Solid State Relay Failing Closed/Short. Not Sure Why Its Failing And How To Protect?

Thread Starter

Mahonroy

Joined Oct 21, 2014
287
Hey guys, I have a pedal (picture something like a sewing machine pedal) that has a micro switch inside of it to signal to the machine that its being pressed down. I then have a solid state relay (SSR) attached to the 2 wires (in parallel to the switch) so I can activate the pedal remotely. The 2 wires generally consist of a voltage source (not more than 15 volts), and a voltage sensor. The current draw is typically less than 20mA. This is the SSR I am using:
https://www.digikey.com/products/en?keywords=TLP3122A(TPLECT-ND
I'm driving the SSR directly with a STM32 GPIO and have a 180 ohm resistor in series with the SSR.

About 1 in 20 of these and the SSR fails in a closed/shorted mode. I can't understand why or how this is happening, or what I should do here? The voltage and current draw is far below the max values of the SSR. Some of them never fail, while others will fail within the first hour of use.

Any ideas what might be the problem, or how I should protect the circuit better? (or how to have it not fail in a closed/shorted mode, but have it open instead)? Thanks and any help/advice is greatly appreciated!
 
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Reloadron

Joined Jan 15, 2015
4,594
The current draw is typically less than 20mA. This is the SSR I am using:
https://www.digikey.com/products/en?keywords=TLP3122A(TPLECT-ND
My question is would the load be an inductive load or resistive load and if an inductive load, like a relay coil or similar, does the load have flyback protection across it? The SSR is sort of an opto MOSFET and likely not going to be forgiving of any inductive kick. I guess if you have a scope you could measure across the SSR when it turns on and off looking for any spikes.

Ron
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
287
My question is would the load be an inductive load or resistive load and if an inductive load, like a relay coil or similar, does the load have flyback protection across it? The SSR is sort of an opto MOSFET and likely not going to be forgiving of any inductive kick. I guess if you have a scope you could measure across the SSR when it turns on and off looking for any spikes.

Ron
Some are inductive and some resistive. The ones that were damaged were surprisingly the resistive loads....hmmm.....

So really the only possible way is that a voltage spike is getting on those wires which is exceeding 60V (max rated voltage for the SSR), or its exceeding the 1.6A current limit?

Could I place a TVS diode across the SSR terminals to solve any voltage spikes? Or would I need to get a ground pin involved too?
 

Thread Starter

Mahonroy

Joined Oct 21, 2014
287
One more question regarding this.... is it possible that the relays have not necessarily failed... but they are leaking current through, which is activating the device, which is giving the illusion of a relay that has failed ON? I was going through the datasheet but couldn't find how much current I could expect to be leaking through. In this case, is there anything I can do to help with this, or beef this circuit up to make it as reliable as possible? Thanks again and any help is greatly appreciated!
 

Reloadron

Joined Jan 15, 2015
4,594
While I never gave it much thought with resistive loads a RC Snubber can be used with DC loads as well as AC loads. You may want to give some thought to using a DC Snubber across your switching terminals. You can also try just placing a Schottky Diode across your switching terminals. My guess based on how this happens is you are getting a high fast transient spike blowing through your MOSFET type DC SSR. Nothing to lose and you will see if the problem goes away.

Now I am not familiar with the SSR you are using beyond the data sheet. Some leakage in a triac design would be normal but I really have no idea in a MOSFET based design DC SSR. Maybe another forum member has a thought on this?

Ron
 

TeeKay6

Joined Apr 20, 2019
461
One more question regarding this.... is it possible that the relays have not necessarily failed... but they are leaking current through, which is activating the device, which is giving the illusion of a relay that has failed ON? I was going through the datasheet but couldn't find how much current I could expect to be leaking through. In this case, is there anything I can do to help with this, or beef this circuit up to make it as reliable as possible? Thanks again and any help is greatly appreciated!
See section 10 of the datasheet. Detector OFF-state current is 1uA max (and typically 2nA).
 

Chris65536

Joined Nov 11, 2019
57
A TVS across the SSR, connected near/at the SSR terminals, is a reasonable approach. Only testing will reveal whether that solves your problem of failing SSR's.
I think post #6 is correct, that it needs to be across the inductive load, not the relay. Across the relay, it will never end up conducting any current.
 

TeeKay6

Joined Apr 20, 2019
461
If the TVS never conducts current and yet the SSR fails, that will be a strong indication that the SSR is not failing due to over-voltage stress. If it does conduct current, then the TVS will keep the SSR voltage at a safe level. One advantage of a TVS across the relay is that the relay will be protected regardless of the actual source of the over-voltage. There is thus far no strong proof that the SSR is failing due to overvoltage; that is still hypothesis and will likely remain so until testing of actual hardware is completed. Also, we have no proof that the relay's LED is being correctly driven (i.e. on all the time when it should be on; off all the time when it should be off). Bottom line: There are things we still don't know, but doing something is better than doing nothing.
 
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