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Registered Member #4146
Joined: Tue Oct 18 2011, 02:01AM
Location: Wisconsin, U.S.A
Posts: 56
Recently built a SSTC and have been running it with fullwave smoothed. From what i've been reading running it halfwave without a smoothing cap puts out some pretty cool looking streamers. Could anyone post a little info on the best way to go and possible differences one would expect using halfwave, fullwave, smoothed and unsmoothed? I really like the look of some of the halfwave streamers out there but i'm also wondering how each method affects the mosfets in the half bridge.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Smoothed DC - huge power draw, shorter dense "brush discharge"/plasma Half-wave unfiltered - decreased power draw, long straight sparks, small/almost none branching, sparks humming at mains frequency Full-wave unfiltered - something between, sparks humming at double mains frequency
Registered Member #4146
Joined: Tue Oct 18 2011, 02:01AM
Location: Wisconsin, U.S.A
Posts: 56
Thanks that was exactly the explanation I was looking for! What about drawbacks concerning mosfet heating and just the amount of punishment each method puts on them in general?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
If you had two identical coils side by side, just one was running half wave unfiltered and the other smoothed DC supply and they both were tuned to the same power input, the half wave one would have higher RMS current running through the transistors so they would heat up more. But the sparks from the half-wave one would be MUCH longer, like several times.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
ScotchTapeLord: The coils are tuned to the same input power. The input current to the bridge of a DC-running Tesla coil is, well, DC. But the current to the bridge of half-wave running TC is pulsed. If a DC current starts to have an AC ripple, the RMS current always goes up if the average current is kept the same. A half-wave rectified pulsating current can be seen as a DC current with huge ripple, so high RMS-to-average current ratio. The half-wave running TC would even draw a bit higher average current, so the rms current is much higher.
Example: Two coils running from 230 volts AC, which has a 325V peak value (for simplification) and the coils are drawing 325W of power: 1) a DC Tesla coil runs from rectified filtered 325 volts, so draws 1 amp RMS and 1 amp average current. 2) a half-wave running TC draws a power with squared-sine envelope (a simplification for resistive load), with every other pulse missing. The peak-to-average ratio of this waveform is 4, so the coil draws a peak of 1300 watts at 325 volts and a peak of 4 amps. The RMS value of this current is 2 amps and the average value is 4/pi = 1.27 amps.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Correct. But if the coils are tuned to the same average power input they're not identical any more. The halfwave one will have fewer primary turns to get the peak power up.
If they really were identical, they would produce the same peak power. So they both would make roughly the same spark length, but the halfwave rectified one would draw less average power and run cooler.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
We both do, we just started from different assumptions. I used the same coil system and just changed the current, Dr. Dark reoptimised the coil to make full use of the silicon.
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SSTC Pro's and Con's of half wave versus full wave rectified.
