Re: KV meter

["Ed Phillips" <evp-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Tesla list wrote:
> 
> Original poster: "David Euans" <davidwe-at-telocity-dot-com>
> 
> I am trying to build a 50 KV high voltage meter for directly measuring
> the voltage from my various high voltage transformers.  I have tried
> using a 100 microamp full scale DC meter with one 250 (32 KV) megohm
> resistor on each leg and an 80 KV diode string (four 20 KV units)
> directly off of one side of a 9 KV neon sign transformer in series with
> one of the 250 megohm resistors, but I get faulty readings each time.
> Using Ohm's law, 500 megohms should yield 18 microamps at 9 KV, but I am
> only getting about 5 microamps of output.  I have tried several
> different diodes, but get similar results regardless.  The NST is
> working properly.  Am I missing something here?   The diodes seem to be
> dropping too much voltage.  If anyone knows how to properly set up a
> voltage divider circuit for measuring AC with a DC meter, I would love
> to hear it.  To illustrate, my circuit is as follows:
> 
> bushing x-----80kv diode-----250 megohm resistor----- +
> 
> 9
> KV
> 100 microamp meter
> NST
> (50 KV full scale)
> 
> bushing x-----------------------250 megohm resistor----- -

	I'm sure you will get a much better answer, but there are a couple of
things wrong.  First of all, you are running a half wave rectifier
circuit which will give you lower output, and your circuit requires that
the diode have a high inverse voltage rating, which is an unnecessary
expense.  Second, you should really have a capacitor across the meter.

	If I were doing it I'd use a voltage divider followed by a full-wave
rectifier with the meter connected directly to it.  Depending on the
power rating of available HV resistors you have, you could use a less
sensitive meter.

Ed