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Solid State Relays

04/11/2016 10:02 PM

I am using solid state relays in a circuit. The relays have four terminals. Terminal 3 is marked as positive and terminal 4 as negative and input can be from 3 to 32 volts DC. Terminal I is marked as AC as is terminal 2. Through put can be 24 to 380 volts AC. The relays are by FOTEK and are SSR-25 solid state modules, made in Taiwan.

When I apply 12 volts DC to the control terminals the red led comes on when the polarity is correct. At this point I think the AC side of the relay should be in the closed state. My trouble is that when I test the relay with or without a DC voltage the relay is in a closed state. Am I wiring it wrong or are the relays just RS. (For those who are unfamiliar with this term just pronounce it as ratshet). These 4 units are brand new (bought off E-Bay) and I hesitate to declare them defunct with out some extra reasons to do so.

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#1

Re: Solid state relays.

04/11/2016 10:10 PM

This relay is designed to switch AC loads. It will not turn OFF a DC load.

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#2

Re: Solid state relays.

04/11/2016 10:31 PM

Put a load on them like a heater and see if the turn on and off with the DCsupply. If they are still on when there is no DC control power they are shorted out and junk.

If they are working when they are on they should exhibit a roughly 1 - 2 volt AC voltage drop across the terminals with a load on them. If they show no drop and don't turn on and off with the DC supply they are shorted full on.

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#3

Re: Solid state relays.

04/12/2016 2:24 AM

As tcmtech says,you need to test them with a load.

SSRs will always leak to some extent, so just measuring output voltage with a high impedance meter (e.g. a DVM) is not a valid test of function.

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#4

Re: Solid State Relays

04/12/2016 10:17 AM
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#5

Re: Solid State Relays

04/12/2016 11:38 PM

Thank you all for your input to this problem. Redfred got my first problem pegged. I was trying to pass a dc voltage through the relay. Then Tcmtech suggested a heater in the circuit. John DG was also correct, I was using an ohm meter to check for a closed circuit. and finally Rixter gave me a web address with pictures (I never realised what a Homer Simpson I am). I rigged up a circuit with a 240 volt light (I used a light as the load I am switching is rather small) and 12 volts DC to test the relays. The four new ones were working fine and one old relay I had not found or tested when I posted was faulty. The light flickered on and off. I imagine at line frequency of 50 Hz. Anyway, the problem is solved and so thanks to you all.

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#6
In reply to #5

Re: Solid State Relays

04/13/2016 4:11 AM

The lesson here is never mix up Electrical and Electronic fault finding.

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#12
In reply to #6

Re: Solid State Relays

04/13/2016 6:35 PM

A valuable lesson, as was the lesson of stopping to think before jumping in to a poorly executed test procedure. This old dog has to learn a new trick or two.

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#7

Re: Solid State Relays

04/13/2016 4:14 AM

Just to sure, you are testing with no "activating" DC supply on the + and - terminals, the relay is still "on" across the AC terminals. Have I understood you correctly?

(Though just measuring with an Ohmmeter across the AC terminals for example, may not be indicative of a problem or not. I am assuming you are using AC voltage and an AC load to test the "ON" condition properly and never connecting the DC terminals to anything.)

If yes, then I believe they are defective. E.g. Always "ON". A common failure mode.

If you are using a DC voltage across the AC terminals, you might be causing the error yourself. As others have said here.....I would never test in this manner.

If defective, then typical ebay!!! Well done in checking them out!!

Do always remember that solid state relays are fantastic, but leakage can be a problem and it should NEVER be assumed that they block all voltage under all conditions, that could be dangerous to life and limb....

Update

I had not read down, thank goodness its cleared up!!

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#10
In reply to #7

Re: Solid State Relays

04/13/2016 6:17 PM

I guess that I was a bit carried away by mixing up between solid state and mechanical relays which I am more use to. Also thank you for the leakage warning.

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#13
In reply to #10

Re: Solid State Relays

04/13/2016 10:24 PM

There are solid state relays designed to switch DC loads. You just happened to not pick one.

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#8

Re: Solid State Relays

04/13/2016 10:43 AM

"These 4 units are brand new (bought off E-Bay)"

Unless these relays are for a hobby project at home, I would not use 'brand new' and 'bought off E-Bay' in the same sentence.

One of the big problems in making reliable industrial machines is weeding out the counterfeit or substandard components. That is done by certifying that the purchased parts have been in an unbroken chain of possession from manufacturer to reputable distributors to the end user. Under those guidelines, 'bought off E-Bay' has the same connotation as 'bought off the back of a truck from some guy in a dark alley.'

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#9
In reply to #8

Re: Solid State Relays

04/13/2016 11:20 AM

Good thought, you reminded me also of the fakes coming out of China of anything electrical.....

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#11
In reply to #8

Re: Solid State Relays

04/13/2016 6:25 PM

Maybe I have been very lucky so far but I have had few problems with anything I bought on E-Bay. But then again maybe I should be a bit more sceptical.

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#14
In reply to #11

Re: Solid State Relays

04/14/2016 9:29 AM

For personal use, E-Bay is fine, but for professional use, when there could be liabilities if someone gets hurt, being able to prove with 100% certainty that the parts used were genuine and proper for the application help to CYA.

I had a similar situation with my old Jeep. A bracket on the front axle housing broke loose, allowing the stabiliser arm to swing free. No reputable shop would weld the bracket back on, because they could not guarantee that the weld will hold even with the rust cleared off the welding site, and they would be liable if that weld failed and caused a crash.

So I had to get a 'new' front axle, and since model year 2000 Wranglers were no longer being made, the mechanic had to do a search through the 'auto graveyard' database to find a new axle assembly for me. My boss was aware of my car problems, and he had a friend who had a junked Wrangler that might supply the axle we needed, but again, no reputable garage would install an 'uncertified' axle from an 'unknown' source.

I's estimate that around 99% to 99.9% of the items sold on E-Bay are legitimate and unflawed, but if I need parts for a project that will get looked at by OSHA if someone gets hurt, I'm going to look to Grainger, Automation Direct, or similar for the parts. Not only to have the CYA in place if the worst happens, but also for my peace of mind; I'll go home knowing that we won't have the scenario of "Mark Smith from third shift lost his hand because I saved a few bucks by using cheap 'Chinese' bolts (actual country of origin unknown) to hold the framework together".

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