Re: HV Measurement - Back to Basics

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

Original poster: "Norman F. Stanley by way of Terry Fritz <teslalist-at-qwest-dot-net>" <nfs-at-midcoast-dot-com>

At 09:13 AM 1/4/03 -0700, Tesla list wrote:
>Original poster: "Matthew Smith by way of Terry Fritz 
><teslalist-at-qwest-dot-net>" <matt-at-kbc-dot-net.au>
>
>Hi All
>
>Could some kind soul give me a hand with this little problem?
>
>I bought (blind) a 15kV voltmeter, which I planned to sit on the end of my 
>MOT-based power supply.  When I first saw it and discovered that the 
>terminals are about 8mm apart, I decided that this is just a meter with a 
>15kV *scale*, not a meter than can be connected to and measure up to 
>15kV...  Never fear, I thought, it's just the question of sizing an 
>appropriate resistor/resistor network.
>
>Looking at the base of the scale, I see some small symbols; the first 
>appears to be an underscore - possibly this is a moving coil (DC) meter 
>(terminals are also marked + and - which would tend to confirm this). The 
>second symbol is a star with a 2 in it - goodness knows what this 
>means.  The third symbol is an upside-down capital T with 1.5 above 
>it.  The fourth symbol appears to be a horseshoe magenet, pointed 
>downwards, with something between the poles.  The fourth symbol is a 
>standard Euro resistor symbol with a very helpful R in it.
>
>If anyone can shed any light on the above, I'd be interested, but the 
>imporant bits followed: 500uA 100V.  Now, I'd read that as being 500uA 
>FSD, and a maximum voltage rating of 100V.  (A bit less than 15kV, eh?)
>
>I canna remember how I'm supposed to wire this up!  I'm fumbling with 
>this:  if FSD is 500uA, I would need a series resistance of:
>
>R = 15,000V/500uA = 30Mw (where w represents capital Omega)
>
>This, however, doesn't sound right because then the whole thing would be 
>dissipating:
>
>15,000V x 15,000V / 30Mw = 7.5W  Wouldn't this be getting a bit warm?
>
>I assume that I'd have to have a potential divider somewhere around here 
>to make sure that the meter never sees more than 100V across it (if, 
>indeed, that is it's rating.)
>
>...and that's where I've come to a grinding halt.  I don't know whether I 
>started off going the wrong way or if I've just got the math wrong 
>somewhere.  Thought it was just basic Ohm's Law...
>
>In a word, help!
>
>Cheers
>
>M
>
>PS - FWIW, the meter is made by Ateliers Pekly of Paris.
>
>--
>Matthew Smith            | Business: http://www.kbc-dot-net.au
>IT Consultant            | PGP Key:  http://gpg.mss.cx
>Kadina, South Australia  | * Tivis Project * Community Connect *

WW2 vintage radars contained meters with KV scales (typically 0-20 KV) and 
1mA f.s. movements.  The series resistors were humongous, up to 3 feet 
long, and dissipated up to 20 W.  Lots of these were available as surplus 
but finding the resistors for them was difficult.  My guess is that you 
have one of these, and you will have to make up a resistor string to handle 
the voltage and wattage, and that the source you want to measure can handle 
the current drain (0.5 mA for your meter).  Oh, and put the meter on the 
low voltage (ground) end to avoid getting zapped.

Norm