Author Topic: High voltage, high impedance voltage measurement?  (Read 886 times)

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Offline uskiTopic starter

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High voltage, high impedance voltage measurement?
« on: September 04, 2024, 05:47:10 am »
Hi!

I am currently playing with geiger muller tubes. I would like to verify what is the voltage that i am powering them with.
They use a high voltage, high impedance power supply. None of my voltmeters have a sufficiently high input impedance and they dramatically affect the voltage / power supply circuitry.

Does anyone have a good idea as to a way to measure DC voltages in the range of 0-1000V (ideally 0-1500V) that does not cost me one kidney?

Thanks!
 

Offline Fungus

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Re: High voltage, high impedance voltage measurement?
« Reply #1 on: September 04, 2024, 06:26:23 am »
You could put a very high impedance resister divider across it (something like 1000:1 ratio) and put an op-amp voltage follower on the junction to buffer the voltage for taking measurements.

Cost: A couple of dollars plus a power supply.
 

Offline Hydron

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Re: High voltage, high impedance voltage measurement?
« Reply #2 on: September 04, 2024, 06:37:00 am »
I had an answer for you until I got to the bit about a kidney. I'd do it using my Keithley 237 SMU which does up to 1100V, but even such old units are not cheap used (this one used to be a 236 before I upgraded it, though while that means the purchase price was a lot less than most 237s the upgrade process isn't very cheap or easy either).
 

Offline tautech

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Offline Someone

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Re: High voltage, high impedance voltage measurement?
« Reply #4 on: September 04, 2024, 08:13:13 am »
Does anyone have a good idea as to a way to measure DC voltages in the range of 0-1000V (ideally 0-1500V) that does not cost me one kidney?
1% of kidney:
https://www.fluke.com/en-us/product/accessories/probes/fluke-80k-40
or try someone "cheaper"
https://www.bkprecision.com/products/component-testers/HV%2044A
https://www.bkprecision.com/products/accessories/210/PR%2028A
 

Offline guenthert

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Re: High voltage, high impedance voltage measurement?
« Reply #5 on: September 04, 2024, 08:16:54 am »
To measure very high voltages from a high impedance source directly one could use an electrostatic voltmeter.  Those might be available on the secondary market for less than a kidney (well, I didn't get anything other then eternal gratitude for my right one, so YMMV).

Another, more precise method, would be using the potentiometer method, i.e. adjust a high voltage, low(er) impedance supply to match (verified with a null detector) the unknown voltage, then the low impedance supply can be measured with a (not-so-high impedance) meter directly or via voltage divider (which would unduly load the high impedance source).  Handling high voltages from a low impedance source isn't without danger though.
« Last Edit: September 04, 2024, 08:19:42 am by guenthert »
 
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Offline Gyro

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Re: High voltage, high impedance voltage measurement?
« Reply #6 on: September 04, 2024, 08:33:24 am »
Yes, an electrostatic voltmeter is the ideal tool for the job. Ebay, patience, and a bit of luck may save the kidney. You need to be careful about the maximum voltage spec as the scales are very non-linear and you quickly loose resolution at the low end. I keep a 1kV and 10kV for such purposes, but 1.5kV and 3kV are common values too.

https://www.eevblog.com/forum/metrology/characterising-and-electrostatic-voltmeter/
Best Regards, Chris
 

Offline tggzzz

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Re: High voltage, high impedance voltage measurement?
« Reply #7 on: September 04, 2024, 08:42:45 am »
Does anyone have a good idea as to a way to measure DC voltages in the range of 0-1000V (ideally 0-1500V) that does not cost me one kidney?

My Fluke 893A goes to 1kV and, at balance, draws zero current. ISTR it cost me £30. If you can't get one of those ( :) ), either look at or make a null-voltmeter. All that needs is a 1kV PSU and a means of dividing that voltage down, e.g. a KVD.

Alternatively, make your own 1000:1 potential divider using a couple of known resistors, with the value chosen so that minimal current is drawn. Measure the 0-1V with any convenient meter. Be aware that resistors can have a value that varies with voltage.

Alternatively many manufacturers make a "high voltage probe" for use with a 10Mohm DVM.
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Offline Fungus

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Re: High voltage, high impedance voltage measurement?
« Reply #8 on: September 04, 2024, 08:51:39 am »
You could put a very high impedance resister divider across it (something like 1000:1 ratio) and put an op-amp voltage follower on the junction to buffer the voltage for taking measurements.

Cost: A couple of dollars plus a power supply.

Take a look at this, you could probably make something similar with the "resisters and op-amp" method. The 9V battery inside one is easily enough to power the op-amp: DT830 hack
 

Offline Gyro

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Re: High voltage, high impedance voltage measurement?
« Reply #9 on: September 04, 2024, 08:52:33 am »
The only other method that comes to mind is the potentiometric approach (as used for measuring standard cells without pulling any current). Build a small low current adjustable 1.5kV PSU, connect a sensitive current meter (DMM on volts range) between it and the GM supply, adjust for zero reading and then measure the PSU voltage.
Best Regards, Chris
 

Offline David Hess

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Re: High voltage, high impedance voltage measurement?
« Reply #10 on: September 04, 2024, 08:18:46 pm »
I am currently playing with geiger muller tubes. I would like to verify what is the voltage that i am powering them with.
They use a high voltage, high impedance power supply. None of my voltmeters have a sufficiently high input impedance and they dramatically affect the voltage / power supply circuitry.

Most DC voltmeters have a 10 megohm input resistance at high voltages, so I will assume that is the case here.

Quote
Does anyone have a good idea as to a way to measure DC voltages in the range of 0-1000V (ideally 0-1500V) that does not cost me one kidney?

What kind of loading is permissible?

For cheap, I would buy a pair of 5 gigohm resistors, place them in series for 10 gigohms, and put those in series with the 10 megohm input of my voltmeter making a 1000:1 divider and presenting a 10 gigohm input resistance.  Now the 2 volt range on my multimeter measures to 2 kilovolts with 0.1 volt resolution.

I would use two 5 gigohm resistors in series simply because a similar 10 gigohm resistor is much more expensive.

A high voltage high impedance buffer, like that used in an electrometer to make high voltage measurements at high impedance, would be a fun project but requires a high voltage power supply which is a project all by itself.

« Last Edit: September 04, 2024, 08:23:42 pm by David Hess »
 
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