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>

  The fuse in large majority of meters is there to protect the meter's
current range and is in series with the low current jack, some times another
fuse with the high current jack, Fluke, Beckman, and Wavetek do this with
there meters.  Also the long length of the High Mega Ohm, High voltage
resistor protects the resistor from flash overs, unless it gets dirt, finger
prints and gunk on it.  It's important to keep them really clean, this
prevents surface tracking and flashover.  This resistor just sits inside of
the high voltage probe, usually spring loaded to make sure the top of the
resistor firmly touches the top connection, the out side of the probe
protects you and keeps the resistor safe and clean.  I have seen commercial
high voltage probes that donnot have the second resistor across the meter
jacks, others do , I guess I like the ones with the extra resistor too, but
I've used probes without.

Shaun Epp


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, March 03, 2002 1:29 PM
Subject: Re: HV voltage divider


> 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

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