SSR not working as expected


I have a standard dual channel Arduino solid state relay module that is powered by 5volts.

I tried connecting it to Arduino to test it. Dc+ to 5v , Dc- to gnd and one of the inputs to pin 4. The relay is getting powered and all voltages read fine at terminals. The program toggels the relay every 5 seconds and I can see it working as the led on relay against the input comes on and goes off in 5 seconds. However if I use multimeter to check continuity at output terminals I get nothing. I tried both relays on the module and still nothing. What could be wrong ?

Your expectation that a multimeter can measure continuity on an SSR. It simply can't, and if you don't know that I suggest you shouldn't be playing about with mains voltages.

Why do you say multimeter cannot measure continuity on terminals. I have never seen a case where it hasn't till date. This isn't my first project though. I would like you to elaborate on your point if possible.

I have always used mechanical relays. This is the first time I am trying to see how a SSR works

So yes the buzzer works. The multimeter shows very minute disturbance. Not like a proper sound in the mechanical relay. I attached a separate buzzer to the terminals and I do get sound there.

A SSR works by triggering a device called a Triac. This behaves nothing like a conventional relay contact.
A continuity meter works by putting a small voltage on the probes and measuring any current through it. This voltage is very much below the threshold that a Triac will trigger. What is more a Triac once turned on will only turn off when the voltage across it is zero. So removing the trigger from a Triac will not turn it off. This is why by and large a Triac is only used on AC voltages not DC ones.

Also there is a thing called holding current, a Triac will only stay on if the holding current is being drawn. This is a minimum value.

Unfortunately there is another arrangement that is packaged up and is known also as an SSR. This type is often called a DC SSR. This is an arrangement of two Power FETs. This will work on DC, but you will still not see this register on a multimeter, again because the voltage it puts out is way too low.

So what type of SSR do you have? You can't tell anything from your photograph.

Thanks for the explanation. That was very helpful. This is what I have.

Yes it says:-

Control 2 loads of up to 2 amps each at 120V or 240V AC.

So it is a Triac and will work with AC voltages only. Which is why as I said I would leave mains circuits until you know a bit more about things.

Agreed. I will spend time studying this more before actually using it for ac mains. Thanks.

OK, just one more thing:-

Never measure resistance or continuity on a circuit that is powered up. The currents already flowing in the circuit make the readings meaningless and it has the potential to damage either your circuit or your meter.

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Oh yes totally agreed. I normally tests relays with battery voltages till am sure of what I want. I never test anything on live voltage.:pray:

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