Re: Testing Isolation Transformer


"Nomen Nescio" <nobody@xxxxxxxxx> wrote in message
news:53036a7da92233454dfcd189310b20bc@xxxxxxxxxxxx
>I just got an isolation transformer for restoration of my 1950's RCA
>Victor
> TV. The power cord is non-polarized. Up until now, I've just been
> using a
> Fluke 77, so I didn't bother with the isolation. But I'm now moving
> into a
> phase where I need to view waveforms on an oscilloscope. And from
> what
> I've read, without an isolation transformer, fireworks will be the
> result
> of the first probe. Now that I've explained the scenario, can anybody
> explain how I can test the isolation transformer so that I know it is
> doing
> the job it was designed to do? I took some measurements with the
> Fluke and
> this is what I got:
>
> wall outlet: 118VAC
> iso: 118VAC
> Hot_wall - Hot_iso: 67.5VAC
> Neutral_wall - Neutral_iso: 60VAC
> Hot_wall - Neutral_iso: 162VAC
> Neutral_wall - Hot_iso: 34VAC
>
> With this information, can it be concluded that the isolation
> transformer
> is working as designed?
>

Very likely the voltages you see are just capacitive coupling from
Primary to secondary.
The impedance of a Fluke 77 is so high it will see all sorts of stray
voltages.
The only simple way to test the isolation is to measure the resistance
between the primary and secondary with a high voltage Megger or similar
insulation tester.
--
John G

Wot's Your Real Problem?


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