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?
Thanks,
Joe McGuckin
Depends.... on where you put the transformer The overvoltage limts of your scope Whether you have line transients What probes you use
The safe way to do it is to use a probe isolator. There's one for sale at the link in my sig.
Next best is to use a differential scope with probes rated for the voltage in use + any transients etc.
Don't float the scope. Aside from the obvious safety issues... there are some not obvious safety issues. mike
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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