Measurement with Voltage Divider (was: strange vttc)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Matthew Smith <matt-at-kbc-dot-net.au> 

Tesla list wrote:
>Original poster: "Randy & Lori" <rburney6-at-comcast-dot-net>
>Instead of a HV probe, couldn't one just wire several resistors in
>series, read the drop across one of them and just apply Ohm's/
>Kirchhoff's law? Maybe those 10M resistors used for draining the primary
>caps? Three resistors will give 1/3 of the applied voltage across any
>single resistor; 4 resistors-1/4.  If you think the 4000V label might be
>correct, go with 5 resistors, a 1000V meter could do that one.  I don't
>know why I haven't done this myself, I have been putting 120VAC into the
>HV windings and measuring the output of the low voltage side, and figure
>my ratios.  I'm not sure how accurate this is, but it seems to be pretty
>close.

Points to watch:
1) Make sure that the resistors you use can handle the voltage across them
2) Watch the power ratings (square of the voltage across a resistor divided 
by the resistor value should not exceed resistor power rating except for 
really short runs).
3) Measure across a resistor with a series potentiometer to allow for 
callibration.  You don't need to measure the resistance of ever single 
resistor - just the total of the ones which the meter is across and the 
total of all the others.  Tweak the pot until you've got a ratio which 
means you don't need to grab your calculator/slide rule/Palm Pilot every 
time you take a reading ;-)

I once acquired four high power/high resistance/high voltage resistors that 
looked like they'd be perfect for a divider to increase the range of a 4kV 
electrostatic meter.  Three of the four had sufficiently large differences 
in resistance that a fair bit of compensation would be required; the fourth 
was so far out as to be unbelievable.

Cheers

M

-- 
Matthew Smith
Kadina Business Consultancy
South Australia
http://www.kbc-dot-net.au