[Test-Equipment] HV probe resistor selection

[Bill Fuqua]

   That just reminded me of an infinite resistance DC volt meter from long ago.
It used a metallic box with the HV going into one side  to a metal plate,
and a metal plate on the other side attached to a connector. Between them 
was a grounded half disk
that rotated. This would interrupt the electric field between the two 
plates generating
an AC current on the measurement side which was measured by an AC VTVM.
   No resistor but complicated.
A type that I have also seen is one where the output side was connected to
a vibrating reed excited by a oscillating current thru a electromagnet. 
This one was called
a vibrating reed electrometer.

73
Bill wa4lav


At 09:34 AM 2/23/2013 -0500, djed1 at aol.com wrote:
>Here's what I think are the design issues:
>
>1. You want the impedance of the probe to be say 100 times the impedance 
>of the circuit under test. So unless you're measuring exotic very high 
>impedance circuits, I would think 25 megs would be OK.  75 megs would be 
>better.
>
>2. For a voltage division of 1000, the choice above defines the shunt 
>resistor:  25meg/25K or 75meg/75K.
>
>3. You want the impedance of the meter to be constant and say 100X that of 
>the shunt resistor.  This will give you accuracies of 1-2%- if you need 
>better accuracy then put a trimmer in series with the shunt resistor.  You 
>can also tweak the shunt value to compensate for a lower Z in the 
>meter.  Of course, this assumes you have an accurate high voltage reference.
>Ed  W2EMN
>
>
>
>
>
>
>
>-----Original Message-----
>From: DaveC <davec2468 at yahoo.com>
>To: Discussion of Electronic Test Equipment <test-equipment at mailman.qth.net>
>Sent: Sat, Feb 23, 2013 5:12 am
>Subject: Re: [Test-Equipment] HV probe resistor selection
>
>
>
>
>Sent from my iPhone
>
>On Feb 23, 2013, at 1:57 AM, David <davidwhess at gmail.com> wrote:
>
> > Automatic ranging or even manual ranging on a correctly designed
> > digital meter will not allow an overrange situation to cause damage
> > unless it exceeds the meter's voltage rating.  For example if the
> > meter says it is rated at 1000 VDC and 750 VAC which is typical, then
> > that should apply on any voltage range without damage.
> >
> > The input impedance should be constant, usually 10 Megohms shunted by
> > 30 to 100 picofarads, with the exception of some bench or labratory
> > meters which may have a higher input resistance on the lower DC volts
> > ranges.  My HP 3478A is greater than 10 Gigohms on its 30 millivolt,
> > 300 millivolt, and 3 volt ranges which I was not really aware of until
> > I used it with my Fluke 80K-6 and the readings did not match with any
> > of my 10 megohm input meters.  Some meters will have a lower input
> > impedance on AC voltage ranges (that pesky HP 3478A again is 1 megohm)
> > and they all have a fair amount of shunt capacitance which will affect
> > higher frequency measurements.
> >
> > As Peter point out, the Fluke 80K-6 only has an input resistance of 75
> > megohms which will be too low to make accurate measurements of high
> > impedance circuits.  Sometimes with a little math you can figure out
> > what the unloaded circuit voltage really is.
> >
> > Given the selection of resistors at Digikey that you linked, the
> > easiest replacement would be 3 x 25M units in series assuming the 75
> > kilohm shunt resistor in the probe is ok.
>
>Thanks for your comments.
>
>The 25 M resistor is 4 inches long. No room in the probe body for 3x.
>
>I'm leaning toward the 1 G resistor and selecting an appropriate R2 plus 10-
>turn pot to calibrate.
>
>But still open to suggestions.
>
>Thanks
>Dave
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