Re: KV meter

["Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

The diode string will have a drop of only a few tens of volts at the most.

What's the power dissipation of the resistor? 250 Meg at 100 uA is 1E-4^2 *
250E6 or 2.5 Watts... That might be enough to get the resistor hot enough
to throw off the value (although not as much as you report).

Recall that the series impedance on a NST is pretty high. If that was a 9
kV/30 mA unit, the series impedance is 300K which is less than 1% of the
series resistance on your meter, so it shouldn't have a huge effect, but
it's something to be aware of.

And, of course, you're only getting 4.5 kV relative to ground, so you'd
expect around 9 uA

You've also got a half wave rectifier there, so the average value that you
are measuring is going to be 50% of the real value (half the time, there
isn't any voltage on the meter).... 4.5 uA expected.  Also, there is the
RMS (what the rating is) to average or peak voltage (what the meter reads).
 If you have a small cap across the meter, it will read peak, if not, it
sort of depends on the mechanical time constant of the meter.

Finally, most meters are rated in terms of accuracy of the full scale
reading.. 5% of FS is typical, so, 5 uA in the case of your 100 uA meter. 
This is why you want to make your measurements in the top third of the
scale.

Off hand, you're measuring about what you'd expect....

I'd try calibrating against something else, like the 110V wall current,
using another meter that has calibration you trust.
  

----------
> From: Tesla list <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: KV meter
> Date: Sunday, October 08, 2000 10:55 AM
> 
> 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----- -
> 
> 
> 
> 
>