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Registered Member #3640
Joined: Sat Jan 22 2011, 12:16PM
Location: Germany close to Heidelberg
Posts: 39
Hi together, I have lately been reading about secondary MMC's here, one case for a small DRSSTC and one for a QCW type coil.
Is this concept thinkable for normal mid sized DRSSTCs? It would probably be possible to cover a voltage of approx 80-100kV. No clue what voltage a normal mid size (60cm) DRSSTC typically reaches. My coil has about 200kHz resonance - with some internal MMC I could get this down for the real big brick IGBTs.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
My main concern with the secondary MMC is that if the topload arcs heavily to ground, the capacitor bank will transmit a big pulse straight to the bottom of the secondary. I think with big high-powered coils this could lead to horrific flashovers between the secondary base and anything else nearby.
A mid-size DRSSTC will have an output voltage somewhere between 250 and 500kV.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Indeed. For small coils, this seems to be just the ticket. Steve Ward's coil runs at about 50kV, and mine certainly doesn't run much higher.
For regular DRSSTC use this is mostly an untested method of lowering fres. I have no idea how long the caps will last in such a setup (especially cheap disc ceramics).
Is this concept thinkable for normal mid sized DRSSTCs? It would probably be possible to cover a voltage of approx 80-100kV. No clue what voltage a normal mid size (60cm) DRSSTC typically reaches. My coil has about 200kHz resonance - with some internal MMC I could get this down for the real big brick IGBTs.
If you want to reduce frequency I'd go for more secondary inductance. That will also reduce secondary Q, easying tuning.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
plazmatron wrote ...
For regular DRSSTC use this is mostly an untested method of lowering fres. I have no idea how long the caps will last in such a setup (especially cheap disc ceramics).
Lets remember that Steve Wards 3 main reasons for adding the secondary MMC had nothing to do with lowering the resonant frequency, but mostly preventing streamers from loading the secondary out of tune.
Steve Ward wrote ... Another interesting feature of this system is a secondary MMC. That is, there is a string of 72 x 1.2nF 700VAC capacitors INSIDE of the secondary coil. This capacitor adds about 17pF to the coil. There are a few reasons for this. 1) due to the gigantic streamers this thing produces, detuning is a serious issue, so the coil needs to have a pretty decent amount of self capacitance. 2) more Csec drops the impedance of the coil, which seems to improve spark growth behavior. 3) adding more energy storage to the secondary coil means slightly less circulating current in the primary. The way i think of it is that the system Q probably stays about the same, but since there is more energy in the secondary, there is less stored in the primary. This capacitor also provides convenient measurement of the toroid voltage by measuring the current through this capacitor only. This is where i obtained the 55kV number, which is in good agreement with previous direct HV measurement of my last QCW system using a Jennings HV vacuum cap divider. So indeed, the voltage is low.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Simulations I have done with my large sized DRSSTC (7 feet tall) at 10 feet of arc I except around 450Kv note that the voltage changes drastically with frequency and secondary specs. The best way i think to figure out the voltage would be to simulate then try it in real life and see if your cap blows up. Other than massively expensive supper awesome volt divider + scope. (kidding)
My thoughts on a secondary cap for a mid to big DRSSTC would be kinda pointless in my mind. The way i see it the main reasons to add a secondary cap are to lower the secondary resonant fres or to compensate for capacitive spark loading or both....
Capacitive spark loading really only becomes a problem when you are trying to push a massive amount of spark out of a very tiny secondary like a QCW as the spark capacitance overwhelms the secondary self capacitance.
Lowering the secondary capacitance is usefully for like what plazmatron did when winding a small secondary and not wanting to use a bunch of tinny wire.
Other than these two reasons I don't see any reason to add capacitance to the secondary circuit. A mid sized or large sized DR doesn't have ether of these problems and on top of this I agree with Steve in that it would add a massive amount of energy that could be very bad when it has a heavy ground arc. (I already flinch when I get big ground arcs, I would probably be missing chunks of strike rail if I added a secondary cap hehe)
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
Goodchild wrote ...
...The best way i think to figure out the voltage would be to simulate then try it in real life and see if your cap blows up. Other than massively expensive supper awesome volt divider + scope. (kidding) ...
Terry Fritz built an interesting probe that is able to rather accurately mesure TC's output Voltage and Current. Click on pic for his project page:
But I noticed something wrong in Terry's calculations : he used "pi R^2" to calculate the surface area of a sphere, when it is "4pi R^2". We have to take this into account when calculating the probe to coil distance that gives accurate current measurements.
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Secondary MMC - thinkable for normal mid-sized DRSSTCs?
