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Old January 5th, 2010, 11:32 AM   #1
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Troubleshooting help needed: 1746-OA16 (staying on)

A client of mine asked about a beacon light that wasn't coming on. I toggled the output on the card, and found that the fuse between the card and the light had failed.
After turning off the output I replaced the fuse and noticed that the light had come on. I checked the logic, and saw that the output O:14/0 was low. I pulled the fuse, and checked line side to ground and found 124VAC. There were other outputs with the fuses removed due to various modifications... some on this card, others on output cards 12 and 13 (all 3 the same 1746-OA16).
In all three of these cards there are outputs that should be low that are high. At the same time there are outputs on all three cards working properly.
What are the likely causes of this and what can be done to minimize future problems?
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Old January 5th, 2010, 11:50 AM   #2
Ron Beaufort
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a good place to start would be a forum search for "TRIAC" and the term "leakage current" ... that will probably take care of 99.9% of your questions ...

to go further, we need to know something about the beacon light ... is it incandescent - or LED - or ??? ...

note that if the lamp USED TO work properly, but now it stays ON even when its control bit is at ZERO status, then the output circuit components (inside the output module) might actually be damaged ... in simplest terms, the circuit can act like a set of "welded" contacts in some "damaged" cases ...

check the wiring between the output module and the lamp CAREFULLY for an intermittently shorted condition ... that situation might be the root cause of your problem ... the rest of your confusion (about the OFF" outputs still being "ON") will probably be taken care of once you fully understand how the TRIAC circuit works ...

hope this helps ...

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Last edited by Ron Beaufort; January 5th, 2010 at 11:59 AM.
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Old January 16th, 2010, 01:03 PM   #3
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Thanks Ron! When I first came across this situation I was perplexed as the one failed output was driving a rotating beacon light.
I replaced that card. Without a load it's still reading 120VAC, however with the load connected it drops to zero.
I'm working through the other outputs now to confirm that they're experiencing the same thing. Is it common to see this phenomenon on upwards of 1/3 of the outputs?
Also, the power supply for the rack is set up with a floating neutral. (instead of L1-120VAC and L2-Neutral it's set up L1-60VAC, L2-60VAC). The voltage at VAC1 and VAC2 on the output cards are both 120VAC.
Could the power supply's lack of a neutral reference be having any effect on triac leakage?

Thanks again for your help.

ps... what's fascinating is that when a load is placed on one output exhibiting leaking, not only does that output's leaking go away, but adjacent outputs also exhibiting leakage recover

Originally Posted by Ron Beaufort View Post
a good place to start would be a forum search for "TRIAC" and the term "leakage current" ... that will probably take care of 99.9% of your questions ...hope this helps ...

Last edited by Russ; January 16th, 2010 at 01:12 PM.
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Old January 16th, 2010, 04:56 PM   #4
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Floating Neutral? Best to check the original design intent to verify that is correct. Could be the reason for many unusual problems.
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Old January 17th, 2010, 07:41 AM   #5
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I agree that the problem is most likely a loose or missing neutral connection... I had one of these trip up one of our service techs on Friday...
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