stuck output

sweetjohnny · Aug 15, 2012

I have an output on a slc 500, card is a 1746sc-OAP8I,the output will work for a day or two then quit coming on when toggled, I change the card and it works again for a day. These are fused outputs and it isn't blowin the fuse. Any suggestions?

JeffKiper · Aug 15, 2012

Is the output being fired in the program? Can you see the indicator light on the front of the card on?

bernie_carlton · Aug 15, 2012

What is the load current you are switching? A very light load may switch intermittently. The card shows 5ma minimum.

sweetjohnny · Aug 15, 2012

Jeff, yes and yes. Bernie .25 amp

Eric Nelson · Aug 15, 2012

What's it controlling? Did you check for voltage directly at the terminal block on the card? Maybe it's not making it out to the load due to a bad connection somewhere?...

-Eric

Gil47 · Aug 16, 2012

Just a wild possible guess

The bottom terminal on the card for most 1746 AC cards is a neutral terminal that should have a wire attached, the card will work without it but not always reliably.

Check and see if you have this on your card I think its normally labelled Com or N

Edit
I googled and found a data sheet and it appears the terminal I refer to above does not exist

sweetjohnny · Aug 16, 2012

I am testing voltage right off the card. there is a neutral and it is hooked up. I can move the output and it will work for a day. There is a switch inline that will force the valve onand it is fused and works fine. This output is controlling a solenoid valve, fyi. It has to be something in the field, but why isn't it smoking the fuse, either one, instead of smoking my card.

Ron Beaufort · Aug 16, 2012

you're probably in the market for a "snubber" ...

There is a switch inline that will force the valve on and it is fused and works fine.

depending on how you have that switch wired up, the old "flyback" effect may be causing your problem ...

see page 49 ...

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1747-um011_-en-p.pdf

basic idea: while the output (the actual COIL device) is turned ON, the DC voltage causes a magnetic flux field to be built up around the coil ... when the DC voltage is suddenly removed, the magnetic flux field quickly COLLAPSES back through the windings of the coil ... this meets the definition of a "generator" ... the voltage that is generated can be many thousands of volts – even though the original (applied) DC voltage was only 24 volts ...

(think about your 12 volt battery in your car – and ask: "where does the 20,000 volts required for the spark plug come from?" ... answer: "the ignition coil" ... bingo! ...

so ... if the manual control switch/button can cause the sudden "magnetic field collapse" that I'm talking about, it is quite possible that the extremely high voltage produced by this "flyback" effect could be damaging the electronic components of your output module ...

It has to be something in the field, but why isn't it smoking the fuse, either one, instead of smoking my card.

the flyback effect creates a very high VOLTAGE – but not much CURRENT ... it takes current to blow the fuse – but voltage (even with very little current) can easily damage the electronic devices in your output module ...

also see Section 2.7 in this manual (shown below) ...

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/sgi-in001_-en-p.pdf

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stuck output

sweetjohnny

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