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Isolating Transformer

I was working on an CRT TV using an isolating transformer. I happened to measure the voltage between the chassis 'dag' and a nearby metal fixture and was surprised to find 75Vac. Without the transformer the reading was 110Vac.

Something is wrong, right?

BobH

Reply to
pfm
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You shold be able to trace it out. I'm glad you said isolation transformer rather than isolation transformer. You want ground isolation because the primary is referenced to ground. You can check out some resistances with the AC removed.

greg

Reply to
GregS

r

he primary

r

he primary

Thanks Greg but I'm not sure what you are saying. I use the terms 'isolating' and 'isolation' interchangeably. I thought the purpose of an isolating/isolation transformer is to ensure that no potential difference can exist between the device under test and a grounded object such as a concrete floor or a water pipe. The risk of shock is restricted to within the device under test.

Reply to
pfm

primary

primary

There's always capacitive and inudctive coupling. You were probably measuring with a high-Z DMM. Put any sort of load between the meter probes and the voltage will go to 0. If it does not, you do have a serious safety issue.

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Reply to
Samuel M. Goldwasser

"pfm"

** Wrong.

DMMs typically have 10 Mohms impedance across the leads when in any of their AC volts ranges - so a reading of 75 volts AC indicates there is leakage of about 7.5 uA of AC current.

Equates to about 600 pF of capacitance ( primary to secondary) in the iso tranny.

Try connecting a 0.1 uF cap from the TVs chassis to ground - that should swamp the effect.

.... Phil

Reply to
Phil Allison

eir

akage

hould

Thanks Sam and Phil, you were right, it was just stray voltage.

Reply to
pfm

I was suggesting that you may be using the wrong kind of transformer. We went all through this recently. Most all isolation transformers sold to the public are NOT isolated. They are for noise reduction. You seem to be correctly talking about a transformer that does not have the secondary tied to ground. It can easily be checked by resistance measurment. I was just trying to troubleshoot your problem, and wanted all the facts.

greg

Reply to
GregS

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