Power measurement

[sfmir]

I've got the following problem :
I have a plasma discharge between two capacitive electrodes (covered with glass as dielectric). In the literature it is suggested that for measureing the power dissipated in the plasma (plasma power), one should put a capacitor in serie with the capacitive plasma (connected to the low powered electrode) and measure the charge traversing the plasma by Q=CV. Then using Lissajous curve of V-Q, the power dissipated is the aire of the lissajous curve multiplied by the frequency of the applied voltage for generating a plasma.
I've been using this method but if I use different capacitor values (in the range of 0.05-10 micr0Farad which is much higher that the plasma capacitance, both being in series that means the total capacitance is close to the plasma capacitance) I get different power values for the same applied voltage.Is that due to the difference of the voltage drop across the capacitor?
any other idea will be welcomed.
thanks
sfmir

 

[IRstuff]

You didn't say how much higher the capacitance was. A factor of 5 would result in 20% of the voltage dropped across the series capacitor.

Different sized capacitors have different values for the effective series resistance, which is particularly critical if your plasma generator is running at high frequencies. Ditto for lead inductance and other parasitics. TTFN

 

[sfmir]

Thanks IRSTUFF.

The Capacitance of the plasma is about 100 PF while the Capacitance for measurement could be as follows :

C= 222 nF ----> Max Power at 12kV = 60 W
C= 1 microFarad ----> Max Power at 12kV = 150 W

What appears to me strange is such a big difference in power only by changing the capacitance. I was wondering if I can trust these power values or not?

 

[sfmir]

By the way the frequency of the applied voltage is 35kHz.

 

[electricuwe]

The difference might be caused by different loss factors tan delta of the capacitors used. Mica capacitors have very low loss factor but might be expensive for the capacitance needed. Polypropylen might work as well at the frequency used.

 

[electricuwe]

sfmir,

can you give the reference you have found describing this setup.

 

[sfmir]

electricuwe,

In general this kind of plasma is called a "Dielectric Barrier Discharge". If you type this in google with "pdf" you will get lot of pdf articles about this. The setup is composed of a function generator, an audio amplifier, a HV step-up transformer and two electrodes at least one of which is covered with a dielectric. The electrodes are seperated of 1-3mm.