In this post we'll study a simple high current mosfet based solid state relay, which can be used in place conventional bulky SPDT mechanical relays. The idea was requested by Mr. Abu-Hafss.

The Design

A simple high current SPDT solid state relay or an SSR can be constructed using a couple of mosfets and an optocoupler, as shown in the digaram above.

The idea looks self explanatory.

In an absence of an external trigger, the lower mosfet stays switched OFF allowing the upper mosfet to conduct through the 10k resistor connected across the positive and the gate of the mosfet.

This enables the N/C contact to get active, and a DC load connected across the supply positive and the N/C gets activated in this situation and vice versa.

Conversely in the presence of an input trigger, the mosfet connected with the opto emitter gets an opportunity to switch ON, switching OFF the upper mosfet.

In this situation a load connected between positive and N/O points gets activated or vice versa.

The Circuit Diagram

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