Re: high voltage measurement w/ divider

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

Original poster: "David Sharpe by way of Terry Fritz <teslalist-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

Dave

Timely response, with my posting on a divider design with a Optoisolated
High Voltage Monitor (OHVM).

I would be really leery of double dissipation thermally on resistors even if
intermittent duty cycle.  You're stressing them with HV anyway, adding
stress of thermal runaway maybe a good way to blow away a meter
or put yourself at risk.  :^O  One my OHVM posting, I size at no more
then 50% of V and thermal rating load on resistors in a divider, you
want it to be reliable for sure on HV, especially if the string opens up
and the meter displays '0' and everything is safe, right?

Moral of story; NEVER assume anything around HV.

Regards
Dave Sharpe, TCBOR
Chesterfield, VA. USA

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.
 >
 > 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?
 >
 > Thanks for any help,
 > David Rieben