Control Logix relay output trouble

LED · Sep 15, 2003

This is my second Control Logix project unfortunately it’s giving me fits. The worst part is it’s with something as simple as a relay output card. My trouble is the outputs are on when there is nothing telling them to be on (all 16 outputs). Even if I force them off they are on. The indicator lights on the output card are off but the voltmeter reads 115.6 volts AC for OFF and 117.5 VAC ON or FORCED ON.
I’ve replaced the Output card and the harness connector but get the same result. Funny thing is the relay output card right next to it works just fine.
???

rsdoran · Sep 15, 2003

Are the devices attached to it actually on?

Is the meter reading some form of leakage..can get funny with plc cards.

Have you checked the backplane connector etc etc to make sure card isnt getting "shorted" to power? Are there any substances that could get to rack or ? and create a short?

Have you attempted to use another card in that slot to see if it will work?

These are all general ideas to help determine the problem, I am not familiar with ControlLogix but other AB systems show these kind of problems from time to time,

Pierre · Sep 15, 2003

Sometimes just using an old meter with a needle will tell you a lot. Try it.

Those nice LCD electronic meters are not always giving us the real McCoy.

See? When in doubt, accuse the techy

Alan Case · Sep 15, 2003

Put a bit of load on one of the outputs, I suggest a household light globe and then check the voltage. You will find it will be 0. (I hope)

gbradley · Sep 16, 2003

Misery Loves Company!

LED.... Don't feel bad!
Once I changed out an entire SLC150 before I caught on.

LED · Sep 16, 2003

Yep you guys were right, it’s leaking voltage. I used a panel light with a transformer and it pulls the output down to about 2 volts. The leaking voltage is enough to create trouble with my DC motor controllers so I’ll have to resort to pull down resistors.
You would think with relay output cards it would be like an open circuit with no voltage leaks. Oh well.
Thanks for the help!

93lt1 · Sep 16, 2003

Are you sure it's a relay card?

Whant is the part number?

Pierre · Sep 16, 2003

LED said:
You would think with relay output cards it would be like an open circuit with no voltage leaks.

EXACTLY!

Relays will not leak.

Investigate firther.

LEN · Sep 16, 2003

I know AB PLC 2 and 5 contact outputs have surge protection across the contacts of the outputs. I assume the Logix relay outputs have this also and the leakage is a result of the RC suppression wired in parallel.

93lt1 · Sep 16, 2003

I did some diggin...

I found the manual for an "isolated contact output module" and though I didn't see anything about surge protection, I did see a "maximum off-state leakage current" in the specifications, so there must be some kind of protection.

hesham · Sep 16, 2003

ground connection

Led
I think that all what are you measuring is strays .Please check your ground connection if it's not connected all the pointa will be floating points and will give you what you read in your meter.
i hope it will be so.
good luck

Danyan · Sep 16, 2003

relay outputs

This got me the first time also. If you check the card my guess is that it uses triacts for the relay. These cards will display a false voltage when no load is applied. If the output light is off than more than likely you actually have no usable voltage. Try hooking up a device such as a relay or solenoid. You should be able to easily see if everything is working right by forcing the output on and off. My guess is that the card that you replaced is good. I hope this fixes your problem.

93lt1 · Sep 16, 2003

Re: relay outputs

hesham said:
Please check your ground connection if it's not connected all the pointa will be floating points and will give you what you read in your meter.

Remember that this would only (kind of) apply if the "meter" was using ground as the reference. If he is checking from the "com" terminal on the card to each individual output, then there wouldn't be any "floating points"

Ken Roach · Sep 16, 2003

The 1756-OW16I doesn't use totally dry contacts like the 1756-OX8I does. There is a built-in RC network for surge suppression, and that's what is leaking the 1.5 milliamperes or so, and causing your confusing meter readings, and misleading your DC motor controllers.

You can see it in the user manual (though it's not in the Installation Instructions):

[attachment]

Control Logix relay output trouble

LED

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