Re: Measuring HV capacitor voltage (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 19 Jul 2007 14:33:28 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Measuring HV capacitor voltage (fwd)

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Thu, 19 Jul 2007 15:15:51 +0000
> From: David Rieben <drieben@xxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Cc: drieben@xxxxxxxxxx
> Subject: Measuring HV capacitor voltage
> 
> Hi all,
> 
> I'm having a little issue with measuring up to 10 kV across a capacitor
> on a standard panel mount, analog 0-15 VDC voltmeter from Radio 
> Shack. Obviously, I was trying to make it read 1 volt for each measured
> 1000 volts. I tried using a voltage divider of 10 seriesed 10 meg resistors 
> and a 100 K pot but found that the 100 megs was just too much resist-
> ance to allow for sufficient current for reliable movement of the volt meter. 
> I then tried a series-parallel arrangment of 10x10 of the 10 meg resistors 
> for a total of 10 meg and then still used the 100 K pot and varied it to try 
> to bring in the proper calibration. I was checking the voltage measurement 
> with my HV probe and DVM. The resistors didn't like being bunched up 
> 10 at a time and started flashing over externally. Also, I was having trouble 
> getting the analog meter readings to be accurate throughout the 0 - 15 volt 
> range according to the measurement of the HV probe. I think the meter is
> designed for 10 meg impedance and it also comes with a 15 K resistor that
> is supposed to be in series with the voltage source. Anyone have any sug-
> gestions for reliable and reasonably accurate measurement of high voltage
> through a low voltage analog voltmeter via a voltage divider network? 
> 
> Thanks
> David Rieben
> 
> 
> 

Hi Dave,

You apparently have a 1 mA Full Scale (1 mA FS) meter movement. To 
convert it to read 0-15 kV, you need to connect a 15 megohm HV resistor 
in series with the meter. You don't need to use any divider circuit. The 
HV resistor(s) must be rated for a total of 15 watts, and MUST be 
capable of withstanding 20 kV (use at least 2X the max anticipated 
voltage for safety margin, ESPECIALLY if you are measuring across an 
energy discharge cap). Use "tiger stripe" type HV resistors that are 
made to accurately handle HV with minimal resistance change. You can 
remove the internal 15k resistor in the meter if you wish, but it's 
really not necessary since its presence doesn't affect the reading by 
more than ~0.1%.

Bert
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