Re: High voltage probe, odd NST measurements

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

CORRECTED PROCEDURE stated below ( correction in CAPS)

Gerry R.

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Hi Ed,

Another solution that I forgot to mention is to do a 1000:1 divider.  For a
10pf load the aggregate stray can now be .01pf and the stray per resister
(assuming 10 resisters) can be 0.1pf

Also, the number of resisters used can be increased to get the total stray
down as well (with a corresponding reduction in individual resistance or the
lower leg resistance..

I dont think you need a 100KV square wave to calibrate,  all you need to do
is find the capacitance per resister and calculate what is needed.  This
doesn't need to be perfect as 60 Hz measurements are less susceptable than
say 10KHz ones.  You could also use a lower voltage squarewave to determine
what this capacitance is, using one of the resisters in the upper leg and
only the measuring device in the lower leg.


Example:  Use ONLY a scope probe that is10Mohm and 10 pf as the lower leg of
the divider.  Put one of the 1Gohm resisters in the upper leg.  You now have
a 100:1 divider and can use a 10V squarewave and measure a 100mv signal to
the scope. This obviously will be undercompensated.  So start adding 1G
resisters in parallel (KEEP INTERCONNECT AS SHORT AS POSSIBLE) with the
original.

As the number of resisters is increased, the stray capacitance of the upper
leg will increase and the divider ratio will decrease (larger signal to the
scope).  ONCE THE NUMBER OF UPPER LEG RESISTERS THAT GIVES GOOD COMPENSATION
IS FOUND, SIMPLY DIVIDE THIS NUMBER INTO {10pf/FINAL_DIVIDER_RATIO} and this
will be the capacitance per resister.  Now you can design the real divider
using the appropriate number of resisters FOR A 10pf LOAD and CHOSEN DIVIDER
RATIO and adjusting the lower leg resister that is in parallel with the
meter/scope appriopriately to get the divider ratio needed.  FINALLY, MAKE
SURE THAT THE VOLTAGE and POWER PER RESISTER IS OK.