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Charge Leakage and measuring Voltage with an Electroscope

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ThereIsNoSky

Mon Oct 18 2010, 04:02AM

Registered Member #2423 Joined: Tue Oct 06 2009, 02:49AM
Location: Oklahoma City
Posts: 10

Hello all,
I finally got the nerve up to make my first post. I hope I don't sound like too much of an idiot.

Basically I am trying to charge up a capacitor ( with HV! ) and observe the voltage on it over an extended period of time. I realize that it is not possible to do this with an ordinary DVM, because as you measure it, you will be draining charge and decreasing the voltage. After some time fiddling with a crazy and moderately successful idea for measuring DC voltage stored on a capacitor without consuming any of the charge, I have decided to go with an electroscope submerged in oil.

I have read some of the posts dealing with using oil as a "potting" material to reduce coronal discharge as well as most of the posts suggesting wax. Currently I'm thinking I'm going to go with the wax option; however, I'm not married to that idea.

My question to the forum is how much can I expect submerssion in oil or wax to prevent charge leakage into the environment, and how accurately do you think the electroscope can measure the voltage / charge?

Currently I am using a computer monitor to charge an HVWiki rolled foil capacitor through an HVWiki diode string. ( Gotta love that -How To- section )
I am using a spark gap to limit the voltage delivered to the Cap and "calibrate" my electroscope, and it seems to be accurate to some degree.
I can see the voltage dissipate after a matter of 1 or 2 minutes, but I would like it to dissipate MUCH more slowly.

Thanks,
NoSky

Proud Mary

Mon Oct 18 2010, 07:57AM

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

Do you mean a gold leaf - or similar - electroscope, where bodies with like charges repel eachother through electrostatic force?

Antonio

Mon Oct 18 2010, 01:56PM

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

Any oil is a much poorer insulator than air, and is only used due to high breakdown voltage, not because of less leakage. A capacitor immersed in oil would discharge much faster than in dry air. Wax is probably better for the capacitor, but the electroscope must be free in air. It's perfectly possible to calibrate an electroscope to a good degree of precision, if you keep friction low on its construction and keep it on a closed, conductive, box. Note that foil electroscopes leak by the edges of the foil. For voltages above 20 kV it's not trivial to make a low leakage electroscope.

Dr. Dark Current

Mon Oct 18 2010, 01:57PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I would say your homemade cap is "leaking"... AFAIK my homemade cap also didn't store the charge too long, maybe it has something to do with internal corona or something...

Does the cap retain its charge if you disconnect the electroscope?

ThereIsNoSky

Mon Oct 18 2010, 03:54PM

Registered Member #2423 Joined: Tue Oct 06 2009, 02:49AM
Location: Oklahoma City
Posts: 10

@Proud Mary - that is exactly what I'm talking about. Currently I'm using 2 .005" thick brass strips. They are pretty ridgid but still very light weight.

@Antonio - It is completely counter intuitive that a high voltage element will bleed off its charge more quickly immersed in oil than if it was in air. I can hear the hiss from the charge leakage when it is exposed.
When I submerged the electroscope in oil it seemed to function just as well and it increased the time it took for the cap to discharge. I will yield to experience, but that still doesn't make a lot of sense to me.

@ Dr. KV - I am thinking the same thing. The cap will retain a good amount of energy judging by the spark length and intensity when discharging. It is pretty similar with or without the electroscope now that it is in oil.

Assuming the electroscope functions fine in oil, would it not reduce charge leakage since there isn't also a ground submerged in the same cup of oil?

Antonio

Mon Oct 18 2010, 04:57PM

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

Of course, if you can hear the corona the losses are huge. Try an electroscope ended in balls. Foil electroscopes leak enormously after just a few kV. Something as this, using foam balls painted with conductive ink (China ink) with a scale, leaks much less:

Henley electrometers are good too, but the terminal ball must have at least 1 mm of radius for each 3 kV of maximum voltage.

IntraWinding

Mon Oct 18 2010, 05:07PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

What's your application? If you just want a capacitor to hold charge for a long time a lower voltage would help.

Patrick

Mon Oct 18 2010, 11:35PM

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

the ball-electrometer is a way better device, then the foil one, also you might consider a MPF102, 2n3819 based electrometer circuit. i would like to nkow your purpose too, as capacitence behaves the same at any voltage(with the exception of minor losses which only become visible at HV)

anyway, you can make intermittent measurements with a DVM then differentiate the rates of change to graph the difference for the meter's off time, in this way you can take into account minor losses like corona, extremely high oil insulation resistence, and so on.

you can put 90M ohms in series with the 10M DVM, and have 100 Mohms of resistence from your charge to ground, this would be better then just the native 10M DVM.

EDIT: did you fill the cap with oil and vacuum or just pour it in?

Russell Haley

Tue Oct 19 2010, 02:11AM

Registered Member #2478 Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47

How often do you plan to use this instrument? If you only need to measure the leakage of one or two capacitors, you could still use the DVM by connecting it only for a second or two to take a measurement. Charge, wait 5 seconds, measure. Charge, wait 10 s, measure. Charge, wait 15 s, measure, and so on.

Or, you could measure the leakage current directly, with a current-to-voltage converter using something like the MAX4238. It might be best to short the feedback resistor when charging the capacitor, to avoid killing the op-amp with overvoltage. Don't forget the shield trace around the inverting input.

Measure the leakage over a range of capacitor bias voltages (it's almost certainly not linear) and you can interpolate, solve the differential equation for v(t), and figure out how long it takes your cap to discharge.

Patrick

Tue Oct 19 2010, 02:41AM

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

Russell Haley wrote ...

How often do you plan to use this instrument? If you only need to measure the leakage of one or two capacitors, you could still use the DVM by connecting it only for a second or two to take a measurement. Charge, wait 5 seconds, measure. Charge, wait 10 s, measure. Charge, wait 15 s, measure, and so on.

Or, you could measure the leakage current directly, with a current-to-voltage converter using something like the MAX4238. It might be best to short the feedback resistor when charging the capacitor, to avoid killing the op-amp with overvoltage. Don't forget the shield trace around the inverting input.

Measure the leakage over a range of capacitor bias voltages (it's almost certainly not linear) and you can interpolate, solve the differential equation for v(t), and figure out how long it takes your cap to discharge.

is this not what i said? except for the part about current to voltage conversion, that sounds like an invitation to murder some silicon.

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