Odd SSR behavior

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stratslinger

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I'm in the final stages of getting my control panel up and running, and I'm running into some weird behavior from a pair of SSR's.

I'm driving everything off of a Raspberry Pi running StrangeBrew Elsinore - and in prototype everything appeared to work great. The Pi drives 2 channels on a Sainsmart 4-channel relay array for operating my pumps, and it drives 2 SSRs for my HLT and BK. I have a manually controlled main relay to power up down the whole panel, and a pair of relays set up to cut power to either of the elements in the event the SSRs fail.

Which brings me to my problem: The SSR's are behaving oddly. When they're off (no signal is sent to them from the Pi), if I test the resistance across the high voltage contacts, I get 0, indicating an open (disconnected) circuit. When I send a signal - whether a constant signal from the Pi's +3.3v or +5v pins or a variable signal from one of the GPIO pins, the indicator light on the SSR lights up, and the resistance across the high voltage indicators goes up, indicating a close (connected) circuit. These are things I expect.

But as soon as I apply 120V AC to the SSR, whether it's active or inactive, both high voltage contacts measure 120V AC. It's as if, despite all other measurements indicating that the circuit is open, 120V is jumping across the circuit.

I've verified that there is no way that I'm back-feeding 120v to either lead - one lead is getting 120v from the main relay, and the other lead is sending 120v to the element relay, and that's the only thing connected to that side of the element relay - so there's no potential source of voltage other than the single source coming from the main relay.

Full disclosure, these are FOTEK SSR's I bought off of Amazon before I started to read about their reputation for being under-specced - but these are both spec'ed to be 40A SSR's that will drive 4500w elements - I'm not too concerned about pushing them too hard.

Now, are these both just failed SSR's? I would expect that if they failed closed then they'd measure resistance across the leads no matter what, so that's the only thing that hasme confused here.

Thanks in advance for any help.
 
This is normal for SSRs. They aren't really so much "open" and "close" as high resistance and low resistance. But if you don't have current flow, there is no voltage to be dropped, so there is always line voltage there. As soon as there is a load, the minimal amount of current flowing will drop the voltage to near 0.
 
Interesting...

So what's throwing me for a loop (I think) is one video Kal has on theelectricbrewery.com - he shows his BK running a given duty cycle, and you see the element LED pulse on and off when the SSR cycles on and off.

I have an LED wired to each element in essentially the same manner he does, but the LED lights up whether I have the element set to constant ON, or I switch over to GPIO control and the GPIO has the SSR inactive. Granted, I also don't have any load on it yet - would having the load in the circuit change that behavior?

* When I say the LED is wired essentially the same was as Kal's, it's because I went a slightly cheaper route and I have pigtails hanging off the panel that my elements will connect to, instead of having the much nicer looking outlets installed in the panel. So where Kal wires his LED's to the outlet, I wire mine to the output side of the element relay.
 
What voltage less are you running? I had a similar issue with running 220 V less with the elements. Due to the leakage they lit whenever it was energized (not just firing) I rewired them for 120 V (and swapped leds) and the problem went away
 
The LEDs are both 220v. I may wind up trying the 120 route - gotta check these ones, I think they may work both ways. Or I just drop a couple bucks and pick up a couple new 110v ones to try.
 
Granted, I also don't have any load on it yet.

That explains all. Don't fiddle with the LED. It doesn't know or care what you are doing; if it has voltage applied, it lights. That is its only job and it is doing it. The only LED that would behave differently would be a defective one - it wouldn't light, ever.
 
FWIW Even with the load mine still lit when they shouldn't, One leg is always hot and the small act of leaked current caused mine to stay lit. I rewired them to 120v and the problem was fixed. Definitely try it with a load first though it could solve the problem as well.
 
Thanks again to everyone who chimed in here - I'm feeling a ton better about the measurements I was getting and the behavior of my control panel. With a little luck, I should have my kettles this coming weekend and I should be able to test this thing out under load, verifying everything here and telling me for sure if my element LED's will work as wired or if I need to swap them out for 110's.
 
with the fotek ssr you will have some current leakage and the led will stay on but should be very dimly lit when the element is off and brightly lit whan the element is on. thats the way mine where /are on the one element I still use the fotek with.
 
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