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 23:10:46 -0600
Approved: tesla-at-stic-dot-net

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From:  Thomas McGahee [SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent:  Thursday, March 05, 1998 7:32 PM
To:  Tesla List
Subject:  Re: High Voltage Test Equipment

Homer,
You can't just throw a full wave bridge in series with the resistor
string to convert the AC to DC. The best way to get the AC converted to
DC is to have a "two" resistor divider (where "one" of the resistors
is actually the 200x10meg resistor string, and the other, bottom resistor
is such that the ratio is, say, 9,999 to 1. That will give you 10 volts
out for 100KV in. The bottom resistor should consist of both a fixed and
a variable resistor in series, each about 3/4 the size of the calculated
resistance needed for the bottom resistance. Adjust for maximum
accuracy.

Now take the 10 volt signal at the bottom and buffer it using a high
impedance operational amplifier. You want a unit that draws minimal
current and has a high bandwidth. Connect the op amp as a buffer, also
known as a voltage follower. Use a conventional digital meter to
display the result. Use the AC scale to read AC. 

This approach is far superior to the one you originally proposed, and is
accurate enough for most work. If extreme accuracy is needed, then there 
are some additional things that would need to be done to tweak the
circuit.

Note that at very low input levels the accuracy will be less than that
at the higher levels, assuming the unit is calibrated at or near
100KV. If you calibrate at say 10KV, then that will be the most accurate
section. It is useful to produce a calibration chart if you desire
greater accuracy.

Oil immerse the main HV resistor string to reduce errors due to corona.
The lower "resistor" can be OUTSIDE this oil case.

Hope this helps.
Fr. Tom McGahee
 
> > 
> > ----------
> > 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
>