[Test-Equipment] HV probe resistor selection

[djed1 at aol.com]

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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