SSR Interference

Troubleshooting
1

I’ve just been testing my second fridge setup today. All my SSRs and 12v supplies are mounted on a metal sheet. I have a 240v SSR (heat) on output B of my first SSR control board along with a 240v SSR (compressor) & DC SSR (condenser fan) on the two A outputs. I then have another DC SSR on a second control board (chamber fan). The second control board is daisy-chained off of the first.

The two AC SSRs seem to behave fine. But the two DC SSRs seem to interfere with each other. Enabling the chamber fan enables the condenser fan. Disabling it makes the switching of the compressor unreliable. The issue isn’t there when the either of the DC relays is disconnected from the control boards. I isolated the DC SSRs from the metal sheet they’re mounted on using double sided foam. This seemed to help when testing without load but doesn’t 100% solve it once a load is connected. I imagine that’s because the shared 12v supply might be providing an interference path once in circuit, or my make-shift isolation has stopped doing its job.

I have replacement DC relays on their way from Farnel. I’ve learned my lesson about ebay SSRs. I’m hoping more reputable SSRs won’t have any of their pins connected to the heat-sink. But I was wondering if there’s any other gotchas in the way the SSR control boards are designed that might be contributing to my issues?

2

Can you provide some photos and/or a schematic on how you have connected things?

We also have very good SSRs in our store, why didn’t you order those? I checked and they do not have their bottom side connected to any of the pins.
The fact that this is the case on your SSRs is worrying because the SSRs should be galvanically isolated between control side and AC side. I doubt that’s properly done on your SSRs too.

3

Sorry I never replied to this. Bit of a mad week.

The new SSRs have done the trick. I didn’t buy from the store originally because I was tempted by the price on ebay. I guess you get what you pay for, though… I went for Farnell for these because I could get free next day delivery and hoped to get it sorted sooner. I have no idea why I didn’t just get good quality to start with. Given how much I’ve spent on the conical, a few £ savings on relays is negligible.

Don’t worry. The AC SSRs are fine. Had my meter on them too. This was the DC ones. I think the grounding issue is down to whatever mosfet they use inside and if the tab is connected to any of the pins, which some can be.