Re: high voltage measurement w/ divider

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

Original poster: "davep by way of Terry Fritz <teslalist-at-qwest-dot-net>" <davep-at-quik-dot-com>


>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?

         yes.  but.

>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

         think is nice.  meter specs give the answer.
         check the manual, or the manufacturers web site.
         sometimes its molded in the back of the meter.

>most DVMs have at least 20 kOhms /volt

         this is a typical value for an old moving
         coil meter.  dvms typically come in higher.

>deflection

         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.

         1) rule of thumb for voltage divider is 10x
         that is 10 m 'sense' resistor should have a
         meter input of 100m.

         2) check the power dissipation

         3) indepedently, be sure the resistiors will
         not arc over.  (a non obvious spec...)


>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?

         I would not do it.
         usual rule of thumb is to run 1/2 W resistors
         at 1/4 W.  Yes, intermittent duty allows a derating.
         Consider that a failure of the bleeder leaves a
         safety hzard.  Failure of the divider can blow the
         meter.  If overrating the resistors, make sure
         they are cooled, either passively (not enclosed)
         or with a fan...

-- 
         best
         dwp

...the net of a million lies...
         Vernor Vinge
There are Many Web Sites which Say Many Things.
         -me