Author Topic: How do you measure high frequency high voltage?  (Read 1128 times)

Offline nsled

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How do you measure high frequency high voltage?
« on: June 24, 2023, 02:09:05 PM »
Let's say low current 20kv at 50kHz.

From what I'm reading you cannot use a high voltage probe like the Fluke 80k-40 since its only rated at 50-60hz. Is measuring high frequency just too complicated for a general purpose meter?

Offline MRMILSTAR

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Re: How do you measure high frequency high voltage?
« Reply #1 on: June 24, 2023, 05:04:18 PM »
I used a full wave bridge rectifier and a COTS HV DC meter to measure the 28 Khz output of a HV power supply. The rectifier uses fast HV diodes and a HV smoothing capacitor. The meter can read up to 40 KV DC. The DC measurement is the peak output of the AC waveform. If you need RMS AC simply multiply the DC value by 0.7071. I have attached a picture of my measurement apparatus.
« Last Edit: June 24, 2023, 09:55:46 PM by MRMILSTAR »
Steve White
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Offline nsled

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Re: How do you measure high frequency high voltage?
« Reply #2 on: June 24, 2023, 09:32:14 PM »
Thats neat.

Are they 2CL2FM or 2CL3FP diodes?

Offline MRMILSTAR

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Re: How do you measure high frequency high voltage?
« Reply #3 on: June 24, 2023, 09:49:09 PM »
2CL20KV. They have a 80 ns reverse recovery time and are rated for 20 KV.
« Last Edit: June 24, 2023, 09:53:58 PM by MRMILSTAR »
Steve White
Cedar Rapids, Iowa
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Offline nsled

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Re: How do you measure high frequency high voltage?
« Reply #4 on: June 24, 2023, 10:10:55 PM »
Thankyou, well I hope 2CL3FP will do I'd purchased before waiting for response. 30kV 100ma 100ns.

Offline MRMILSTAR

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Re: How do you measure high frequency high voltage?
« Reply #5 on: June 25, 2023, 05:23:57 AM »
Those should work great.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline TizianoBll

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Re: How do you measure high frequency high voltage?
« Reply #6 on: June 25, 2023, 06:08:44 PM »
The issue with the bridge rectifier (rather than a single diode) is that AC and DC rectified don't have the same common. This means that you must be careful with insulation (if the AC side is not well insulated from ground the ground of the HV probe could reach dangerous voltages).

Using a single diode wouldn't work at lower frequencies (60Hz) but at a few kHz's should be equally good, smaller and would be easier to manage insulation.

Offline MRMILSTAR

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Re: How do you measure high frequency high voltage?
« Reply #7 on: June 25, 2023, 09:44:03 PM »
Using only a single diode will only yield half-wave rectification. This will require a larger HV capacitor to get the same level of voltage ripple as the full-wave bridge rectifier, otherwise the measurement will not be as accurate. Your point is well taken however. I never touch any of the apparatus using the full-wave bridge rectifier while I am making the measurement. I only read the meter.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline davekni

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Re: How do you measure high frequency high voltage?
« Reply #8 on: June 25, 2023, 10:45:39 PM »
Quote
This will require a larger HV capacitor to get the same level of voltage ripple as the full-wave bridge rectifier, otherwise the measurement will not be as accurate.
At the initially listed 50kHz, and presuming 100M HV probe impedance, 20pF is sufficient for 1% ripple half-wave.  That is discounting effect of diode capacitance, which is probably a larger source of ripple than is meter probe impedance.

If a scope is available, another option for measuring AC is a capacitive divider.  1pF to 1nF makes a ~1000x divider.  For the small HV ~1pF capacitor, I've used just a section of HV-insulated wire taped or twisted to an adjacent section.  Such a divider does require calibration.  Measure a lower-voltage (perhaps 100V) signal of similar frequency, perhaps primary of transformer generating 20kV.  Measure with a known-good scope probe and again with home-made divider.
David Knierim

High Voltage Forum

Re: How do you measure high frequency high voltage?
« Reply #8 on: June 25, 2023, 10:45:39 PM »

 


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