Re: High voltage probe, odd NST measurements

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Michael S <mike5904@xxxxxxxxx>

Well looking at the meter specifications I see a 10M impedance, which
would lead to a ratio of 11000:1.  I wouldn't know how to go about
measuring the effect of meter capacitance on frequency response but at
least the measure for the NST was "close enough" for my purposes.

On 9/4/05, Tesla list <tesla@xxxxxxxxxx> wrote:
> 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.
>  >
>  >
>  >
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