High voltage, high impedance voltage measurement? - Page 1
Forum section: electronics / testgear

author: uski
author_rank: Frequent Contributor
timestamp: September 04, 2024, 05:47:10 am
content: 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!

author: Fungus
author_rank: Super Contributor
timestamp: September 04, 2024, 06:26:23 am
content: 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.

author: Hydron
author_rank: Super Contributor
timestamp: September 04, 2024, 06:37:00 am
content: 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).

author: tautech
author_rank: Super Contributor
timestamp: September 04, 2024, 06:37:44 am
content: https://www.aliexpress.com/w/wholesale-1000x-voltage-probe.html?spm=a2g0o.productlist.search.0

author: Someone
author_rank: Super Contributor
timestamp: September 04, 2024, 08:13:13 am
content: Quote from: uski on September 04, 2024, 05:47:10 am1% of kidney:https://www.fluke.com/en-us/product/accessories/probes/fluke-80k-40or try someone "cheaper"https://www.bkprecision.com/products/component-testers/HV%2044Ahttps://www.bkprecision.com/products/accessories/210/PR%2028A

author: guenthert
author_rank: Frequent Contributor
timestamp: September 04, 2024, 08:16:54 am
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2024, 08:33:24 am
content: 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/

author: tggzzz
author_rank: Super Contributor
timestamp: September 04, 2024, 08:42:45 am
content: Quote from: uski on September 04, 2024, 05:47:10 amMy 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.

author: Fungus
author_rank: Super Contributor
timestamp: September 04, 2024, 08:51:39 am
content: Quote from: Fungus on September 04, 2024, 06:26:23 amTake 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

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2024, 08:52:33 am
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2024, 08:18:46 pm
content: Quote from: uski on September 04, 2024, 05:47:10 amMost DC voltmeters have a 10 megohm input resistance at high voltages, so I will assume that is the case here.QuoteWhat kind of loading is permissible?For cheap, I would buy a pair of5 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.