I have a BrewPi setup with 2 x AC SSR’s and 1 x DC SSR switching 12v. The AC relays work fine for example when controlling mash temp. I have an issue though that two DC SSR’s have failed closed when trying to control a 12v pump using the wizard-based glycol cooling setup. As far as I can tell they have failed immediately. One relay was a working unit from another project and the other fresh out of the box. Any troubleshooting tips? Relay is 100A 30vdc unit switching a 12v 19w pump in my fridge pumping cold water. Thanks.
Where did you get the DC SSRs?
I have used them before and they seemed fine and ran for years in another project. I didn’t think I could be so unlucky to get two DOA?
Posting the link to the supplier may have helped me answer my own question. It states inductive loads must have diode suppression. The datasheet cites that the relay is suitable for DC motors and I think of AC induction motors as an inductive load but a quick google shows me DC motors are both inductive and resistive loads. That seems like it could be my problem? I clearly don’t know what I am doing here!
Yes, that is exactly what I would recommend and what probably caused this.
It seems that the relays have no internal protection diode.
An inductive load cannot suddenly change its current. As a result, when the SSR closes, the unchanged current will be pushed through the now-closed SSR, causing a very high voltage.
Placing a diode in parallel with the pump, in blocking orientation for normal operation, provides a path for that current to flow when the SSR closes until there is no stored energy left in the inductor.
This is called a flyback diode.
We also have this board for DC pumps and small heaters:
It has the required diodes on the board. Because it communicates over OneWire, you cannot use fast PWM (like 100Hz) for the pump speed with this board. The minimum period is 1 second. This works just fine for glycol though.
Great, thank you for confirming and the explanation. I’ll look at my options.