Solid State Relay Module

This solid state relay propels a puny logic signal into the big time, switching up to 20 Amps of mains electricity.

Magic Smoke is 1

This very basic project shows how a solid state relay (SSR) can turn power appliances on and off in response to a low voltage pulse. It will be a stepping stone for future demonstrations of PWM (pulse width modulation) and PID control —bringing things full circle, because the very first post on this blog featured the enduringly popular PID controlled flower pot smoker. Since winking into existence exactly 1 year ago Magic Smoke has given rise to 38 posts, 0.05 million page views, 7 projects picked up by Hack A Day, 13 featured posts on Dangerous Prototypes, 8 free development boards, at least 19 repurposed household utensils, and 44 spilled cups of coffee. And it has been worth every minute, especially for the raccoon that broke into the hen house and ate my chickens. If you have enjoyed reading Magic Smoke and would like to show your appreciation, please hit the Paypal button top right.

What Is Inside A SSR?

The kind of solid state relay that switches AC typically contains the parts shown in the diagram below. An optocoupler, containing a LED that triggers a photosensor, isolates the low voltage DC control from the high voltage AC. The signal may be gated by a zero-crossing detector circuit that delays switching the AC until it next reaches 0 V. This helps prevent radio noise and damage to the load from sudden voltage surges. A triac acts as the AC switch, with a snubber circuit across the output terminals to prevent false firing of the triac from voltage spikes generated by inductive loads.


Building The Case

I have had very good success with the inexpensive 40A Fotek SSR which keeps cooler than the 25A model. But this project was all about the box.

It would have been nice to use a Carlon 4" x 4" x 2" junction box and just drill a hole in the side for a PG11 waterproof gland to secure the power cable. What I had was a 2 gang PVC switch box and cover left over from a garage wiring project. I decided to chisel off the conduit connection socket and insert a cable clamp, sealed with silicone and grounded using a crimp ring attached to the clamp screw. Awkwardly, the position of the hole near the floor of the box meant that the retaining ring did not fit so I had to cut a section of it out in order to thread it onto the clamp.

For the NEMA 5-20 receptacle I drilled a 1-3/8" hole, the largest width on my step bit, plus 2 little holes for bolts to fix it in place. Red and black binding posts were attached on the opposite side to carry the low voltage signal to the SSR.

A junked PC supplied a large heatsink which was mounted externally on the cover after cutting an appropriately sized hole. I drilled holes in bottom of heatsink to mount the SSR with self-tapping screws and just enough silicone grease to ensure good thermal contact.

stripped some 12 AWG cable for the wiring inside the box and attached crimp terminals to attach to the screw fittings on the SSR. My crimper can apply plenty of force but for applications like this one that are not subject to a lot of vibration I like to solder crimp terminals, just to be on the safe side. The final touch before joining all the wires was an industrial grade plug on the power cable.


Mains electricity is potentially Dangerous so before attempting this build please make sure that you are competent to wire power circuits safely.

Connect the components as shown in the diagram on the right. The ground wire (green) is wired to both the green screw on the receptacle and the back plate of the SSR. The hot wire (thick black) attaches to the brass screw on the receptacle via the AC terminals on the SSR. The neutral wire (grey) connects to the white metal screw on the receptacle. The thinner low voltage wires join the positive and negative signal terminals on the SSR to the red and black binding posts, respectively.

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