1771-OD Card Trouble.

Im having trouble with a start up horn on a belt. The output quit energizing. The logic shows it should be, but nothing is happening. I test for voltage on the card and get noting on the B side, and 120volt on the A side.

There is a spare output on the card, so i run a 120volt lead to the A side, and the voltage is now showing on the B side with no programming. I force the output off, and it doesnt change anything.

Am I missing something, or is just most likely a bad card?

connect a load (not a meter) to the "new" output and see if that works ...

if that works OK (turns off and on appropriately) then look up the terms "TRIAC" and "leakage current" to see what's making the output be ON when there's a ZERO status in its bit/box ...

TIP: if the module's original output went "bad" - then the first worry on your mind should be: "WHAT damaged that output?" ... if there's something wrong with the HORN (drawing too much current) - or if there's something wrong with the wiring (shorted by moisture in the conduit, etc.) - then you're probably going to damage the "spare/new" output too ...

The 1771-OD outputs are individually fused, are they not? If they are then I think there should also be an indicator for each fuse state?

How many indicators are on the module?
Are any indicators on? (besides normal output indicators)

The fuses should be accessible under a cover if memory serves me correct?

If there is, and you do have to replace a fuse, then I would suggest you test on a known good load of say 100 mA or there abouts. That should be sufficient to switch the triac output without stressing it.

It should be good for a couple of amps but no more. Perhaps the horn has recently gone faulty?

*Don't forget to POWER OFF if changing a 120VAC fuse!*

Regards,
George

This morning I found this...

AC (120V) Isolated Output Module (Catalog Number 1771-OD) -

Installation Instructions: http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1771-in078_-en-p.pdf

Note the "Pre-installation Instructions" on page 4 -

Maximum load per channel = 2A
Minimum load per channel = 60mA
Maximum load per module = 6A

On a fully used module you cannot load the 6 output channels at 2 Amps each as that would be 12 Amps for the entire module. Therefore, the general rule of thumb is to limit each of the 6 output channels to just 1 Amp or less.

Also note the "Suppression" comments on page 4 -

Even though there is a MOV across each output channel to protect against voltage transients, you are also advised to fit additional suppression devices to the outputs for inductive loads, such as your horn. This helps dissipate arcing at the output contact, which can degrade its switching performance over time.

It could be your horn has gone faulty and blown the output fuse or it could be the output contact has degraded to the point of failure. Ideally the output should be switching a suitable interposing relay which then switches the horn ON and OFF.

The fuses on each output I mentioned are described on pages 10 and 11, including how to replace them. The front cover clearly states what the bottom group of LEDs are for - "FUSE BLOWN". So it should be very easy to see if your particular output has blown its fuse or not. The fuse is a 5A, 250V Bussman MTH5, IEC 127 Type F, according to the literature.

Here's a picture to illustrate what I mean with the LEDs...


Regards,
George

It took a while, but I was finally able to make my way back to this problem.

I read up on triac and leakage current. That pretty much helped me understand my readings. I hooked up a small contactor to the output to actually test it. Everything worked perfect.

Yes, that is the card I have. You are correct, that the indicators light up when fuses blow. You just have to remove 2 screws on the back up the card. I had voltage on the output, so i didnt bother checking the fuses.

Hi,

I'm glad to hear you are "back in action" but do you mind telling us what exactly was the problem? I can't decipher the root cause from your last post?...

Was it a fuse that had blown?

Did you hook up the contactor to the original output or the second output you were testing?

Is the horn working and if so, which output is it now on?

It's just for closure on the thread and saves the "what was it in the end?" type question from somebody in the future reading this looking for similar information.

Regards,
George

The original output has went bad. Im assuming that the horn that was attached caused damage to the output. Only about have of the I/O is fused here. Big problem, I know. The horn was found to be full of mud, so it may have pulled higher amps than normal trying to make a sound.

The new input worked just fine. I just wasnt used to the output having voltage without the output being energized. As Ron mentioned above, a search of TRIAC and leakage current helped me out a lot.

Most important thing I learned from this....make sure you have a load on the output in order to test is. A meter wont really tell you anything.

Most important thing I learned from this....make sure you have a load on the output in order to test is. A meter wont really tell you anything.

Click to expand...

to use a "regular" voltmeter when testing TRIAC outputs, this little adapter comes in handy ...

https://www.fluke.com/en-us/product/accessories/adapters/fluke-sv225
.

Thanks for the added information. That gives us a clearer picture.

Ron (Hi Ron) has you well advised on the "trickier triacs" and what to expect when placing meter probes across them.

Just a further reiteration of something I mentioned earlier...

HoldenC said:

The original output has went bad. Im assuming that the horn that was attached caused damage to the output. Only about have of the I/O is fused here. Big problem, I know. The horn was found to be full of mud, so it may have pulled higher amps than normal trying to make a sound...

Click to expand...

As I mentioned, to further protect PLC outputs which may be controlling relatively higher loads, it is good practice to have the outputs switch an interposing relay of a relatively lower load. The relay's contacts then indirectly switch the heavier load, such as an inductive load horn. This way, the output is only ever subjected to the lower current required to energize the relay's coil. The other advantage is that the wiring for the output is contained within the PLC enclosure and is therefore not exposed to potential external faults, such as catastrophic device failures, short circuits or cross voltages due to cable damage, voltage drops, and anything else possible "out in the field".

If the field device does irreversibly damage the interposing relay contact then only the relatively cheaper relay need be replaced and much more quickly, rather than having a now damaged, unusable single output channel, or sometimes more expensive complete module failure.

Of course, if not using interposing relays, then adequate fusing of directly wired output field devices really should be done as a standard practice, else you run the risks.

It is also good practice to not only add suppression to the interposing relay contacts for arc protection against the higher field load, but to also add suppression to the relay's coil to suppress potential arcing at the coil which may degrade the output's contacts.

All in the ideal world of course. But, with the mention of mud getting into this particular horn, I would strongly consider retrofitting an interposing relay, or at minimum a fuse, to protect that now at similar risk replacement output channel.

Regards,
George