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Re: High voltage probe, odd NST measurements

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Check the input impedance of your meter and take this into account when figuring the final "1Mohm to ground resistance.

If you are really going to go to 50KV with 10 1Gohm resisters, each resister will have 5KV across it and breakdown may be a problem. My HV probe is 1000:1 so a 50KV input will give a 50 volt output which should be fine for scopes and DVMs. The probe requires a 10Mohm load which is a typical 10:1 scope probe or a DVM.

I just got a thought. Your 12KV allanson is center taped meaning you have 6KV on any one of the two bushings. I'm thinking that with the correct connection and no reading that something is opening up or the 1Mohm is being shorted (maybe by the meter if it is somehow grounded). Maybe a bad 10 Gohm resister. Check the voltage rating of your resisters. You might try shorting out (only one at a time) each of the 1 Gohm resisters to find the bad resister (assuming this is the cause) and/or try the correct connection with the meter on the PVC insulator (incase the meter is shorting out the 1 Mohm resister)

Gerry R.


At 03:53 PM 9/4/2005, you wrote: 
I just built a simple resistor voltage divider for the purpose of
direct measurement of high voltages in the 1-50kV range.  It consists
of 10 1Gohm resistors in series with a 1 Mohm resistor.  Voltage is
measured across the 1 Mohm resistor, which should create a 10000:1
divider.


I decide a simple test would be measuring one of my NSTs, a
12kV/60mA Allanson unit.  Using a fairly disposable meter I had around
I hook up the low voltage end of the probe, connect the probe across
ground and one of the transformer legs.  I apply about 50V to the
transformer, and get no reading.  Confused, I try to draw an arc off
of one of the terminals to a wire connected to the transformer case
(not the best idea), which works fine.  Now even more confused, I
decide to switch the terminals on the probe.  I insulate the meter on
a sheet of PVC first, since it will now be at the potential of one of
the NST legs, and power the transformer up again.  Now I get a reading
of .55, which would correspond to a voltage of 5.5kV from leg to
ground.


Here's what I'd like to know:
-Why do I get no reading when measuring from ground to leg, when there
is obviously a potential difference there?


-Why do I get a completely different reading when reversing the probe
(and putting the meter in an unsafe position)?


-Why is the second reading twice that of what I would expect for
ground to leg? (since I am only applying about 50V)