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Measuring HV with mA meter?

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EEYORE

Sun Jan 17 2010, 10:39PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello all,
Everyone seems to use a divider for measuring HV. I was wondering if I could use a mA meter (analog) to measure voltage instead. (Using ohms law). A HV divider just means more to order, put together, and fidget with. Ive seen people put an ammeter in series with a HV circuit to measure current, so I figure I could do the same and figure voltage via ohms law (since resistance and current are known).

Sound good? I plan to use this to figure the voltage output from a voltage multiplier with a resistor in series to limit current to my x-ray tube.

Any problems to consider?
Thanks,
Matt

StevenCaton

Sun Jan 17 2010, 11:05PM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

I just did this last week to measure the output of a voltage doubler. I wanted to verify that it gave 30KV. So I put 200 million ohms in series, and measured the current. It worked fine.

Any problems to consider?

Maybe shielding the meters to keep the field from screwing with it? I'm not sure how important this is.

You might want to consider investing in a high voltage probe, instead of using a meter. A meter might be the cheapest way to do it, but if you have a really nice meter, I don't know if I would recommend it. (but it worked for me)

radiotech

Sun Jan 17 2010, 11:24PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

A good meter to put in series with iine to measure current is a thermocouple RF ammeter. Been used forever, and it cant lie, no matter what the waveform. I think you may even see them in aerial wires from spark gap transmitters of old.

Steve Conner

Sun Jan 17 2010, 11:33PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Matt, what do you think is inside a high voltage meter?

Moving coil analog meters are all fundamentally the same, the only difference between a voltmeter and an ammeter is that the voltmeter has a large resistor in series with it, but the ammeter has a small resistor shunted across it. And the scales are different too, I guess.

The divider makes sense if you're using a scope or DMM, but for an analog meter all you need is a series resistor. The more sensitive the meter, the higher a resistance you'll need, and the less current it'll rob from the HV source. 50uA FSD is about the best meter you can get, and that would measure 50V FSD with a 1M multiplier resistor. For 50kV FSD you'd need a gigaohm, and you start having to worry about corona and leakage spoiling the accuracy.

If you go up to a 1mA FSD meter, then to measure 50kV you need a 50M resistor, and it'll draw 1mA (obviously) and dissipate 50 watts.

Sulaiman

Sun Jan 17 2010, 11:38PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

There are two main problems, the first is of course safety which I'm sure you will cover, the second is the power that you take from the circuit being measured;
e.g. 50kV measured with a 50uA meter-
The circuit will have to supply 50kV x 50uA = 2.5 Watts

Will this significantly affect the circuit?

The resistor(s) will have to dissipate this power as heat.

Also, common resistors have a voltage rating of a few hundred volts so you will need either an expensive hv resistor or many normal ones in series.

EEYORE

Mon Jan 18 2010, 12:27AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello,
I got a 1mA current meter since I plan to run around 500uA. I dont need a lot of accuracy, just a good idea of where I am. This isnt the main reason I got the meter. I got it to measure current through the tube, but thought it might serve a good way to know voltage as well. I have no idea how it is constructed (such as shunt size, and working voltage) as I ordered it.

Matt

Antonio

Mon Jan 18 2010, 12:34AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Just take care to not do a mistake that I did: I was measuring HV using a string of precision resistors in series with a digital meter in the uA scale. Works fine. But then I turned the system off and turned the meter off too. When I returned for more measurements I turned the HV on before turning on the meter on the uA scale. In the off position the probes were disconnected internally on the meter. The result was internal sparking that destroyed the meter.

radiotech

Mon Jan 18 2010, 01:26AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

There are also electrostatic voltmeters; i've got 2 . and on DC they dont draw any current as long as the pointer isn't moving.

Think of a balanced tuning capacitor.

radiotech

Mon Jan 18 2010, 02:17AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Electrostatic meter here

Wolfram

Mon Jan 18 2010, 08:52AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Check out this

ebay auction if you need HV resistors. On that board there are 18 200M resistors. I've bought some of these myself for the exact same purpose, they are rated for 20kV each, so with all of them in series with a 500uA meter you can measure up to 360kV. For your purpose I suppose it would be more suitable to use five of them in series with a 100uA FSD meter to get a 100kV full scale deflection.

I would use a seperate meter to monitor current as it can be convenient to both monitor voltage and current at the same time.

Anders M.

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