Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 07:43 AM 2/2/2006, you wrote:
Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx> Adam, Jim, Steve, Sean, Thanks for the helpful hints. Actually, I had built a somewhat crude quarter shrinker a few years ago but ended up dismantling it and selling the caps. I am now trying to build another one with over twice as much discharge energy (over 20 kJ as opposed to a "mere" 10 kJ with my frst setup). I had noted the same aspects that Adam mentions with charging up his, but was using 2 seriesed 7.2 kV PTs as the high voltage source. I was also using 2 MOTs with their secondaries shorted as an input ballast for the PTs. I don't have the MOT ballast in this setup, but may just use my 3 kOhm, 250+ watt ceramic resistor, which can be seenon the second picture down at: http://dawntreader.net/hvgroup/resistor.html in series with the charging supply to the caps as a current limiter. Hopefully, I can secure the other (2) 10 kV, 120 µFd Maxwells in the next few weeks to complete the capacitor bank. David Rieben
There's a couple reasons why you might want a variable voltage for capacitor charging: 1) If you charge from a "stiff" (constant voltage) source, and there's an RC circuit, the energy dissipated in the R will be equal to that stored in the C. So, having a voltage that gradually ramps up, so you have something closer to constant current, will reduce the heating in the series R.
2) Surge current through the rectifier diodes on initial power on. There's a couple clever ways to solve the problem:a) Use a voltage multiplier with a fair number of stages. The terrible voltage regulation of the multiplier helps here. When the energy storage cap is low voltage, the multiplier is heavily loaded so its output voltage is close to the transformer's Vpk (or 2Vpk)
b) Put a lightbulb or resistor in series with the transformer primary. When the current drops below a certain amount, a relay shorts the resistor.
c) Use a SCR/Triac type phase control dimmer on the primary. With this one (you can get it as a nice "brick" from mfrs like Carlo Gavazzi.. looks a lot like a solid state relay), you could actually do a feedback scheme that gradually ramps up the primary voltage as the secondary voltage increases.
Just to keep this all TC related, this kind of thing works well for charging that huge filter capacitor on a DC coil. Personally, though, I think the optimum strategy for a DC tesla coil supply is some sort of 3 phase supply (or even better, a switcher) to reduce the required stored energy.