[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: High voltage probe, odd NST measurements

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

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 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 is obviously undercompensated. So start adding 1G resisters in parallel 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 gives good compensation, simply divide this number into 10pf and this will be the capacitance per resister. Now you can design the real divider using the appropriate number of resisters and adjusting the lower leg resister that is in parallel with the meter/scope appriopriately to get the divider ratio needed.

Gerry R.


Original poster: Ed Phillips <evp@xxxxxxxxxxx>

"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"

As I said before, all you really need is a 100 kV square wave generator
to calibrate with.  Certainly adjusting the shunt capacitance would work
fine if you had a means of knowing when you had the correct value.

Ed