High Voltage Test Equipment

To: "'Tesla List'" <tesla-at-pupman-dot-com>
Subject: High Voltage Test Equipment
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 6 Mar 1998 22:57:59 -0600
Approved: tesla-at-stic-dot-net

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From:  Mike Harrison [SMTP:wwl-at-netcomuk.co.uk]
Sent:  Friday, March 06, 1998 7:58 AM
To:  Tesla List
Subject:  Re: High Voltage Test Equipment

On Thu, 5 Mar 1998 19:11:52 -0600, you wrote:

>
>----------
>From:  Homer Lea [SMTP:HomerLea-at-aol-dot-com]
>Sent:  Thursday, March 05, 1998 2:05 AM
>To:  tesla-at-pupman-dot-com
>Subject:  Re: High Voltage Test Equipment
>
>In a message dated 98-03-05 00:46:30 EST, you write:
>
>> From:  terryf-at-verinet-dot-com [SMTP:terryf-at-verinet-dot-com]
>>  Sent:  Wednesday, March 04, 1998 5:22 PM
>>  To:  Tesla List
>>  Subject:  Re: High Voltage Test Equipment
>>  
>>  Jim,
>>          Having just gone through the HV voltage divider design zoo with my
>>  equipment, I have one caution.  At 60Hz the long string of resistors will
>>  work fine.  However, if you wan't to measure voltage waveforms at say
>200KHz
>>  the frequency response will be bad.  I am now persuing vacuum molded epoxy
>>  encapsulation to get small size, reasonable power dissipation, and high
>>  frequency response.  Your oil bath should do the same just a little more
>>  "fluid" :-)  Do try to make it very compact.
>>  
>>          Terry
>>  
>  Terry:
>I just sent off new specs:
>-------------
>post script: I ended up getting 400 15meg resistors. They are small (1/8)
>watt. My plan is to string them in series, put them in a Tygon tube and fill
>with oil. I will coil up the tube so it isn't 20 feet long. I assume the
>inductive reactance will be zilch compared to the 6 billion ohms of resistors
>The resistors will be in series with a  50 or 100 ua meter connected through a
>full wave rectifier bridge. Does anyone know how much voltage I can put across
>each resistor?? I am hoping for at least 500 v each and dreaming of 1000 v.
Metal-Glaze resistors have a much higher voltage rating than
conventional ones - e.g. Philips VR25 series 0.25W are rated 1150VAC
or 1600VDC, VR37 0.5W are 2500/3500V.  
Small carbon film R's tend to be rated for around 200V, and at high
resistances you may have problems running them over their voltage
rating. 
If you're measuring AC, you also need to consider stray capcitance -
if your divider is 6Gohm, you don't need a lot of C at 50Hz to be a
significant proportion of this. At 50Hz 1pF has a reactance of about
3Gohm. 
Capacitance of a bridge rec will almost certainly be a problem - I'd
use a half-wave rec, but even then, reverse capacitance could result
in very poor accuracy.   

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