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Old 22nd Nov 2019, 9:20 am   #1
Martin G7MRV
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Default Measuring a High Voltage, very low current supply?

I have need to measure the voltage produced by a simple, flyback and diode multiplier type high voltage supply, up to about 500V. This uses a 9v battery and a 555 timer plus inductor to drive the multiplier, and is is very low current, perhaps just a few tens of uA at the HT terminal (its to drive a G-M tube),

I dont have any dedicated high voltage test gear, only normal modern multimeters.

Can anyone advise how best to set up a suitable resistor divider to accomplish this measurement without loading the circuit? The only suggestion ive seen so far says a 1Gohm resistor!?

I appreciate that I should be able to work this out from first principles, but would rather defer to experience!

Cheers
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Old 22nd Nov 2019, 9:41 am   #2
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Default Re: Measuring a High Voltage, very low current supply?

The classic method is to have a second, variable, supply and use a centre-zero voltmeter to bridge the two supplies. This allows you to adjust the variable supply to match the one under test, taking zero current from the supply under test. The variable supply can be loaded with a normal voltmeter, the loading not affecting your accuracy.

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Old 22nd Nov 2019, 9:50 am   #3
Martin G7MRV
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Default Re: Measuring a High Voltage, very low current supply?

Thats not a luxury I have im afraid, although once I have one working HT supply circuit, I suppose I could use that method then to set up subsequent ones,

For now I need to be able to measure it on a DMM.
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Old 22nd Nov 2019, 10:02 am   #4
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Default Re: Measuring a High Voltage, very low current supply?

Hi Martin, you could try an electrostatic voltmeter, but their accuracy is not the greatest and they are normally for higher voltages but are available at this level.

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Old 22nd Nov 2019, 10:05 am   #5
Martin G7MRV
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Default Re: Measuring a High Voltage, very low current supply?

Working the numbers, if my meter is 1M input (im not sure, so will need to check the spec), then I need a 100:1 ratio to give a 1kV range (10V on meter), which is enough for this job, and of course is just a 100M series resistor (or chain of lower values up to that),

The current then would be similarly divided?

Im sure though there is something im missing! Now I 'think' that its the last stage of the divider! - if ive got this right, R2 of my divider (center to ground) is not the meters internal impedance, but a lower value resistor to measure the voltage drop over? So i'd want R2 to be 100:1 lower than R1, and also lower, say 10:1 than the meter impedance? So, 100M R1 and 1M R2, if my meter is 10M?

I know I should know this, but since qualifying about 20years ago, ive rarely had to go over 48V!
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Old 22nd Nov 2019, 10:27 am   #6
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Default Re: Measuring a High Voltage, very low current supply?

Since you only want to measure DC the job is not too difficult. As you say, you need a resistive divider.

The easiest approach is to assume that the digital multimeter you have really does have the input resistance (often 10Mohm) that its manufacturer claims. If you also assume that the high-value resistor you have bought has the resistance that it claims then you can put the resistor in series with the meter and there's your divider.

As an example here is a 100Mohm 1% resistor https://cpc.farnell.com/cgs-te-conne...-8w/dp/RE06352. Three of these in series plus the meter resistance would give you 310Mohm, which would draw just 1.6uA from your supply. 500V at the supply terminals would give 16V at the meter.

I've chosen the division ratio (31:1) to give me a meter voltage quite close to the range maximum. Many modern meters are '3.5 digit' so the 20V range will measure up to 19.99V with a precision of 0.01V. By making the measured voltage as large as possible I can make full use of this precision. It's one part in 1600 for a 16V measurement whereas it would only be one part in 500 (say) if I was measuring just 5V.

Dividers work on the principle that all of the elements (individual resistances) in the divider have the same properties. So if they happen to be a bit nonlinear (the resistance changes with voltage) then as long as all the resistors change by the same amount that won't matter - the division ratio will still be the same. Using a meter in series with commercial fixed resistors obviously breaks this rule. We don't know whether the meter and/or the resistors are nonlinear and we certainly don't know whether they have the same nonlinearity.

If we are worried about nonlinearity then the best approach is to make a chain of identical resistors and to ensure that the meter, usually placed across the bottom resistor, draws a very small current relative to what's flowing down the chain. That way the different nonlinearities of the meter and the resistors won't matter (much). The problem then is that the individual resistance element needs to be low-value compared with the meter resistance - say a 1Mohm resistor with a 10Mohm meter across it. But if your resistor element is only 1Mohm then you need 300 of them to build a high enough resistance not to load your supply ! When I used to do this for a living I did, on occasion, build dividers with 50-100 identical resistors, but 300 would have been too many even for me. You can economise on numbers by combining two dividers - say with 30 10Mohm resistors, the lowest of which then has 40 1.5Mohm resistors across it, with the meter put across the lowest of the 1.5Mohms. Now the resistor count is down to 'just' 70 (still quite an enterprise though).

Personally in your case I don't think I'd go to the trouble of long resistor chains. I'd stick with my first design (3 x 100Mohm in series with the meter). Then I'd check its calibration using another power supply which could deliver enough current not to be loaded down significantly if connected to the meter directly. The procedure would be to check the supply voltage directly with the meter then check it again using the divider and confirm that the two readings were compatible. When you know you can trust the divider you can then use it on your low-current supply.

Cheers,

GJ
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Old 22nd Nov 2019, 10:32 am   #7
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Default Re: Measuring a High Voltage, very low current supply?

Be careful measuring high voltages generated from a switching, or oscillating, power supply with a DVM. I blew up my first good DVM measuring the cathode voltage on a CRT in a scope.

I think the problem comes with the high frequency, any capacitance within the DVM is much lower impedance than the resistive dividers, hence the applied DC voltage ends up right on the pins of the chip. Fizzle pop!
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Old 22nd Nov 2019, 10:35 am   #8
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Default Re: Measuring a High Voltage, very low current supply?

There are many occasions where you need to measure over 400V in a workshop and see the waveforms on a scope. The usual 400V rated x 10 scope probes are not really up to it, unless you want to damage the probe or worse, your scope's input circuitry.

In fact if you do not have a cheap common garden x100 probe in your kit of workshop test gear, you are somewhat remiss. These are not expensive items these days, dozens of clones of them come out out China. I bought my x100 probe from RS, but there are many like these coming out of China, just one example:

https://www.ebay.com.au/itm/BNC-Male...MAAOSwMN5dzjtl

Sure, you could goof around trying to make a resistive divider, but you better be darn sure that the high value resistor is at least 1 to 2kV rated or it will fail on you, and if you did make a divider with resistors, its HF response would be useless (without elaborate compensation networks) and only any use for a DC measurement, not to inspect any AC waveforms. So it would be a very limited test device.

So the solution is simply buy a x100 scope probe. You can also use it with a DMV (if you don't have scope) with a BNC to twin banana adapter if the DVM has the typical 1M Ohm input resistance.

I could also add:

It is never a smart move to attempt to find a way around having proper test gear, whatever the motivation, either want a quick solution or cost issues.

The compromises of doing that will foul up your measurements and lead to all kinds of grief you could do without.

Mind you if a x100 probe cost many hundreds of dollars, I might be tempted to get around it myself, but now these probes are as cheap as chips.
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Old 22nd Nov 2019, 11:34 am   #9
Martin G7MRV
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Default Re: Measuring a High Voltage, very low current supply?

Thanks GJ, great write up,

The meter i'll be using is a AN860B+. From what I can find out its 11.5M input on most DC scales, which are in 6s (6v, 60v, etc), but intruigingly ive seen the mV ranges specified as 'Hi-Z' and seen a value of 20Gohm?

As the circuit to be tested is very very low power, and I require to test it tomorrow, i'll go with a resistive divider chain built up from whatever the highest values I have in the workshop.

Im wondering now which range to select to measure on? If I go with the 60v range then im looking around a 10:1 for 600v, or 20:1 for 1.2kv
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Old 22nd Nov 2019, 11:54 am   #10
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Default Re: Measuring a High Voltage, very low current supply?

Use the supply to charge up a 1uF or more high voltage non electrolytic capacitor. Plonk your meter on a suitable range across the capacitor. The time constant should be long enough to give a reasonable initial indication if your meter isn't too slow. A 20k/volt analogue meter on a 500V or higher range would be just as good if not better.
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Old 22nd Nov 2019, 12:03 pm   #11
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Default Re: Measuring a High Voltage, very low current supply?

As Argus25 says, if someone else has solved the problem and you trust them and you can afford the price and you aren't in a hurry then the right answer is always to buy the solution.

But if any of those things doesn't apply then you will have to come up with a solution yourself. At work I was generally trying to measure peak pulse amplitudes of 20kV-1MV with 10ns-1us risetimes. There were no realistic commercial solutions, at least not to the early stage divisions, so we really did have to build our own.

A very important principle was testing and calibration. I made it a rule never really to trust any monitor that I couldn't calibrate two independent ways. If you're not sure about the input resistance of your meter then I'd suggest temporarily connecting a known resistor of comparable value across it. If you think the meter is 10Mohm then see what happens to the reading if you put a 10Mohm resistor across it (it should roughly halve - you can calculate the precise change with a pencil and paper). Or try 4.7Mohm if that's what you've got. Simple checks like this can be very reassuring, or can reveal unknown issues before you get very far down the measurement pathway.

Cheers,

GJ
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Old 22nd Nov 2019, 12:05 pm   #12
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Default Re: Measuring a High Voltage, very low current supply?

My Fluke 177 on the 1kV DC range is showing 10M Ohm, question is, do I believe it?
( I should spec says >10M, //<100pF)

My Fluke 325 Clamp Amp measures 1M Ohm switched to DC V, spec seems to have nothing to say on the matter.
OK for measuring the mains and car batteries then.

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Old 22nd Nov 2019, 12:14 pm   #13
Martin G7MRV
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Default Re: Measuring a High Voltage, very low current supply?

At the moment I cant justify the costs of more test gear, so it is a homebrew job.

Now, taking the 11.5M input impedance specified as accurate, if ive worked this out right (my maths is hopeless), with an input impedance of 11.5M, for a 50V reading with a 500V input, I need to total resistance of 115M, so a chain of 103.5M. Bit of an odd set of values but workable.

That is a very good idea to check the meters impedance with a resistor in parallel.

If this was a higher current supply i'd be happy to use the meters higher ranges, its just that I cant load it due to the very small current capability
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Old 22nd Nov 2019, 12:16 pm   #14
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Default Re: Measuring a High Voltage, very low current supply?

Quote:
Originally Posted by GrimJosef View Post

There were no realistic commercial solutions, at least not to the early stage divisions, so we really did have to build our own.

Cheers,

GJ
I agree with this and I have had to design my own high voltage, high frequency compensated probes that go to 50kV, because any available probe that got even close was insanely expensive.

But, with a 100:1 scope probe now they have got so cheap, less than a hamburger & chips in some cases, you would require a sanity check to ignore them, than the alternative, to start from scratch to make one, unless you were stuck on a spacecraft or on a desert island and fancied yourself as Macguyver.
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Old 22nd Nov 2019, 12:18 pm   #15
Martin G7MRV
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Default Re: Measuring a High Voltage, very low current supply?

Quote:
Originally Posted by Herald1360 View Post
Use the supply to charge up a 1uF or more high voltage non electrolytic capacitor. Plonk your meter on a suitable range across the capacitor. The time constant should be long enough to give a reasonable initial indication if your meter isn't too slow. A 20k/volt analogue meter on a 500V or higher range would be just as good if not better.
Ive nothing of suitable value with a high enough voltage!

As I say, its very rare that I go north of 48V!
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Old 22nd Nov 2019, 1:04 pm   #16
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Default Re: Measuring a High Voltage, very low current supply?

Quote:
Originally Posted by Argus25 View Post
... But, with a 100:1 scope probe now they have got so cheap, less than a hamburger & chips in some cases, you would require a sanity check to ignore them, than the alternative, to start from scratch to make one, unless you were stuck on a spacecraft or on a desert island and fancied yourself as Macguyver.
I mentioned above that one of the conditions I put on test gear is that I trust the manufacturer. As the requirements get tougher and tougher the need for trust gets higher and higher.

Here's a test of the 4100 probe https://www.youtube.com/watch?v=v1jJ1Z_fgFU. The person doing the testing does seem biased against it from the start. But he's happy to admit that it performs well enough when it's not pushed hard. It fails when it is though. And the internal construction isn't exactly up to Jon Barth's standard https://barthelectronics.com/wp-cont...Model-2440.pdf (I love the way the spec sheet includes a drawing of where your fingers have to be when you're holding the probe ). Barth's stuff is not cheap. But I guess you never get what you don't pay for.

Cheers,

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Old 22nd Nov 2019, 1:11 pm   #17
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Default Re: Measuring a High Voltage, very low current supply?

Quote:
Originally Posted by Martin G7MRV View Post

Ive nothing of suitable value with a high enough voltage!

As I say, its very rare that I go north of 48V!
As I have said with the risk of repeating myself, this is exactly why you need to get a 100:1 probe, its gets the voltage right down to a manageable level in an insulated & safe manner and therefore helps to eliminate many of the risks associated with measuring voltages over a few hundred volts and drastically reduces the chance of electric shock and test equipment damage testing levels in the 1000v to 1500 V range.
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Old 23rd Nov 2019, 11:44 am   #18
Martin G7MRV
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Default Re: Measuring a High Voltage, very low current supply?

HV divider chain now built, and tested with bench PSU to ensure ratio is correct. Built on perfboard with whatever resistors I had in stock, its a bit of a mishmash, but will do for this job. Even found a 4M7 preset to put in at the meter end of the chain to allow calibration!

It still needs some insulation around it before I put it to HV use though. This is only for bench testing of breadboarded projects - its never going to be used to probe inside an equipment chassis!
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Old 23rd Nov 2019, 2:27 pm   #19
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Arrow Re: Measuring a High Voltage, very low current supply?

My opinion - FWIW - is in agreement with Argus 25: get a 100:1 'scope probe. That is what I use.

Al.
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Old 24th Nov 2019, 9:39 am   #20
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Default Re: Measuring a High Voltage, very low current supply?

Agreed, a 100:1 probe is worth having.

With a battery powered DSO, you can even work on live-chassis sets, subject to some precautions.
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