jim rozen wrote:

In article , Ted Edwards says...

If your scope is dual trace, you can do what you want with a current
transformer. You need a 120V to 12V transformer rated for a dozen or so
...

I don't understand how he is going to display voltage
with this setup, unless he ties one side of the
incoming line (240, yes?) to the scope common rail,
which is typically by definition power ground. That
will cause smoke to leak out someplace.

The transformer provides isolation so the "12V" current sensing winding
can go in either motor lead as dictated by convenience. The transformer
and resistor converts the current to a proportional voltage. One probe
and ground goes across the resistor which is across the "120V" winding
and isolated from the other winding.

The other channel of the scope is sensing voltage. The _probe_ goes to
the hot lead, the scope ground goes to ground. You may have to allow
for the small difference between neutral and ground but this can be
measured separately.

If you still have doubts, I could post a circuit to the dropbox.

BTW, this method is commonly used to measure currents in lines kilovolts
above ground. The common clamp on ammeter also works on this scheme.
The clamp contains the core and the wire which it encircles forms a one
turn winding.

Ted