Re: HV voltage divider

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Shaun Epp by way of Terry Fritz <twftesla-at-qwest-dot-net>" <scepp-at-mts-dot-net>

You may not even need an extra resistor to make a high voltage probe, the
input impedance of you VOM you said was 10 Mega ohms.  Check your meter with
another meter on all DC voltage ranges to make sure the input impedance
remains the same, probably 10 Mohms, if it does you'll have a 100:1 DC high
voltage probe.  Rh = 990Mohms (your stick resistor) , Rm=10Mohms (you meters
input impedance).

Vin / Vout(meter) =  (Rh + Rm)/Rm          --  in the case for a 10Mohm
meter, this gives a  perfect 100 : 1 divider

I'd check it with a known source too.   Also for AC, most meters use a lower
input impedance like 1 Mohm or 2.2 Mohm and if you use a really high
resistance in series with it like 990 Mohms, you have to take into account
the input capacitance of the meter which of course if frequency dependant.

Hope this helps,

Shaun Epp


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Friday, March 01, 2002 5:28 PM
Subject: Re: HV voltage divider


> Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<davep-at-quik-dot-com>
>
> Tesla list wrote:
>
> > Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <jimlux-at-earthlink-dot-net>
>
> > Exactly how you'd do it....
> >Is the meter impedance really 10 Meg, or is it 11Meg?...
>
> Concur.
> with one minor comment:
> Suggest checking the power rating of the high meg resistor.
> (yes.  The resistance is high, so the currents low, BUT THE
> Voltages high.  Also look at the sparkover possibilities
> 'along' the multiplier R.  (One such probe i ahd worked the
> resistor in liquid freon...)  If the high r resistor fails,
> or sparks over, the voltmeter is toast....
>
> > You can always calibrate it with a known source...
>
> > Tesla list wrote:
>
> > > Original poster: "Richard Williams by way of Terry Fritz
> > <twftesla-at-qwest-dot-net>" <richardwwilliams-at-earthlink-dot-net>
>
> > > I have a 990meg ohm resistor salvaged from a HV DC probe whose
> >>meter was shot.  Can I add a resistor in series, 10 meg ohm?, to
> >>ground to measure HV? Haven't tried it since I'm leary of inflicting
> >>damage to my digital meter since I've never done this before. Input
> >>impedence of the meter at the 700vac range is 10meg so would that
> >>"look" like a 5 meg resistor overall?
>
> best
> dwp
>