Creating high voltage components is challenging because of the parasitic capacitances to the surroundings. The electrical field at 10KV requires careful isolation of any conducting surface of about 1”; furthermore, any sharp niches can trigger an arc between the component and the surrounding lower potential items.
I needed to measure high voltages around 30KV; so I undertook the task to create a “reasonably accurate” voltage divider. The goal was to divide the voltage by a factor of 1000 and still present an input impedance of less than 1M to the lower part of the divider.
As it turns out, the accuracy of divider does not depend only on the tolerance of the individual resistors – the parasitic capacitances to ground play a much more important role than the resistors themselves, as shown in the figure on the right. One way to mask-out the parasitic capacitors is to add much larger capacitors linearly with the resistor.
I used 100 pieces of 1206 chip resistors of 10M each. As these resistors are stringed together, a total impedance of 1,000 MOhm or 1 TOhm is possible. But how to you solder 100 chip resistors when each one of them is about a 0.6mm in width? The answer: line them up using a razor blade – use scotch tape to hold them temporarily until a tiny drop of solder flows between them! Stringing 15 of them at a time was not that bad – it took about 2min per segment. Then, after enough segments were made, I used 1/4” of 30AWG wire to solder the segments together – this way the overall string can maintain its flexibility for thermal expansion and mechanical stress. I was about to solder capacitors in parallel with the resistors, but at the end I thought that he capacitance of the soldered sides will be just enough to reduce the parasitic effect.
After all resistors were soldered together, I used an one foot long glass tube of 1/8” of diameter to place them. To ensure that the resistors live in a non-humid environment, I used the heating gun to overhead the glass tube – while hot, I dipped the two ends in silicon glue which was sucked in as the inside air was cooling down…
I used a HV generator of 5kV which could create positive or negative DC voltages – amazing enough the accuracy of the divider was excellent: when I applied 3,735 volts, the meter was reading 3.72 volts, i.e. an error or 0.4%. This was hard to believe since, all components: the DVM, generator, and the divider are not more precise than 1%.
Finally, I used a HV generator, called “the ionizer” to test the voltage rating – I would only go to 30kV; so far, so good.