In this post we'll study a simple high current mosfet based SPDT DC relay, which can be used in place conventional bulky SPDT mechanical relays. The idea was requested by Mr. Abu-Hafss.
A simple high current SPDT solid state DC relay or a DC 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
The above design can be configured in the following manner, and in fact this appears to be technically more correct, and therefore is the recommended one.
This design will work regardless of the opto input switching voltage, right from 3V to 30V DC.
Upgrading the SPDT Relay to DPDT Version
Creating a DPDT version of the above DC solid state relay is actually not too difficult. We can do this by adding a couple of more MOSFETs as shown in the following illustration.
Here, although the poles appear to be a single pole, connected with the positive line, it could be simply separated and integrated with the different DC supplies for operating two individual loads, and implementing a DPDT SSR function.