[Test-Equipment] HV probe resistor selection

[Peter Loron]

Note that for some HV systems (e.g. geiger counters), you need a very high input impedance on the probe to avoid clobbering the power supply. They have very low power ratings and will sag or collapse completely under much of a load. Many HV probes for this use have 1GOhm main resistors.

-Pete

On Feb 22, 2013, at 9:10 PM, 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
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