Re: high voltage measurement w/ divider

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

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Yes, this will work.. I'd suggest putting something like a 10 K pot at the 
bottom of the string and measuring the voltage there.  1000:1 ratio would 
be nice. You can adjust the pot to make the ratio come out right.  Your DVM 
probably has a 10 Meg, or so, input impedance, so measuring 10 volts across 
a 10 K resistor shouldn't have any issues with the impedance of the meter.

Yes, you'll be overloading the resistors, a bunch.  They get hot in a 
hurry, which changes their value, ruining the accuracy of your measurement.

If you've got more resistors, why not string 20 together... That will get 
you down to 500V per resistor, and only 1/4 watt, much better.. You can 
then use a 25 K pot at the bottom... (15 resistors would also work, etc.)


20k/volt would be for an old style 50 microamp movement analog multimeter.

Pay real attention to grounding the bottom end of the resistor string.  If 
you get an open resistor, you don't want your meter case floating up to 10 kV.

At 06:35 PM 2/3/2003 -0700, you 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