[sdiy] diy HV probe?

[R. D. Davis]

Quothe Mark Romberg, from writings of Sun, Jun 27, 2004 at 02:44:50AM -0500:
> i have an o-scope thats on the fritz, so i find myself in need of an HV 
> probe, as my multimeter can only handle 600V.  but, i dont want to 
> spend more than about $10 for something im only rarely going to use, 
> and the commercial HV probes seem to be far above that.  would it be 
> safe/possible to build an HV probe?

Yes... IIRC, hypothetically spealking, you'll need a plastic drinking
straw, heat shrink tubing, about ten 10M-ohm 1/2W resistors, soldered
closely together in series, and a roll of electical tape to wrap the
heat-shrink tubing covered straw with, multiple times.  As you've
probably guessed, the resistors go inside the straw.  For the sake of
thriftyness, perhaps it's possible to use a very short portion of a
resistor lead can be used as the probe tip.  I used heat-shrink tubing
around the resistors as well for added insulation... they just barely
fit in the straw that way.  There are a few examples of doing
something like this to be found on the WWW.  Do this at your own risk
and don't skimp on the layers of insulation... mistakes can zap you
into a permanent otherworldly existence!  This is not offered as
advice, but as a hypothetical example of measuring high voltage at a
low cost.  It's also possibly not a good idea to use it to measure the
output of the multiplier with, but rather the lower (3KV to 5KV or so)
outputs of the HV supply with before the multiplier; either way,
playing with high voltages can be dangerous.

All that having been said, you might be able to do with out building
that.  Are there any signs of HV being present, such as a staticky
sound when powering the 'scope on or off, or will a piece of paper
stick to the CRT surface?  What about the low voltage circuitry in the
power supply, such as the HV oscillator?  Any signs of life there?

Good luck!

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