A Low-Cost
High Impedance
High Voltage Probe
for Digital Voltmeters
by Charlie Thompson
Buda Seismic Observatory

Measuring the high voltage on those CDV700's and other geiger
counters can present a real challenge.  Ordinary voltmeters including
DVM's draw WAY too much current to allow you to accurately measure
the HV on your gieger or scintillator power supply.  My low-cost solution
was to use a DVM that has a 10Meg input impedance.  Many DVM's have
an input impedance 10Megs and you can usually measure this with another
meter.  Assuming you are fortunate and your meter is one with 10Megs of
input impedance you can construct my simple 10X probe below.  It is a
simple 10:1 voltage divider using a string of NINE 10Meg quarter watt
resistors (an excuse to visit Radio Shack is now in order).  I checked the
accuracy of my 10:1 probe against a FLUKE 343 voltage calibrator and using
5% resistors (as you might imagine) it is accurate to about 5%.  Not great
but MUCH better than loading your HV circuit with a mere 10Megs.

If you get compulsive you can select 10Meg resistors to get better accuracy but
you'll need a voltage calibrator to verify the accuracy.

To use the probe set your DVM to the 100V or 200V range (or equivalent).
Then multiply the DVM voltage reading by 10.  That's it!