A solid state relay (SSR) is an electronic switch, which, unlike an electromechanical relay, contains no moving parts. The types of SSR are photo-coupled SSR, transformer-coupled SSR, and hybrid SSR. A photo-coupled SSR is controlled by a low voltage signal which is isolated optically from the load. The control signal in a photo-coupled SSR typically energizes an LED which activates a photo-sensitive diode. The diode turns on a back-to-back thyristor, silicon controlled rectifier, or MOSFET transistor to switch the load.
Operation
Voltage applied to the control line of an SSR causes the LED to shine on the photo-sensitive diode. This produces a voltage between the MOSFET source and its gate, causing the MOSFET to turn on. An SSR based on a single MOSFET, or multiple MOSFETs in a paralleled array works well for DC loads.
There is an inherent substrate diode in all MOSFETs that conducts in the reverse direction. This means that a single MOSFET can't 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 then are alternately reverse biased in order 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. There is also commonly some circuitry to discharge the gate when the LED is turned off, speeding the relay's turn-off.
Advantages over mechanical relays
SSRs are faster than electromechanical relays; their switching time is dependent on the time needed to power the LED on and off, typically on the order of nanoseconds
Increased lifetime due to the fact that there are no moving parts, and thus no wear
Clean, bounceless operation
Decreased electrical noise when switching
Can be used in explosive environments where a spark must not be generated during turn-on
Totally silent operation
Smaller than a corresponding mechanical relay.
Disadvantages
Fail short more easily than electro-mechanical relays
Increased electrical noise when conducting
Higher impedance when closed (-> heat production)
Lower impedance when open
Reverse leakage current when open (µA range)
Possibility of false switching due to voltage transients
Often more expensive than comparable electromechanical relays
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