Re: High voltage probe, odd NST measurements

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

On 4 Sep 2005, at 17:36, Tesla list wrote:

> Original poster: Michael S <mike5904@xxxxxxxxx>
>
> 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)

What is the input impedance of you meter? It will be connected in
parallel with the 1M resistor in the probe which will alter the probe
characteristics. The impedance may not be purely resistive either
(e.g. there might be a capacitive component). That will affect the
measurement of AC voltages since several GOhms shunted by even a tiny
capacitance will form a low pass filter.

Malcolm

> Thanks for any help.
>
>
>