Go Down

Topic: Relay + SSR. (Read 589 times) previous topic - next topic


What if a small (cheapest SSR) and a Relay were wired in parallel?

They both fire off at the same time, the cheap SSR will fire off and power the item, 40ms seconds later the Mechanic Relay connects.

If the SSR is allowing the current (and at 40ms, I presume little heat even if it was a 2000watt appliance) and starts to conduct, now
at the very point in time the mechanical relay metal contacts meet?

What happens?, will it ark? or just make contact and nothing more because the current's going through the SSR?


No, it will not arc. The relay will start conducting all the current, because the voltage drop over a relay is 0V, and the voltage drop over a SSR is 0,7V or something.

When a lot of current is flowing, the relay may arc when it is switched off.

Note: I think your relay will be faster than 40mS.


The SSR has most likely a noise suppressor (C + serial R) inside.
That will reduce the spark of the relay contacts when switching off.

It will work, but a SSR for high current is cheap.
Search for SSR-40DA and the like.


I was thinking of a way to prolong a mechanical relay's life and the ssr in combination, 1 to stop an arc from damaging the contacts 2 to turn off the ssr and let the 1st relay handle all the current (only the theory was to protect the relay from arching and extending its life, since little heat build up occurs, the ssr can then be switched off?

depends on if it arcs...


A snubber circuit will do. A capacitor and resistor in series for each contact. About 10...220 ohm and 10... 470nF. I use 47 ohm and 100nF. The capacitor must be high voltage.

I have seen relay contacts melting away over a period of a couple of days by arcs, so it can be a real danger.

If you have two outputs, you could make the SSR the first to close and the last to open. With a delay of 500ms or so.
But then again, a single heavy SSR will solve this problem.


I've seen this "relay contact in parallel with SCR/Triac strategy" used in very expensive large industrial UPS systems (think 10-50KW units) in their output switching circuits for say switching in and out of service or switching to an alternate AC input feed. This switching circuit was in it's own cabinet and was called a "static switch" and it was operated both automatically by the UPS system or manually when the UPS was needed to be taken out of service for maintenance without disrupting current flow to the protected loads.

If gives the switching speed of solid state followed with the low voltage drop of contacts Vs semiconductors.




snubber circuits (which looks like a low/high whichever pass filter)

thanks for the comments :)


In AC appliances most relays can be built to be zero crossing just like an SSR.

Go Up