Re: High voltage probe, odd NST measurements

["Tesla list" <tesla@xxxxxxxxxx>]

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

Hi Ed,

OK I'm beginning to see your concern, I went back to your original 
post and interpreted the stray capacitance of 0.26pf as being the 
stray per resister meaning that with 10 resisters in series the 
effective capacitance would be .026pf.  I also calculated the 
correctly compensated probe (with a 10pf load across the 1Meg 
resister) and found that the stray per 1Gohm resister needs to be 
0.01pf.  So if your .26pf stray is for each of the 10 1G resisters, 
then the probe would be overcompensated by a factor of 26.  Resisters 
for proper compensation are of the HV type that are tall and skinny 
that also reduces the parisitic capacitance between the resister 
ends.   With the stray capacitance included, there are two 
dividers:  a resistive and a capacitive divider.  The resulting 
voltage (the affect from both dividers) requires superposition to 
calculate.  Even with 0.26pf of stray per resister, the resisters 
dominate impedance wise by a factor of 10 (0.5% error).  I did a 
spice with 0.026 pf across the 10Gohm aggregate and at 60 Hz, the 
effect was negligible.  At higher frequencies, the capacitance 
becomes dominate and the error goes up.

If the 0.26pf is the aggregate stray of 10 resisters, then (as you 
say) the error would be 41%

I dont know what the real stray capacitance of resisters are, but if 
the resisters used have stray that is too high, one should be able to 
minimize the effect by increasing the load capacitance to bring the 
capacitance divider ratio back to 10000:1

Gerry R
> Original poster: Ed Phillips <evp@xxxxxxxxxxx>
>
> "Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
> > Hi Ed,
> >
> > Yes, over compensation will cause high readings.  One might calculate
> > how much capacitance it takes across each resister to properly
> > compensate.  I'm guessing this is an order of magnitude larger than
> > the strays and may be hard to accidently do.
> >
> > Gerry R."
>
>         If I remember the original design correctly it uses a 10G ohm (10^10
> ohm) series multiplier.  At 60 Hz an 0.265 pF capacitor would have that
> reactance and cause ~ 41%  high error.  That's not much capacitance at
> all!
>
> Ed