For my High voltage experiments, knowing the exact potential difference of a given supply value would be extremely useful. So I decided to build a high voltage probe, using a resistive divider. After doing some calculations, I decided to go for a 1:1000 divider ratio, for the many advantages including ease of reading the measured values (10kV -> 10V).

The High voltage goes as Vin, and the measurement is done at Vout.
I needed a 1GOhm resistor, so I purchased 5x 200MO Vishay Dale High voltage resistors from Ebay. Connecting them in series I get the desired value:

To insulate this resistor, I put it in a PVC pipe under oil:

If R1 is 1GOhm, and R2 is 1MOhm, the voltage drop across R2 , Vout will be aprox. 1/1000 * Vin . Instead of R2 I use a multimeter with 1MOhm impedance (However, to protect the multimeter, additional components are required). The results:

The left picture shows a good result: for 30.3V in, the multimeter measures 32mV, very close to the 1/1000 ratio (actually the ratio is 1/1001).

However, trying to use this probe with my 50KV supply, gives very poor results. The multimeter jumps between values, the biggest shown is 12 (12KV). Probably because of the high frequency currents.

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