Potential problem using Solid State Relays (SSR)

Hi All.
There has been mention of using SSRs a bit lately which is all good.
These devices have a built in potential problem which can be best described by example.

Some years ago we had occasion to require on/off switching of a device by switching 240VAC using a SSR. Switch ON… All OK. Switch OFF… No action, the device continued working happily.
The problem turned out to be a built in “snubber” circuit consisting of a capacitor and resistor in series connected across the “contacts” of the SSR. This circuit is part of the SSR and really nothing can be done about it. The device being switched was of the type that is happy with an input voltage between about 80VAC and 260VAC and presented a very light load (milliamps @ 240VAC) to the supply and relay. This in built cap and resistor provided enough leakage to keep our device working!!!. I point out here this circuit is in all SSRs that I have encountered. There must be a reason for it but I don’t know what. It is a technique sometimes used on mechanical relays to minimise contact arcing but with zero crossing switching I don’t know.

The solution was to fit a 240V mains rated capacitor between SSR output and neutral, that is effectively in parallel with the switched device. I don’t remember now what value was used but it increased the load sufficiently to reduce the leakage to a point well below our device operating voltage and thus allowed it to switch off.

This can be a sneaky little problem and I post this to maybe help stop people discarding perfectly good relays for no other reason than they seemingly don’t turn off.
I will confess we tried a couple of SSRs before locating the underlying problem. It was built in. The worst kind.
Cheers Bob


Hi Bob,

Excellent post! Just chiming in with mains is a deadly voltage, if you need a project to control mains power get it installed with a licensed electrician.

Hopefully we see even more discussion and usage of SSRs thanks to this topic!



Google low current ac ssr
You will find low current specific SSR devices.

It sound like the SSR you had was for a larger load like in the Amps range, where snubber leakage current would not be significant.

Tom… :grinning: :+1: :coffee: :australia:


Hi Tom
This was about 15 years ago. The choice then was not as wide as it is now.

The post was a for general consideration. The snubber circuit has been in every SSR I have come across and is still there no matter what the current rating of the relay is so the leakage remains dependant on the snubber values.

Since I posted that I have come across some good info here

Although the author targets a particular small SSR he does provide some hints and advice of a general nature which would apply pretty much across the board.
Cheers Bob


The circuit in that post is very similar to what is on the SSR module for Core Electronics. But I think it uses a different transistor and it does not have the 100nF cap.

Thanks for posting this information Bob.
I am using those modules to switch 24Vac from a plug pack to watering system solenoids. So far it works fine on the bench for a short time. Will see how it goes in use when the project is finished.
Your information has been helpful.

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