Re: HV voltage divider

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

I would always use a resistor at the bottom of the divider.  If the fuse in
the DMM opens, or a lead falls off (you weren't going to solder the meter
into the circuit, were you?), the meter can float up to HV. One scenario is
where the big series resistor flashes over (it happens, why do you think
they have all those funny ribs on HV probes), causing a momentary large
fault current in your meter, blowing the fuse.  Granted, the current will be
limited by the big series resistor so death and destruction is unlikely, but
it's just good practice to anticipate failures and make sure the results are
non-catastrophic.

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, March 02, 2002 4:25 PM
Subject: Re: HV voltage divider


> 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
> >
>
>
>