Solid state relay

From Wikipedia, the free encyclopedia

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, hybrid SSR etc. A photo-coupled solid state relay is controlled by a low voltage signal which is isolated optically from the load. The control signal in an 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.

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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 works well for a DC load.

Bi-directional solid state relay with opto-isolation.
Bi-directional solid state relay with opto-isolation.

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.

  • 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 milliseconds.
  • SSRs provide an isolation barrier between the control logic and a high voltage load.
  • Increased lifetime
  • Clean, bounceless operation
  • Decreased electrical noise

  • Higher impedance when closed
  • Reverse leakage current when open (µA range)
  • Possibility of false switching due to voltage transients

Solid State Relays


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