Solid State Relay (SSR) Circuit using OptoCoupler : how it works ?

The input cicruit is one of the parts which composed a solid state relay.

The input circuit is made up of an optical control called an optocoupler. An optocoupler contains one infra-red light-emitting diode, or LED light source, and a photo sensitive device within a single case. The opto-isolator isolates the input from the output.

The optocoupler is the portal between the trigger and the switching circuit. The input trigger is applied to the LED of the opto which illuminates and makes the photo-transistor conduct.

The voltage from the photo-transistor passes across the collector to the emitter and finally reaches the triac’s gate to operate it. As the LED takes currents from 2 to 30mA , the control voltage can generally be between 4V and 30VDC.

Here is the drawing of an optocoupler :

celduc® has selected the best optocouplers on the market in order to offer high-immunity SSRs. In fact, the lifetime of a Solid State Relay is mainly related to the thermal stress (we have already highlighted the importance of the proper selection of the heatsink thermal characteristics) and ageing of the optocoupler.

For more information about the composition of a solid state relay and its advantages, please watch our video :