Re: high voltage probes

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

Original poster: Mark Broker <mbroker-at-thegeekgroup-dot-org> 

This sounds like a good idea.  I do have a 50's vintage tube scope that I 
view as expendable.  I realize that "ground" is not really at ground 
potential in a Marx.  I suppose I could make two and connect them to A and 
B to make a poor-man's differential probe....  No, I do not have a UPS nor 
access to one that is viewed as expendable.

Calibration is an issue, since I would rather like to figure out how a 32kV 
rectifier was overvolted when only 20kV should have been across it.  We've 
also lost two capacitors to what looks like overvoltage despite never 
getting above about 65% of their voltage rating (22kV).

Would a resistor divider with some 10Meg 1/4W resistors (I have plenty) 
work in place of the HV probe?

Thanks for the replies so far!

Mark Broker



On Sat, 27 Sep 2003 17:32:04 -0600, Tesla list <tesla-at-pupman-dot-com> wrote:

>Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> A water resistor makes a 
>good load/probe for a Marx.  You can stand the pipe
>vertically, and put  the probe in at the side.. Make the water resistor
>something like 50:1 (i.e. if it's 50 inches tall, put the probe 1" from the
>bottom) and then use a normal HV probe to get it in range for your scope.
>
>You can capacitively couple out of the resistor too.. a splotch of copper
>foil tape stuck to the outside of the water resistor works dandy.
>Calibration and compensation's not too hot, but if what you're interested is
>rough stuff like rise/fall time and voltage, it'll work ok.
>
>----- Original Message -----
>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: <tesla-at-pupman-dot-com>
>Sent: Saturday, September 27, 2003 4:05 PM
>Subject: Re: high voltage probes
>
>
>  > Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
>  >
>  > Hi Mark,
>  >
>  > Consider "water resistors".  Or, plane wave antenna.  Just make a little
>  > flat "transmitter" antenna to get a better signal off what you want to
>read.
>  >
>  > Directly connecting the Marx is scarry since the voltages and currents are
>  > so high that no ground can be trusted. Use a cheapo E-bay scope with lots
>  > of big MOVs on the power and stuff ;-))  I would simply "plan" on it going
>  > bad...  But once you get it going, it will seem easy ;-))  If the scope
>  > eats the output, it will also go right into the AC wiring as it happily
>  > cremates the scope.  So work on protecting the AC line stuff real
>  > well.  Might even consider running off a cheap old UPS power source....
>  >
>  > Cheers,
>  >
>  >          Terry
>  >
>  >
>  > At 05:44 PM 9/27/2003 -0500, you wrote:
>  > >I have a desire to hook our small Marx Generator up to an oscilloscope to
>  > >view the waveform across the rectifier during the discharge event.  A
>  > >while back we overvolted a 32kV rectifier we were using at "low voltage"
>  > >with only a little over 10kV cap charge.  In theory, only 20kVmax should
>  > >have existed across the rectifier.  I don't want a similar event with the
>  > >final 55kV rectifier when we run it at full voltage off a 15/30 NST.
>  > >
>  > >I was thinking of just making a 1000:1 resistive divider, which would
>work
>  > >for low frequency/DC, but I don't know how well that would work with the
>  > >MHz hash in the discharge.  Any opinions from the crowd?  I don't expect
>  > >the need to measure more than 25kV.
>  > >
>  > >Thanks!
>  > >
>  > >Mark Broker
>  > >Chief Engineer, The Geek Group
>  >
>  >