RE: differential probes

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

Original poster: "jimmy hynes by way of Terry Fritz <teslalist-at-qwest-dot-net>" <chunkyboy86-at-yahoo-dot-com>

Hello,

Why would there be two separate MOV (or spark gap)
grounds? It seems like having the grounds floating
would just allow bigger spikes through. It seems like
you would want it at the same voltage as the low
voltage ground, but not directly tied, in case the
MOVs see some high voltage.

There is still the problem of getting the probes
connected to the board. Maybe I could get some high
voltage BNC connectors, or grind the most of the pins
off, and cut the board.

How bad of a "nightmare" would it be to add some more
resistance at the end? I would only be measuring a low
frequency sinewave, and I don't care too much about
the accuracy. I would just want to make sure it wasn't
clipping, so I could prevent my IGBTs from exploding
again.


--- "FRITZ, TERRY" <TERRYF-at-aei-dot-com> wrote:
 > Hi Jimmy,
 >
 > -----Original Message-----
 > From: jimmy hynes [mailto:chunkyboy86-at-yahoo-dot-com]
 > Sent: Tuesday, July 15, 2003 12:22 PM
 > To: FRITZ, TERRY
 > Subject: RE: differential probes
 >
 >
 > Hi Terry,
 >
 >
 > --- "FRITZ, TERRY" <TERRYF-at-aei-dot-com> wrote:
 > > Hi Jimmy,
 > >
 >
 >
 > > C12 and C21 should all have their "own" vias going
 > > to the ground plane (high
 > > current at 150MHz!).
 >
 > Ok. I should disconnect the top trace then, to
 > prevent
 > a ground loop, right?
 >
 >
 > - Yep  that sounds great.
 >
 >
 > > There should be three "very" separate grounds.
 > >
 > > 	The low voltage electronics will have it's own
 > > ground plane but that
 > > will end on the low voltage side of the 1Meg
 > > resistors.  There should be
 > > "no" ground plans under the HV resistor areas.
 > The
 > > HV resistors from a very
 > > wide line of isolation separating the low voltage
 > > stuff from the high
 > > voltage stuff.  Only the resistors should cross
 > that
 > > line.  The grounds are
 > > also "very" separate since under a fault they will
 > > be far from zero volts!
 > >
 > > 	The MOV sets each have their own "very high"
 > > current (like 2000 amp
 > > pulse) grounds that go out to ground studs.  The
 > MOV
 > > grounds are meant to
 > > take giant faults and ground them sparately away
 > > from the other grounds.
 >
 > Ok. I'll separate the grounds, and make big fat
 > traces
 > for the MOV ground. Isn't that only two grounds
 > though? All I see is the low voltage ground and the
 > MOV ground.
 >
 >
 > - I was thinking each of the two channels would each
 > have a MOV ground.
 > That ways you could float the two sides ground
 > around some.
 >
 >
 > > The Official "CE" clearances for 2000Vp on a PCB
 > are
 > > 1 inch!
 > > "Theoretically", the high voltage side stuff
 > should
 > > all be spaced 1 inch
 > > appart from anything else.  2000V will easily arc
 > > 1/4 inch between sharp
 > > points over a surface.  So you may need far more
 > > spacing on the high voltage
 > > stuff.
 > >
 > > Not sure BNC connectors are good for the high
 > > voltage.  I was going to go to
 > > the HV fuses (yet to be figgured out) and then to
 > > banana jacks.  In one way,
 > > I like the BNC and coax cables since they "could
 > > fail" and stop a high
 > > voltage event from getting through to the scope.
 > > Sort of a "crowbar" ;-))
 > > However, we should probably try to stop over
 > voltage
 > > events more
 > > diplomatically :o))
 >
 > How were you planning on going from the probes to
 > the
 > fuses though? What's the insulation in the probe
 > itself? I was worried about the whole high
 > voltage/close spacing thing, but didn't know what
 > the
 > spacing had to be.
 >
 >
 > - I am still trying to figure out the fuse thing.
 > Maybe there will not be
 > any fuses after all....  Might go to gas tubes....
 > Don't know...
 >
 >
 > If I added another 9 Megaohms at the end of the
 > probe,
 > I could measure higher voltages (primary caps), but
 > with some reduction in bandwidth right?
 >
 >
 > - With higher resistances, stray capacitance become
 > a nightmare...  Almost
 > gets impossible...
 >
 > Cheers,
 >
 > 	Terry
 >
 >
 >
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 > Energy Industries, Inc.  The dissemination,
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 > Advanced Energy Industries, Inc.


=====
Jimmy

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