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Re: High voltage probe, odd NST measurements

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

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.


The meter itself probably has 10M input resistance too.

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.


Hmmmm.  I can't think of anything that would go wrong....  It should work...


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?

Check the connections and meter input impedance. If the meter is digital, the high voltage may be locking it up.

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

You might try a different meter to verify that. The "fairly disposable meter" might be gettign affected by the nearby high voltage.

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


Not sure.  What you are doing is pretty standard and it "should" all work fine.

Cheers,

        Terry



Thanks for any help.