[Test-Equipment] HV probe resistor selection

[David]

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.

On Fri, 22 Feb 2013 21:10:55 -0800, Dave C <davec2468 at yahoo.com>
wrote:

>I am rebuilding a Fluke 80K-6 HV probe that self- ... well, was the victim of too much DIY curiosity. 
>
>I'm following this FAQ re. rebuilding it:
>
>http://www.repairfaq.org/REPAIR/F_hvprobe.html
>
>The original divider resistors in the probe were 85 Mohm and 85 Kohm which gives a nice divide-by 1000. 
>
>I see that these resistors are appropriate for HV use and in the range I will need: 
>
>http://www.digikey.com/product-search/en?FV=ffec498c
>
>I've been thinking about what meter I want to put near HV. I have an old FET-input Radio Shack meter which might be good for this but it is spec'd at only 10 Kohm/V impedance on AC, and this impedance will change depending on the range selected. A cheap DMM might be a better economic choice -- little to lose if/when HV gets to it. But input impedance is typically low -- one on which I just checked the specs: ">1 Mohm". Preferred meter is a Fluke 117 (I do a lot of mains work) but the impedance is spec'd as >5 Mohm on AC range, not much better. Also, I see that auto-ranging can change impedance and upset the measurement ratio (series probe resistor and meter impedance) which could put the meter at risk of overvoltage(?).
>
>Am I over-thinking this?
>
>I know I can just duplicate the resistor values and leave it at that, but now that I understand the implication of meter impedance I'm trying to make some good decisions re. choosing new resistors and possibly a dedicated meter for its use. 
>
>Suggestions? Ideas? 
>
>Thanks,
>Dave