Re: Bad to connect scope leads to AC line voltage?
- From: "Walter Harley" <walterh@xxxxxxxxxxxxxxxxxxxx>
- Date: Tue, 6 Sep 2005 23:31:04 -0700
<joe@xxxxxxx> wrote in message
news:1125989293.533764.18880@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>I seem to recall reading somewhere that one is supposed to use a 1:1
> isolation transformer when wanting
> to scope the hot side of line AC powered equipment.
>
> Other than the obvious safety issue, is this bad for the scope? Will it
> damage the scope somehow?
If you connect the scope ground lead to the line, it'll short the line to
ground through the scope, which will certainly fry something.
If the scope and probe are rated for 120vrms (or 240, etc., depending on
where you live), then there is nothing whatsoever different about the mains
than any other ground-referenced signal. Most common scope applications
deal with ground-referenced signals.
Where things get tricky is when you want to measure a signal that is
relative to the hot side of the line; for instance, you want to measure the
drop across a fuse. Not thinking clearly, you might decide to connect the
probe tip to one side of the fuse, and the probe ground lead to the other.
Kaboom.
So, the isolation transformer thing is really just a trick, a "poor man's
differential probe". It lets you reference your scope signal to anything
you want. But the cost is that the scope chassis is then hot with respect
to earth ground, and you're liable to zap yourself. Better to use a
differential probe, or two probes and the scope's differencing function if
it has one.
.
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