Re: Problem with common leads in dual channel scopings?

["Tesla list" <tesla-at-pupman-dot-com>]

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.
 >
 > Agreed. However, there is nothing to stop you referencing one side of
 > an H-bridge to ground as long as the power supply rails are all
 > floating. This is done in some bridged audio amps. Caution - don't do
 > this unless you know what you are doing.

Yes, that is correct.  But as you said, you must be EXTREMELY careful and
make sure that your rails are completely isolated.
Otherwise - BOOM!  There goes all through FETs on that leg!

The Captain












 > Malcolm
 >
 >
 >  >
 >  >  > 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...
 >  >
 >  >
 >  >
 >
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