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Measuring capacitance

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LAN_403

Plasma

Wed Feb 20 2019, 01:02AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268

I'm building a device that will have electrodes at high voltage, with the hope it will pull ion's towards and surrounding of the electrodes with a wire mesh that is close but not touching.

I'm trying to workout the capacitance of the two meshes, they will be at high voltage, and I'm not sure I can measure the discharge time constant.

Is there any way to measure the capacitance.

Thanks

2Spoons

Wed Feb 20 2019, 04:33AM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

capacitance meter? Most DMMs will measure capacitance.
Or you could use a neon with a very large resistor to measure discharge time - they will usually extinguish at about 70V. I'm assuming you are charging to much higher than 70v

ritaismyconscience

Thu Feb 21 2019, 05:47PM

Registered Member #62937 Joined: Wed Jun 06 2018, 08:20PM
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Posts: 32

You could also use an oscilloscope by measuring the charge time of the capacitor. (You can probably use the 1M impedance of the probes to charge it).

Sulaiman

Thu Feb 21 2019, 06:25PM

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

I'd expect low pF range so resonance with a known inductance is practical.

Known (as-purchased precision) inductance usually means;
. wind your inductor as designed
. measure unloaded parallel resonant frequency with zero added capacitance,
only your device,
your device in parallel with known capacitance,
known capacitance only.

Then you can calculate your device's capacitance quite accurately.

Plasma

Thu Feb 21 2019, 10:50PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268

Thanks for the input, but I don't think I explained it good,

this picture is the idea, it will be 50 kV DC and if anything that isn't open circuit will arc between the ecltrode and mesh.
It needs to be charged to 50 kV in steps and measurement of capacitance taking at each step. To see if higher voltage makes more capacitance because of ion build up, supposable.

DerAlbi

Thu Feb 21 2019, 11:50PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Use constant current to charge the capacitor. The voltage slope gives you the information you want.

Plasma

Fri Feb 22 2019, 01:09AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268

I've haven't got much equipment :( can I have a capacitor voltage divider on the electrodes and another one on the mesh, would the series parallel arranged make a different voltage depending on capacitance?

2Spoons

Fri Feb 22 2019, 09:12PM

Registered Member #2939 Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615

Frankly, if you want to be able to measure stuff you are going to have to invest in a bit of test gear. Basic digital meters are cheap these days, Signal generators can be had very cheaply from china. And when you decide to get an oscilloscope, its pretty hard to go past Rigol's 2 channel 50MHz one (its easily hackable to 100MHz) for hobby use.

radiotech

Thu Feb 28 2019, 05:46AM

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

Can't you simply calculate the capacitance knowing the spacing, plate area,
and air as the dielectric?

C = (2235 * A * k) / 10^18 * t

When the mesh starts producing ozone, all bets are off.

C microFarads , A area in inches k = 1 for air , t = spacing in inches.

Connect two coils in series with the supply electrodes.

Find the series resonance of these coils and the plates.

At each level of voltage increase, cause a test arc to occur.

If you make it a rotary (toned) ark detect the signal with a radio,
and note the wavelength.

If the effective capacity changes, as voltage increases, so too

will the wavelength, and knowing the coil inductance, the capacity

will be known.

Sulaiman

Thu Feb 28 2019, 02:59PM

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

If you charge the capacitance in step voltage changes
(or ramp-hold-ramp-hold..until maximum)
and monitor the current,
while the voltage is changing current due to C.dV/dt can be monitored,
while the voltage is held constant any current is due to leakage, which includes ionic.

If you do not have a suitable current meter then you can judge the brightness of lamps,
filament for higher currents, led for medium currents and neon for lower currents,

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