Electronics > Beginners

KV/µA source, how do you measure the output?

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Leuven:
I have a 100KV source at 100µA, and trying to measure the output with a multimeter.

I cobbled together a voltage divider, but it reads about 10 times less than what it is supposed to. Can I use an op amp as a buffer?

Ian.M:
No. You can't do it that way.   Either the OPAMP input will rail or it will die as there's nothing to stop the current to the +in through the 100Meg resistor taking the input past its Vcc rail except whatever internal protection network it has.  You *NEED* a lower arm in the divider.   Also a 100Meg resistor with 100KV across it would pass 1mA, which is ten times what your EHT source is rated to output.  You need a minimum of a 1G upper resistor in the divider, which would then draw 100uA, and if you are interested in the EHT voltage when the source is lightly loaded a 10G resistor would be preferable.   Doing that cheaply with reasonable accuracy is going to be challenging.

ArthurDent:
I don't know how accurate you have to measure the voltage but D.C. voltages of that level can be measured by an electrostatic voltmeter and there are videos showing how to make a DIY unit. Calibrating it could be tricky but it is a way to measure high voltage with basically no current being drawn.

https://en.wikipedia.org/wiki/Electrostatic_voltmeter

Another way would be to look up the distance a spark will jump in dry air and do the calculations on how large a gap your supply voltage will jump. Also check articles on Van De Graaff generators to see how they do it.

Leuven:
Thanks Ian, so there's so shortcut to it. I figured the impedance is not high enough, but I clearly don't understand how op amps work. I will add more resistors. Accuracy of the measurement is not crucial, I just need to confirm the KV output is within some 5% of what it displays.

Leuven:

--- Quote from: ArthurDent on June 29, 2019, 03:10:24 am ---I don't know how accurate you have to measure the voltage but D.C. voltages of that level can be measured by an electrostatic voltmeter and there are videos showing how to make a DIY unit. Calibrating it could be tricky but it is a way to measure high voltage with basically no current being drawn.

https://en.wikipedia.org/wiki/Electrostatic_voltmeter

Another way would be to look up the distance a spark will jump in dry air and do the calculations on how large a gap your supply voltage will jump. Also check articles on Van De Graaff generators to see how they do it.

--- End quote ---

Interesting ideas, thanks. I reckon it's more involved compared to a voltage divider. It'd be worth doing as an experiment.

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