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Old 12-23-2011, 02:05 AM   #1
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Fixing a flourescent light fixture


Replacing the bulbs didn't work so I decided to replace the ballast (probably screwed up letting the thing flicker for days-- evidently that's bad).

However, having taken it apart, I notice something odd: when switch is on I get the expected 120 V from the two wires, but when it's off I get 40V AC-- now that shouldn't be, should it?

I'm assuming that this means that one of the switches (it's a 3-way pair of switches) is defective and should be fixed, but I thought I'd stop by here and run it by you guys.

Thanks!

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Old 12-23-2011, 10:10 AM   #2
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Fixing a flourescent light fixture


Just for clarification: A pair of 3-switches. That said, IF the switch controlling the particular light you are working on is in the "OFF" position, and it IS a 3-way switch, then YES you could possibly be getting some "backfeed" via the wiring for the switches. OR--if the controlling 3-way switch is backfed via another switch, then anything is possible. I prefer dedicated circuits for multiple switching devices.

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Old 12-23-2011, 10:46 AM   #3
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Fixing a flourescent light fixture


If you are using a high impeadance meter, you are just picking up induced voltage....the moment you put a load across it, the voltage will drop to 0.
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Old 12-23-2011, 11:13 AM   #4
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Fixing a flourescent light fixture


Quote:
Originally Posted by ddawg16 View Post
If you are using a high impeadance meter, you are just picking up induced voltage....the moment you put a load across it, the voltage will drop to 0.

I can pickup voltage on just about any wire that is a few feet long on my Fluke 88, and not a thing on my much cheaper and lower impedance DVOM.
Little things like this can sometimes cause issues with troubleshooting if you dont keep that in mind.
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Old 12-23-2011, 02:29 PM   #5
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Fixing a flourescent light fixture


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Originally Posted by Thurman View Post
I prefer dedicated circuits for multiple switching devices.
Wow that could get expensive and fill your panel up very quickly.
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Old 12-23-2011, 05:50 PM   #6
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Fixing a flourescent light fixture


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Originally Posted by ddawg16 View Post
If you are using a high impeadance meter, you are just picking up induced voltage....the moment you put a load across it, the voltage will drop to 0.
Thanks for the responses, especially ddawg16. I believe it's a case of stray (induced) voltage. Here's a primer for those interested:
http://support.fluke.com/find-sales/...01_ENG_B_W.PDF

I'm using an el-cheapo Chinese DMM. I connected a cell phone charger to the two leads while measuring, and the AC voltage went from 40V down to 4V.


I've finished replacing the ballast and it works great. Thanks, everyone!

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