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High Voltage Test Equipment
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From: Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent: Thursday, March 05, 1998 10:15 AM
To: Tesla List
Subject: Re: High Voltage Test Equipment
>
>
> ----------
> From: Homer Lea [SMTP:HomerLea-at-aol-dot-com]
> Sent: Tuesday, March 03, 1998 7:41 PM
> To: tesla-at-pupman-dot-com
> Subject: Re: High Voltage Test Equipment
>
> In a message dated 98-03-03 04:33:40 EST, you write:
>
> >
> How high? I just ordered 200x10meg resistors to make a voltage divider. I
> figure to put them in a tube in oil. They should be good for 100 kv
> (500volts/resistor) to give 50 ua at 100 kv and use a 50 ua meter to
indicate
> the voltage. That's only wasting 5 watts of power. If I out a full wave
bridge
> in series I am hoping to measure ac too.
>
> jim heagy
Generally a good strategy, however, watch out for the so-called Voltage
Coefficient of Resistance. This is a particular problem with high value
resistors at high voltages. There is also a significant parasitic
capacitance problem with long resistor strings, which various people have
come up with ways of compensating, minimizing or otherwise dealing with.
So, you are back to the calibration problem. How do you calibrate a 100 kV
divider without a precision 100 kV source (or a reference resistor). Of
course, if you are lucky enough already have a good HV divider to calibrate
against.... Or, if 10% accuracy is good enough...
As to the physical arrangement of your 200 resistors, I have seen a number
of schemes where they are essentially wound in a long helix (sometimes in a
bifilar counter-wound configuration to minimize L). It provides even E
field distribution along the string, particularly if some metal discs are
spaced along the helix (central axis of disk parallel to helix long axis),
although that aggravates the parasitic C problem.
If you assume 1.5 inches per resistor (including leads, solder joint and
resistor body), your 200 resistors will be 300 inches long. Coil it up so
that you have 20 turns of 10 resistors each (15 inches circumference, 5
inches ID (approx)) Space the turns an inch or so apart (5kV/turn voltage
difference) for an overall unit some 6" in diameter and 20-24" long.