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Ripple voltage on high voltage DC power supply

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Advanced Member level 5
Jun 13, 2021
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We have a HV power supply for new innovative microwave oven prototype. We wish to measure ripple voltage on it…it is 30kV DC. We believe to measure ripple voltage we need to use a stack of lower voltage capacitors in series. We then look at the voltage of the lowest one. So ..we don’t want to have balancing resistors across all the caps, as this will give an RC time constant, and will filter out the ripple voltage….so how do we do this? We are expecting the ripple frequency to be around 25kHz, as this is the switching frequency of the HV SMPS.

Hi there,

That might work....

What's wrong with standard resistor divider with "high-enough" resistance so it does not load the source?

Indirect current measurement with current probe / hall probe and oscilloscope and resistor load? There are current probes available for very low current levels...

I am no expert... rather just thinking out loud...

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Thanks, would you agree, measuring the vout ripple would not in fact be possible with the series capacitor stack?...since without bleeder resistors in pllel with each cap, then at least one of the caps will blow up.....and with the pllel resistors, their will be a filtering effect, which will filter out what is being measured.
Would you agree with the above?

The series chain of resistors does seem like the cheapest way of doing it?

simple transformer coupling with blocking caps will do this with ease ....
Thanks, im trying to conjure up the small detail of this method?

Agree about necessity of parallel resistors, but no problem with suitable resistor values. You can either make a perfect compensated voltage divider that has constant divider ratio for all frequencies, or use high sufficient resistor values if you want to measure AC only.
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There's also the option to use a simple capacitive voltage divider with two capacitors, one being 30 kV rated.
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You can resitor devider to reduce voltage before measure, but safety for measurement tool is very importance. We can't lost osciloscope for this.
Because of high voltage, so resistor need serial to reduce voltage drop on each. Resitor value also too high (xx Mega ohm), will using probe it has 10M (not fix) ohm 6pF (not fix) impedance, this can act as low pass filter, and the wave form read on Osc is not real. -> need a pre-amplifier to recover signal and buffer for Osc.
You can take alook in LISN use to measure EMC.
Or use can use wireless module with ADC > 5 x 25kHz (100khz) sample rate, using pre-amp filter only measure ac-voltage, then log data to RAM, then send BLE, Wifi to host. Isolation space up to few meters. -> Don't need worry about leakage current.
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