches on a photo-sensitive diode (photo-voltaic); the diode current turns on a back-to-back thyristor, silicon controlled rectifier, or MOSFET to switch the load. The optical coupling allows the control circuit to be electrically isolated from the load.
A SSR based on a single MOSFET, or multiple MOSFETs in a paralleled array, works well for DC loads.
MOSFETs have an inherent substrate diode that conducts in the reverse direction, so a single MOSFET cannot block current in both directions. For AC (bi-directional) operation two MOSFETs are arranged back to back with their source pins tied together. Their drain pins are connected to either side of the output. The substrate diodes are alternately reverse biased to block current when the relay is off. When the relay is on, the common source is always riding on the instantaneous signal level and both gates are biased positive relative to the source by the photo-diode.
It is common to provide access to the common source so that multiple MOSFETs can be wired in parallel if switching a DC load. Usually a network is provided to speed the turn-off of the MOSFET when the control input is removed.
One significant advantage of a solid-state SCR or TRIAC relay over an electromechanical device is its natural tendency to open the AC circuit only at a point of zero load current. Because SCR's and TRIAC's are thyristors, their inherent hysteresis maintains circuit continuity after the LED is de-energized until the AC current falls below a threshold value (the holding current). In practical terms what this means is the circuit will never be interrupted in the middle of a sine wave peak. Such untimely interruptions in a circuit containing substantial inductance would normally produce large voltage spikes due to the sudden magnetic field collapse around the inductance. This will not happen in a circuit broken by an SCR or TRIAC. This feature is called zero-crossover switching.
|This section requires expansion. (September 2010)|
The relay is characterised by a number of parameters including the required activating input voltage, current, output voltage and current, whether it is AC or DC, voltage drop or resistance affecting output current, thermal resistance, and thermal and electrical parameters for safe operating area (e.g., derating according to thermal resistance when repeatedly switching large currents).
Advantages over mechanical relays 
Most of the relative advantages of solid state and electromechanical relays are common to all solid-state as against electromechanical devices.
- SSRs are faster than electromechanical relays; their switching time is dependent on the time needed to power the LED on and off, of the order of microseconds to milliseconds
- Lower (if any) minimum output current (latching current) required
- Increased lifetime, particularly if activated many times, as there are no moving parts to wear
- Output resistance remains constant regardless of amount of use
- Clean, bounceless operation
- Decreased electrical noise when switching
- No sparking, allowing use in explosive environments where it is critical that no spark is generated during switching
- Totally silent operation
- Inherently smaller than a mechanical relay of similar specification (if desired may have the same "casing" form factor for interchangeability).
- Much less sensitive to storage and operating environment factors such as mechanical shock, vibration, humidity, and external magnetic fields.
- Voltage/current characteristic of semiconductor rather than mechanical contacts:
- When closed, higher resistance (generating heat), and increased electrical noise
- When open, lower resistance, and reverse leakage current (typically µA range)
- Voltage/current characteristic is not linear (not purely resistive), distorting switched waveforms to some extent. An electromechanical relay has the low ohmic (linear) resistance of the associated mechanical switch when activated, and the exceedingly high resistance of the air gap and insulating materials when open.
- Some types have polarity-sensitive output circuits. Electromechanical relays are not affected by polarity.
- Possibility of spurious switching due to voltage transients (due to much faster switching than mechanical relay)
- Isolated bias supply required for gate charge circuit
- Higher transient reverse recovery time (Trr) due to the presence of Body diode
- Tendency to fail "shorted" on their outputs, while electromechanical relay contacts tend to fail "open".
- A DC Fault Protection Circuit for Audio Amplifiers
- Solid state relay glossary of terminology
- National Instruments Solid State Relays
- "Solid State Relays (SSRs) vs Electromechanical Relays (EMRs)". Archived from [=http://www.crydom.com/en/Tech/Newsletters/Solid%20Statements%20-%20SSRs%20vs%20EMRs.pdf the original] on 9 November 2011.
- Solid State Relay Inventor and Early Patents