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Re: Problem with common leads in dual channel scopings?



Original poster: dhmccauley-at-spacecatlighting-dot-com 


You have to remember that each channel on a typical oscilloscope is NOT
isolated.  The grounds on each of the scope's channels are internally tied
together and this ground is also internally
tied to chassis ground which is tied to the 3rd prong on your AC power cord.
This is the reason that you can take measurements without connecting the
ground.  (although a bit noisy)  You're circuit
is somehow connected to ground already through your house wiring etc...

Also, you have to be careful how you connect a scope up to a circuit.  For
example, if you hook up a scope probe across the output of a full bridge,
you will DESTROY that full-bridge since you are basically grounding one of
the hot outputs.  I think you are seeing this problem in your descriptions
below.

To truly measure a signal like that you need a differential probe.


 > This brings me to a question of scope operation: can
 > anyone simply say why grounding is necessary? These
 > scope leads were only monitoring voltages from an
 > alternator stator being under 12 volts or so, so the
 > voltage ranging precautions of the scope were
 > available, BUT it might be true that since it is
 > monitoring a circuit of higher amperage delivery,
 > might any special precautions be necessary?  Here it
 > seems like some kind of short occured, so after the
 > scope was trashed I tried my TEK 2213 on the same
 > circuit. In this case the short starting acting every
 > time, and such a dual channel scoping by that method
 > was impossible.  In fact what strangely occurs with
 > these primaries, what happens is that when a certain
 > volume of amperage rise occurs by increasing the
 > alternator input via DC variac to field, all of a
 > sudden the primary being monitiored simply stops
 > resonating. Could it be possible that the internal
 > impedance of the scope might act differently when
 > observing a 480 hz signal? Doesnt sound too likely,
 > more like a problem involving the probe connections I
 > thought...