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Registered Member #229
Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506
Why don't use copper or aluminium sheets/flashing for the primary coil? I remember that Jimmy Hynes used this idea for his DRSSTC. It is compact and allow more turns and a good coupling.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
vasil wrote ...
Why don't use copper or aluminium sheets/flashing for the primary coil? I remember that Jimmy Hynes used this idea for his DRSSTC. It is compact and allow more turns and a good coupling.
the idea is not bad, though in order to achieve required number of turns, I'd need to use thin copper foil tape would as densely as possible - and I'm afraid this may not dissipate heat too well. If I could sorce some copper foil tape someday, I could try a coil of it under my copper tube primary, with everything in series.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
teravolt wrote ...
That is a tough project with the heating and power dissipation but very cool. The spark looks like a class-E type spark
Mine or steve's project? :P
Cheers,
Marko
[/quote1284004071]
I think that your coil is more difficult because of the amount of power you have to manage. Certainly Steve's is more complex.
It seems to me that your primary is choked up quite a bit on the secondary have you tried pushing it to the bottom yet. In your picture of the primary the enamel seems to pealing off the secondary and it seems to me that having your primary so high is cosing the lower part of the secondary to heat. how do you feel about a tapered primary to the bottom?
I have also noticed that you have no MMC wich I believe is by design. If you had a MMC you may get the higher impedance that you want with the added benifit of a high voltage rise accross your primary. What kind of caps would you choose for the power you are dealing with since you are melting 10awg wire.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi teravolt,
It seems to me that your primary is choked up quite a bit on the secondary have you tried pushing it to the bottom yet. In your picture of the primary the enamel seems to pealing off the secondary and it seems to me that having your primary so high is cosing the lower part of the secondary to heat. how do you feel about a tapered primary to the bottom?
Yes, the primary is choked up as high as possible since high coupling is of benefit for this type of coil. The main reason why I want to lower is is to hog a bit less power at lower voltages. I'd be much better if I had just a few more turns.
You are wrong about the secondary heating, though - it really is just ugly varnish, and I haven't seen the secondary get more than lukewarm. I used the same resin from which the form was cast (boat polyester resin) diluted with styrene to coat the coil, although the resulting mixture was still too thick and didn't spread too well. I'm afraid that lowering the primary will actually cause the bottom of the secondary to heat up more.
Though I don't understand what you mean with "tapered primary to the bottom"?
I have also noticed that you have no MMC wich I believe is by design. If you had a MMC you may get the higher impedance that you want with the added benifit of a high voltage rise accross your primary. What kind of caps would you choose for the power you are dealing with since you are melting 10awg wire.
I'm not sure what you mean by this. I have a DC blocking cap which is a sort of ''MMC'' of four parallel caps, but a series resonant MMC is a no-go in this case. With my current primary it would just turn the coil into DRSSTC. I obviously want to run CW, not with primary current going for hundreds of amps.
The coil was measured to resonate at 145kHz with small spark load.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
Yes, the primary is choked up as high as possible since high coupling is of benefit for this type of coil. The main reason why I want to lower is is to hog a bit less power at lower voltages. I'd be much better if I had just a few more turns.
Though I don't understand what you mean with "tapered primary to the bottom"?
what I ment is a primary where the windings are set up in a tapered angle like my picture that starts at the bottom of the secondary
I have also noticed that you have no MMC wich I believe is by design. If you had a MMC you may get the higher impedance that you want with the added benifit of a high voltage rise accross your primary. What kind of caps would you choose for the power you are dealing with since you are melting 10awg wire.
I'm not sure what you mean by this. I have a DC blocking cap which is a sort of ''MMC'' of four parallel caps, but a series resonant MMC is a no-go in this case. With my current primary it would just turn the coil into DRSSTC. I obviously want to run CW, not with primary current going for hundreds of amps.
I asume that you are basicly using the XLprimary and XCmmc say 10 ohm to controle the current. how about a parallel resonant cap that way the primary circulating current will be high between L and C that way your bridge won't have to work so hard because a parallel tank has high Z at resonance. If you had that set up you could use a parallel L-R network in series with your primary and your bridge like a VTTC to very power. Does that make sense? By the way how much peak current is flowwing in the primary turns?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
how about a parallel resonant cap that way the primary circulating current will be high between L and C that way your bridge won't have to work so hard because a parallel tank has high Z at resonance. If you had that set up you could use a parallel L-R network in series with your primary and your bridge like a VTTC to very power. Does that make sense? By the way how much peak current is flowwing in the primary turns?
A parallel LC circuit has high impedance at it's center frequency, but the inverter puts out square wave with high frequency components for which it would basically be a short. Driving a large capacitor by the inverter is never good at all.
If I added a series inductor to the bridge output, I would get LCLR. Richie B. told me once that such a topology would work for a sstc, but I had little luck with it in low power tests. It seems to take a while for the primary tank to ring up - hence I may benefit from my improved startup circuit.
I also don't have any knowledge on how would this circuit behave at these power levels, and whether I would get any soft switching benefits at all. I suspect the inductor may give me ZVS but I would hae large current to turn off.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Just as a comment:
I have run my QCW coil in true CW mode, but it was with either a 3nF or 10nF tank cap and i had to use a big 18" diameter primary with ~8-10 turns (i think). It resonated at 350khz or so, so its a fairly high tank impedance. The spark ultimately limits the primary current. The benefit is that you can achieve real ZCS this way, whereas an untuned primary never will (even with PLL tricks). This is quite a "big deal" in my opinion, given that switching losses are probably dominant (well certainly at 360khz they are). Since Marko doesnt happen to have a bank of Mica caps to handle such duty, its probably not an option.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Steve,
I actually have a handful of 3.3nF 10kV, 20kVar doorknob caps laying around. I wonder if a bank of them (say 20nF, since my frequency is about half of yours) would be suitable for the purpose?
Regarding the primary, I'm surprised that you got away with ''only'' 10 turns. My primary is about 12inches, and I could fit about 12 turns reasonably on the outside, and then perhaps another 5-6 on the inside of the PP studs (perhaps a re-use of my current primary coil?). I would need less than 60 microhenries with a 20 nF cap so I may even have some margin in there.
The cost of copper is a bit high, but I might invest in another 12 meters of pipe if it proves worthy to use such a topology. I may want to test it small scale first.
I'm too busy with my college now and not with too much time to dream about this until christmas. I heavily appreciate your experimental efforts, since I thought a much higher impedance would be required any I would most likely have to come to it by trial and error.
I want to do some experiments and measurements nevertheless, both on this coil and smaller scale. I'm too concerned with the cold deaths of mosfets in some of my smaller scale experiments, and that has to stop. I have many things to try, and what I want is to build a bullet-proof small scale system before progressing further.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Those caps sound OK, being rated at 20kVARs each they should be good for a few amps RMS each id guess, so with plenty of them in parallel, and not demanding extremely long run times, they ought to work.
Id also highly recommend some sort of phase compensating circuit, either PLL or this "prediktor" type thing to get better ZCS. The penalty for hard switching is pretty bad at these high frequencies, supposing you really want to maximize the capacity of your switches. Ive seen hard switched designs that run 300kHz at work, and they de-rate the IGBTs to maybe 20% of the DC rating to keep it running.
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"Big Bad" the CW coil
