120/12V Step Down Isolation Transformer......

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chevyx92

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Do you ground the common on the secondary? Trying to troubleshoot a lighting problem and existing transformers have a ground wire going to the common side of secondary and it's bonded to EGC of primary and metal can. Old incandescent lights worked fine. Swapped them out with LED light fixtures and as soon as power is applied it blows internal fuse on lights. There is 12V to each light.
 
Daniel malack said:
Swap the positive and negative wires, remember it is a diode only flows in 1 direction, see if that makes a difference.

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If it is a transformer only then he is dealing with AC, reversing polarity will not change anything.

If he has a rectifier incorporated into it then he has a DC power supply and output polarity could be an issue.
 
chevyx92 said:
Do you ground the common on the secondary? Trying to troubleshoot a lighting problem and existing transformers have a ground wire going to the common side of secondary and it's bonded to EGC of primary and metal can. Old incandescent lights worked fine. Swapped them out with LED light fixtures and as soon as power is applied it blows internal fuse on lights. There is 12V to each light.
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What is the open-circuit voltage from the transformer, with no load? Even if it's listed at 12VAC, I'd bet it could float as high as ~18VAC with no load. Halogen lamps provided a low enough resistance to load the transformer and have it supply its nameplate voltage. The LED's may not provide enough load, and are blowing up on overvoltage. I'd bet that the secondary being grounded has nothing to do with it.


SceneryDriver
 
SceneryDriver said:
What is the open-circuit voltage from the transformer, with no load? Even if it's listed at 12VAC, I'd bet it could float as high as ~18VAC with no load. Halogen lamps provided a low enough resistance to load the transformer and have it supply its nameplate voltage. The LED's may not provide enough load, and are blowing up on overvoltage. I'd bet that the secondary being grounded has nothing to do with it.


SceneryDriver
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12.8 volts. Tech support says after 14V they blow.
 
Possibly a ground fault somewhere in the secondary. No ground reference, no current, no current differential.

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chevyx92 said:
We removed the bonding jumper on secondary side of isolation transformer and it fixed the problem. Fuses don't blow anymore.
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There is a ground fault on the opposite secondary lead then. The system is now grounded wherever that fault is.
 
Abdel Rahman Mohamed said:
Do you mean that now there is a current leakage or ground fault? I can not got it.
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You can't bond two different points of the supply system to the same point - that results in short circuit current. By removing an intentional grounding/bonding jumper all you are doing is opening the short circuit pathway, and whatever unintentional connection to ground exists on the other line is now effectively grounding/bonding the other conductor.
 
kwired said:
You can't bond two different points of the supply system to the same point - that results in short circuit current. By removing an intentional grounding/bonding jumper all you are doing is opening the short circuit pathway, and whatever unintentional connection to ground exists on the other line is now effectively grounding/bonding the other conductor.
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I agree with you, but normally the neutral point for isolation transformer is to be grounded in order to eliminate harmonics, That's why I confused.

And I think that there is something else is wrong, kindly double check.
 
Abdel Rahman Mohamed said:
I agree with you, but normally the neutral point for isolation transformer is to be grounded in order to eliminate harmonics, That's why I confused.

And I think that there is something else is wrong, kindly double check.
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How does grounding a neutral point change harmonics?

OP's application is a two wire secondary - there is no neutral in a two wire source.
 
kwired said:
There is a ground fault on the opposite secondary lead then. The system is now grounded wherever that fault is.
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I disagree with your statement. If there were a fault condition then why did the original incandescent fixtures work? Also this pool has 15 lights, 8 isolation transformers fed from multiple GFCI protected breakers. For your statement to hold any water, every circuit would have to have a fault in it because we had to remove the bonding jumper on every transformer(which isn't required on these isolation type transformers)?
 
chevyx92 said:
I disagree with your statement. If there were a fault condition then why did the original incandescent fixtures work? Also this pool has 15 lights, 8 isolation transformers fed from multiple GFCI protected breakers. For your statement to hold any water, every circuit would have to have a fault in it because we had to remove the bonding jumper on every transformer(which isn't required on these isolation type transformers)?
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A wiring fault on every one of the circuits would indeed be unlikely.
But you replaced the incan with LED on each and every circuit. My guess is that (possibly deliberately) one of the two leads connecting to the LED fixture is also connected to some point on the outside that gets grounded to the shell and conduit when you mount the fixture.
If you had a simple transformer secondary with no center tap and you grounded one end of the secondary you would have a 50% chance of grounding the same wire that is grounded at the fixture.
But with a center tap whichever way you connect it you will get half the secondary voltage shorted out at the LED fixture.

That would be the simplest explanation, and is easy to test by measuring with an ohmmeter on a fixture which is mounted but not connected to the transformer secondary.
 
GoldDigger said:
My guess is that (possibly deliberately) one of the two leads connecting to the LED fixture is also connected to some point on the outside that gets grounded to the shell and conduit when you mount the fixture.
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One or two, OK then I would agree but not all 15. I just can't see that possibility on all 15 fixtures especially when conduit is PVC too.
 
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