Measuring panel capacitance

[arend-jan]

I want to measure the capacitance of an electrostatic panel while it is charged (e.g. connected to the HT), but I don't know how to do it.
Does anyone know a good/simple method to do this? Thanks.

Arend-Jan

[oshifis]

If you have a (preferably analog) DC voltmeter with high-voltage probe, you can measure the time it discharges to 63% of its initial voltage counted from the time when it is disconnected from the polarizing HT. You need at least 100 Mohm probe for this and a stopwatch.

[arend-jan]

OK, I was not clear. I want to measure the capacitance at the plates (stators) with a low frequency signal. To compare the capacitance with and without the HT applied.


Hmm... perhaps I can place a known resistor or capacitor in series with the panel and measure the signal across it?

[Conrad Hoffman]

In principle you could measure it by isolating it with a larger capacitor (or two), then either use a capacitance meter, or measure the AC voltage across it in series with a resistor. The problem is knowing that the measurement isn't influenced by the change in voltage across the larger caps, and not blowing up your meter or anything else in the process. The older capacitance bridges like the GR 716C could measure small values of capacitance (up to 1.1uF, I think) with a bias voltage of up to 700V or so. What's the voltage of your HT?

[arend-jan]

Hi Conrad,

The HT is 1500 Volts. I expect the capacitance to be in the order of 300 pF.

The difficulty is indeed preventing the measurement itself from upsetting the values too much, and keeping the instruments healthy at these voltages. I have a 100MOhm probe that I can use (with a 1:1000 voltage devider).

There is also the risk of the stepup transformer influencing the measurement (shunt winding capacitance is 200pF).

There must be some elegant way to measure this, right?

[Conrad Hoffman]

That's a tough one. I have visions of making the measurement through a big oil type HV capacitor, but then you have a fairly deadly high energy capacitor to contend with. IMO, it'll be hard to see the signal with a X100 probe, much less X1000. I'm not up on electrostatic speakers- does the application of the HT offset the diaphragm? If not, you might be far safer to just assume the capacitance doesn't change much with voltage. Here's another intriguing thing. A pure reactance can't dissipate any power, so a speaker that's purely capacitive can't produce any sound. The power consumption has to show up as dissipation factor for the capacitor/speaker. IMO, that DF number would be something very interesting over the audio frequency range. If the efficiency is any good at all, the DF should be fairly high, and the capacitance measurement is thus only half the story. You could wire up a purpose designed bridge for the measurement, but that will require a bit more thought. Sorry I can't be of more help, but this measurement does seem like a tough one.

[Calvin]

Hi,

You only charge up the membrane! If the polarization voltage is connected such that the ground potential is connected to the midpoint of the audio tranny, both stators will be on ground potential (with no audio signal applied). Since only the capacitance stator-stator is of interest (and easy measurable, because these are low ohmic, while the membrane itself is very high-ohmic) there should be no problems with high voltages.
Besides: I find the capacitance change of low interest. Isn´t it rather the value und change of the resonance frequency, that is more interesting?

jauu
Calvin

[arend-jan]

You are a very clever man, Calvin. Thanks.