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Nik

Tue Nov 27 2012, 02:42AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

Out of curiosity I used my cheapo mastercraft DMM to try to measure the current from my van de graff generator. It measured 3uA and when I swapped the leads to make sure it wasn't biased one way or the other it read -3uA. How accurate do you guys think that reading really is?

It can light an LED dimly but without flicker from just the corona so it is supplying a reasonably steady charge to the sphere. I don't know how the meter actually makes its measurement so I don't know if a voltage drop of tens (or hundreds) of thousands of volts would cause it to give false reading. Does any one have any thoughts?

Proud Mary

Tue Nov 27 2012, 11:23AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

If your DMM has a standard 10MÎ© input impdance, then the voltage drop across it will be 30V when 3 Î¼A are flowing.

With no corona losses and little leakage at 30V, 3Î¼A - or 90 Î¼W - sounds believable to me as a derivative of the coulomb seconds being delivered to the sphere by the belt.

Ash Small

Tue Nov 27 2012, 11:56AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

This is a very interesting thread after some of the recent discussions here regarding electrostatics.

I assume you have no return path, you are just 'charging the ball'.

How does this fit in with the requirement that 'Every circuit requires a return path'?

Nik

Tue Nov 27 2012, 12:12PM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

The return path was ground for both measurements. As cheap as it is I wasn't ready to to start hitting it with 8 inch sparks.

Mary, wouldn't the impedance be a lot lower in the current measuring mode? (like ideally 0) I think I have a 10Mohm resistor around here some where from a TV, I might try running the VDG charge current through that and measuring the voltage drop to see if I get results in the same ball park as my original readings.

Proud Mary

Tue Nov 27 2012, 12:48PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Nik wrote ...

Mary, wouldn't the impedance be a lot lower in the current measuring mode? (like ideally 0) I think I have a 10Mohm resistor around here some where from a TV, I might try running the VDG charge current through that and measuring the voltage drop to see if I get results in the same ball park as my original readings.

Oh yes, how daft of me! Yes, I'd expect the impedance on the current range to be somwhere between about 0.1Î© and perhaps 8Î©, depending on the make, model, and range setting.

If you short the sphere to Earth via 10MÎ©, and measure the voltage across it with a 10MÎ© input impedance voltmeter, you will see the voltage drop across 5MÎ©, if there is enough output to get a reading.

Patrick

Wed Nov 28 2012, 02:51AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

wow! proud mary really goofed that one!

he might be better off with a 200M resistor then measuring the curve as the current drops (several seconds if R is high enuff), then use some derivitive magic math to find it out with more useful aaccuracy... if that matters.

if its nearly constant DC (and charge accumulation is) then he can use a smaller 10-50 Mohm resitor to meaure the current via voltage, which might be better for not killing his meter...

Nik

Wed Nov 28 2012, 04:30AM

Registered Member #53 Joined: Thu Feb 09 2006, 04:31AM
Location: Ontario, Canada
Posts: 638

I reworked the upper part of my VDG tonight and effectively buggered it. I had a taboo upper roller outside of the sphere so I reworked it to the more conventional design. Since then the output has been pitiful. Once I get it running better I'll see about measuring the charging current using your (plural) suggestions.

Uspring

Wed Nov 28 2012, 11:21AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Another way to get the charging current is to measure the rate at which it sparks. Put a small grounded sphere say 2cm away from the VDG top. It will spark then every time it has reached about 60kV. The capacitance of your VDG top is about the same in pF as its radius in cm. For example:
Your top radius is 10cm. Then its capacitance will be about 10pF. The time between sparks is then t=V*C/I = 60kV * 10pF / 3uA = 0.2s.
That's ballpark, but should give similar values to your DMM.

radiotech

Fri Nov 30 2012, 03:51AM

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

What model Mastercraft DMM do you have? Those meters are used everywhere in Canada,
simply because of the distributor, and the periodic deep discounts they have.

I use models 52-0052-2 and 052-0060-2 in classes here and have yet to have one ruined.

Patrick

Fri Nov 30 2012, 05:19AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

radiotech wrote ...

I use models 52-0052-2 and 052-0060-2 in classes here and have yet to have one ruined.

as bad as students are on intruements of all types, 10kv is worse...

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