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Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
I would tend to point out that as the Inductor and Capacitor in the tank circuit ring, the ringdown voltage is 2X Vpk so your standoff voltage must be much higher then you might think.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
All-- I apologize for going off topic & getting personal yesterday.
Hazmatt_(The Underdog) wrote ...
I would tend to point out that as the Inductor and Capacitor in the tank circuit ring, the ringdown voltage is 2X Vpk so your standoff voltage must be much higher then you might think.
Where does the 2X come from? If MMC bank is charged to +20 kV when spark gap fires, the ringing carries the voltage momentarily close to -20 kV, but when does the dielectric see an absolute voltage greater than 20 kV? I understand there are interactions with NST output impedance, which should be limited by interposing a filter like Terry's.
[edited to simplify] Question 1 about the AC voltage rating on WIMA FKP1 datasheet, one click away from link in OP: For nominal 2000 to 6000 VDC parts, and any f < 1000 Hz, the spec is uniformly 700 VAC. That's independent of DC voltage rating, capacitance value, and frequency variations, so apparently not due to electrical or thermal stress (see next paragraph). Anybody here know a simple explanation for the low frequency VAC limit? Perhaps related to safety regulations, or just WIMA keeping it simple & conservative?
Question 2: Then a chart headed: "Permissible AC voltage in relation to frequency at 10°C internal temperature rise" shows the limit dropping to about 280 V at 100 kHz, for the 22 nF sizes. The slope of frequency derating curves doesn't quite match a fixed current limit. For example, the VAC limits for 22 nF permit 3.3 amps at 34 kHz and 5.8 amps at 1 MHz. For larger C values at same frequencies, the amp limit is higher but amps per uF is lower. So to Tesla coil experts: For a SGTC is it unusual to consider this heating-related AC voltage limit, figuring RMS voltage from 0.7 * SG breakdown voltage, times sqrt of duty cycle (itself figured from PRF and ringdown damping time)?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
It's because of dielectric polarization, the dielectric constant (which is a measure of the extent to which the dielectric can be polarized) the dielectric loss angle, and dielectric heating, lad.
Dielectric loss is not a constant, but depends upon such things as temperature, moisture content, and frequency.
The power that is dissipated in a dielectric is given in the following expression:
P (W/m3)= 2pi × f × epsilon 0 × epsilon '×. tan(delta ) × E sq.
where:
f is the frequency of the electric field in Hz epsilon 0 is the dielectric constan:-Dt of the vacuum (8.84·10 to the -12 F/m) epsilon’·tan(delta) is the loss factor of the dielectric E : the electrical field strength in the dielectric (rms) [V/m]
Better you wish you'd never asked now!
Google these terms to learn more!
The AC working voltage of a capacitor is always lower than the DC rating, because, mainly, of the dielectric heating caused by the work done on each half cycle to reverse the previous polarization of the dielectric.
If you don't know of it already, ESR - equivalent series resistance - is another very important property of capacitors, which belongs to Google, rather than an over-fed post.
It's all a lot less complicated in practice, and the day will come when you will just be able to go to the drawer and select suitable values without a second thought - just like the skilled chef who knows by eye and by experience what makes for a good recipe!
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