Solid State Relay Simulation, Explained

[SaltyPuglord] needed a solid state relay for a project. We’d have just bought one, but he decided to design his own in LTSpice. Along the way he made the video below, which is pretty informative and a good example of a non-trivial design in LTSpice.

MOSFETs have made designs like this a lot easier, to the extent that it should be as easy as putting a pair of beefy fets in-line with the AC source and load. However, that has a few ramifications that [Salty] covers in the video.

The biggest concern comes in isolating the DC supply from ground. He used a transformer which is tricky to simulate in LTSpice. Beyond that the design of the power supply is quite simple, and as he mentions in the video, you don’t really need this complex of a regulator just to feed the gates of the MOSFETs.

The other issue is that the wires between the two MOSFETS really have to float and could handle quite a bit of current. The problem is, LTSpice can become confused if there’s no ground reference for the FETs. Thus there is a ground in the simulated circuit. In fact, the default LTSpice solver doesn’t like the circuit anyway. The video shows how to change to the alternative solver which works well.

It looks as though a lot of what the video talks about is also mentioned in a linked document from TI. If you want to try a light version of the circuit without installing LTSpice, try Falstad. Or try Micro Cap if you want to install something different. If you do stick with LTSpice and you want to learn more about modeling transformers, we’ve got you covered.