Re: high voltage measurement w/ divider

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <teslalist-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi David,

On 3 Feb 2003, at 18:35, Tesla list wrote:

 > Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
<tesla729-at-cs-dot-com>
 >
 > Hey all,
 >
 > I was wanting to build a voltage divider out of some spare 10
 > meg resistors that I had laying around to safely measure the
 > charge voltage of my 200 uFD, 10 kV energy discharge cap bank
 > for my can crusher/quarter shrinker assembly. I was wondering
 > if I could accurately measure the known fraction of the total
 > voltage by placing 10 of these resistors in series and measur-
 > ing 1/10 of the total voltage across just one of the resistors?
 > I know this principle works because of Ohm's law and all, but
 > what my real question is would the 100 megs be too much resis-
 > tance to get an accurate and reliable reading on my Sperry DVM?
 > I think most DVMs have at least 20 kOhms /volt deflection so
 > it seems that measuring up to 1000 volts (1/10 of the 10 kV)
 > should be ok since 20 megs (20 K X 1000 volts) is greater than
 > the 10 meg for each resistor. BTW, these are the Digikey 10 meg
 > resistors that many of you are using as bleeders for your MMCs.

The ideal situation is to replace the resistor (call it R1) across
which you want to measure the voltage with another (call it R2) such
that the resistance of the voltmeter and R2 in parallel = R1. That
ensures that an identical voltage drop will be present across the
measuring combination. DVMs really have a fixed input resistance for
a given range (if not all ranges on an auto-ranging meter) so the
20k/Volt thing doesn't count.

 > Also, I think these are 1/2 watt resistors and if my math is
 > right, they should be dissapating 1 watt when the caps are
 > charged to the full 10 kV (10*4 V/10*8 Ohms= 10*-4 amps or
 > 0.1 mA and therefore 10 kV X 0.1 mA = 1 watt. I think this
 > doubling of their wattage rating on such an intermittent ba-
 > sis should be ok?

Depends on how reliable you want the setup to be and also on the
ability of the resistors to withstand the voltage. Personally I go
for a safety margin every time but that's me.

Regards,
Malcolm