Help me understand Solid State Relays

[wbyrd01]

I just finished building my 240v control box with a DSPR320, 5500 watt element, using a Leviton 3032 switch to allow power to the SSR. I have a 240 LED indicator light wired to come on when the SSR is on. My problem is that as soon as I turn my switch on the LED comes on and I read 230v across the SSR, even with nothing hooked up to DC side of the SSR. I've tried three different SSR's and all three do this. Should I be getting power across the SSR with nothing on the DC side? The odd thing is, when I do put DC power on the SSR to activate it, the LED on the SSR turns on and my LED indicator light gets brighter, but my voltage reading drops to near zero. To say I'm a little confused by this is an understatement. My understanding of how this should work is that there should be no voltage across the SSR when there is no DC power to activate it. Then when I do apply DC power my LED indicator light should come on and I should read 240v across the SSR.

Is my understanding of how an SSR works completely wrong?

 

[BeardedBrews]

You're very close to right...

The only piece missing is that SSRs leak a little bit. If you connect the element the indicator light thing will fix itself and the element will be fine since it isn't enough juice to heat it up.

I can't remember 100% if that fixes seeing voltage on a meter, but I think it might.

Now the question about seeing low voltage when the SSR indicator is on is a bit more curious. Make sure the meter is on the right setting and check each SSR leg to neutral to confirm 120v. If one is not working then you're not going to get 240 across them.

 

[day_trippr]

wrt seeing voltage on the unloaded switched side of an SSR: ^correct*. It's all about RDSon - the same specification that (along with load) dictates heat sink size
Open SSRs with non-zero RDSon (which is all of them) will "bleed" voltage at very low current (microamperes). A load will drag that quickly to immeasurable (outside of a lab) value...

Cheers!

 

[wbyrd01]

Well on the bright side I now have three perfectly fine SSR's!

 

[processhead]

Here is one way to compare an SSR vs. a mechanical switch or contactor.

A mechanical switch has near zero resistance when closed and infinite resistance when open.
This works fine and is very predicable with almost any load you connect to it.

In contrast an SSR is a less than perfect switching device. It has very low resistance when on (or closed) and very high, but measurable, resistance when off (or open).

These differences are insignificant when switching a low resistance load like a heating element and both the SSR and mechanical switch will work just fine for that purpose.

Where things get confusing is when you connect a high resistance device like a voltmeter or an LED lamp across an SSR without a load while in the off state.

In the off state, and SSR will "leak" enough current through its high resistance to give a voltage reading with a meter, or illuminate an LED indicator lamp.

This does not happen with a mechanical switch because its resistance is infinite when it is off.

 

[doug293cz]

I just finished building my 240v control box with a DSPR320, 5500 watt element, using a Leviton 3032 switch to allow power to the SSR. I have a 240 LED indicator light wired to come on when the SSR is on. My problem is that as soon as I turn my switch on the LED comes on and I read 230v across the SSR, even with nothing hooked up to DC side of the SSR. I've tried three different SSR's and all three do this. Should I be getting power across the SSR with nothing on the DC side? The odd thing is, when I do put DC power on the SSR to activate it, the LED on the SSR turns on and my LED indicator light gets brighter, but my voltage reading drops to near zero. To say I'm a little confused by this is an understatement. My understanding of how this should work is that there should be no voltage across the SSR when there is no DC power to activate it. Then when I do apply DC power my LED indicator light should come on and I should read 240v across the SSR.

Is my understanding of how an SSR works completely wrong?

If you are measuring between the two SSR power terminals when you trigger the SSR (by applying DC to the control terminals), you should not measure a significant voltage if the only load is your "element firing light." If the element is connected (be sure its covered by water first) and you apply control voltage to the SSR, then you should read something around 1V across the power terminals.

Brew on

 

[doug293cz]

I posted some additional information that explains the behavior of SSR's in another thread here. This may be of some help in improving your understanding.

Brew on