Measuring high voltages

Things at the limits.

Re: Measuring high voltages

Postby johnf » Sat Jan 14, 2012 2:00 pm

Chris
your dmm's are going black because of the electric field, LCD is an electric field device ie it is the volts /meter that is doing it.
Its because of this our instruments (we build)at work use LED displays
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Re: Measuring high voltages

Postby Doug Coulter » Sun Jan 15, 2012 10:08 am

Yup, John's got this one. LEDs are also easier to read in the dark. You're asking for trouble floating a DMM at half the supply voltage too - then it's hot to everything. Ground the meter minus input, put the divider near the HV and only run the low volt lead back to the meter (preferably in coax with a grounded shield) - keeping the meter fairly far away (a few feet), and your troubles are over. Measure current in the ground lead! Then you can do things like touch the meter to change the range.

I've had issues with moving coil meters in this use too - same issue - the field. If they are sensitive, particularly, because the twitchy ones use such a weak return spring. They'll follow your fingertip across the window and do all sorts of interesting but not useful things.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Measuring high voltages

Postby chrismb » Sun Jan 15, 2012 11:09 am

Doug Coulter wrote:Measure current in the ground lead!


(As mentioned somewhere, I think!)... I'm working on a fully DC isolated supply. Whatever ground current there is is just leakage. I can fix either end (or in the middle, or the meter itself) to ground, but that doesn't quite test the thing I've made. If there is an exclusive leakage path through the meter, then the meter is at ground, but in any case it is not completing a circuit. It is not 'at a potential' in the usual sense, excepting for the inevitable leakage routes that cause charging up of the terminals.

Incidentally, as a side issue, I think this module of mine is a very simple demonstration that charge-carriers are negative - if I just run the thing, nothing attached to the terminals, then over time it is the negative terminal that builds up a charge with respect to ground, whilst the positive stays floating near ground.

Anyhow - thanks - yes, I now see that it was 'just' a big fat electric field disrupting all those teeny liquid crystals. Just moving the meter away brought [some] immediate relief to the poor thing.

Yes, I think the benefit of an illuminated LED is, itself, an advantage! But I'm going to build a moving-coil HV meter in any case. I'm losing faith in electronics near this stuff at +50kV!!
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Re: Measuring high voltages

Postby Doug Coulter » Sun Jan 15, 2012 1:42 pm

Yes, me too. We've had perhaps 5 LCD's and and 2-3 LED panel meters fail here due to simple static electricity buildup in the back, where that tiny microwatt chip lives. The worst was in the pre proto version of the standard counter where we had the HV supply (850v in that case) inside the same box with the LCD. Just dust flying around in there seemed to be enough to xfer enough charge to the chip to fry it in either case.
CMOS chip-on-board...not so robust.

I'm not so sure you proved anything at all about charge carriers there - if negatives are it - then the negative one should have been pulled to ground by emitting those charge carriers...and in air, it's really no contest - both are there aplenty, from ions of both sexes as well as the electrons, in varying amounts. Remember all that crap about "good" negative ion generators to reduce the bulk of "bad" positive ions on dust in the air?
I'd assume that scam went worldwide.
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Re: Measuring high voltages

Postby chrismb » Sun Jan 15, 2012 4:36 pm

Doug Coulter wrote:I'm not so sure you proved anything at all about charge carriers there - if negatives are it - then the negative one should have been pulled to ground by emitting those charge carriers...and in air, it's really no contest - both are there aplenty


Well, I figure this is quite different when there is no circuit to ground. All the module does, effectively, is shift charge from one terminal to the other. It can't pull charge from anywhere else, unless it is fed, or loses, charge. So it'll start by simply pushing charge from one terminal to the other. Leakage currents will, clearly, play a part, but not until one side of the module is at a high potential wrt ground. I figure this will mean the -ve terminal will more likely end up with a pile of electrons with 'nowhere to go' because they are 'real charge', whereas at the +ve there are no 'real charges', there is simply an absence of electrons.

You might be right - the logic may not be as simple as I am making out. But in any case, it is not a symmetric process - and so it has proved to be!

I should say - just to keep this on the thread subject line!! - it is impossible to measure as 'a voltage' (except and exclusively between those terminals) because neither terminal has any circuit to ground. If you bring an earthed link from ground up to the +ve terminal, then there is a very teeny discharge, if at all. Whereas if you bring the ground link up to the negative [first] you get a goodly spark as the -ve output stage jumps up to earth. After that, the -ve terminal is at earth potential, of course, and if you then take the earth line off the -ve and on to the [now high] +ve any time in the next few minutes you'll then get another spark as the output jumps back down again! So if you were to ever try to measure the voltage wrt ground on the terminals, you'll get an initial current flow, then it'll just read '0'.
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