On Fri, 10 Feb 2006 13:43:44 -0800, Eric R Snow
wrote:

On 10 Feb 2006 19:52:40 GMT, (Chuck
Sherwood) wrote:


The safe way to do this is to make sure your generator is grounded
because the ground lead on your scope will be grounded and you
don't want current flowing in the ground lead or you will burn up
something or shock yourself. Unless you REALLY know what you are
doing you should not float the scope or alternator and obviously
you would not be asking how to do it here if that was the case.

Next is to use a 10x or 100x probe and connect it to the alternator
output. The probe choice will depend on the max input voltage of
your scope. Read the manual to make sure that you don't overvoltage
the input, lest you fry it. My guess is you will need a 100x probe.
Even then, you need to be careful that you do not exceed the max
rating of the probe. There are two little tiny resistors in there
that are used as a voltage divider. If you exceed the power dissapation
of the resistors, they burn out and your probe is now worthless.

This will allow you to check one leg. If you want to make leg to
leg measurements, you will need a two channel scope and two identical
probes. Put one probe on each leg and invert one channel and put the
scope in ADD mode. Now you can safely look at the combined voltage.

chuck
All the generators I have seen are floating. If that's what is meant
by not being connected to ground. They only get grounded when
something is plugged into them and that something is grounded. As far
as the scope being grounded, it is grounded when it's plugged into the
AC mains. So if I use it to check the AC generator output it will be
grounded and the generator will not be. Unless it needs to be. If it
should be grounded to the scope ground then that's what I'll do. If I
can just touch the probe to one of the wires on the generator to
measure the frequency then that's what will get done.
ERS

I would either ground the genny to the outlet box that powers the
scope, or I'd just power the scope off the genny. If the genny is
floating you could have a common mode voltage that could cause
problems.

Another safe approach would be to drive a transformer with the genny
and observe the secondary with the scope. That'll reduce the voltage
and provide common-mode isolation. It'll also probably strip off any
high frequency content present on the genny wave, but there probably
isn't any there anyway.