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Sigma delta modulated QCW- bang without the buck?

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Steve Conner

Mon Nov 21 2011, 09:42PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well here are the HV lab test results as promised...

The weird thing is that the sparks fundamentally don't look that different to how they did without the ramp! Either the ramp generator is really bad (the scope shows the primary current, and as you can see it is quite bad

) or the 220kHz resonant frequency affects the spark appearance so strongly that no amount of envelope shaping can undo it.

They look pretty big and gnarly though, and no components blew up, so I think it's at least somewhat of a result. I intend to go on and improve the modulation somewhat, then tighten the coupling and add more DC bus caps and see if the sparks can be got any bigger that way.

Then I'll build a new resonator with a frequency above 320kHz, which according to Steve Ward is the magic frequency where the sparks suddenly go swordlike. If I don't get sword sparks, this will prove that the modulator isn't good enough.

The tank cap used in this coil is 2 strings of 10 470nF, 1kV PP film/foil caps, for a total of 0.1uF. I've managed to get hold of 10 conduction cooled mica caps rated at 22nF, 1kV RMS, 75A RMS, to make a new "MMC From Hell".

Stay tuned for more updates, if you pardon the pun!

(Moved pics and video into top post as per Projects forum rules.)

Marko

Mon Nov 21 2011, 09:47PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Lol that makes a good bang... reminds far more of zillipoper coils than a QCW... can see more clearly now - your primary current seems to be rising really fast, more DRSSTC-like, notching and then settling to stable level... hm, control circuit malfunction?

Marko

Steve Conner

Mon Nov 21 2011, 09:58PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, I'd class Zilipopper's coil as a QCW too.

The control circuit isn't necessarily "malfunctioning", it's just not doing what I expected! I can sometimes see the primary current drop almost to zero at the end of the burst and then pop up again.

I have to find out whether it's some sort of instability caused by nonlinear spark loading, or whether noise is just getting into it and tripping it prematurely, which would be a malfunction.

Edit: The best explanation I can think of is that the DC bus cap is emptying out, and a half cycle of mains is coming along and recharging it partly.

Avalanche

Tue Nov 22 2011, 01:24PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Awesome, yes it certainly is loud! The only time I get that kind of sound from my SSTCs is when they explode

Look forward to seeing what happens when you add more DC bus capacitance, should straighten out the ramp a bit especially as the loop is closed on current.

teravolt

Wed Nov 23 2011, 03:01PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

a tube is a natural chioce for a high voltage high frequency modulator. would it be pausible to do delta sigma with a VTTC

Marko

Wed Nov 23 2011, 03:36PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

teravolt wrote ...

a tube is a natural chioce for a high voltage high frequency modulator. would it be pausible to do delta sigma with a VTTC

Well, I was thinking of either a delta sigma topology (sensing the primary current with a CT, and controlling the tube either in "staccato" by a mosfet or by toggling the screen grid voltage);

or, in case this still fails, screen could just be modulated linearly with a ramp to produce the same effect! Not as efficient but I see no reason why wouldn't it work!

Marko

Dr. Dark Current

Thu Nov 24 2011, 09:29AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Yes, the modulation of the screen should work. The parallel resonant primary circuit behaves close to a constant voltage sink (as opposed to constant current sink for a series resonant primary), so the modulation shouldn't be that inefficient.

Steve Conner

Thu Nov 24 2011, 10:39AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It doesn't matter if the modulation is inefficient anyway. The plate of a vacuum tube should be able to handle a huge pulse of heat for a short time- it's not like a silicon die that could crack from thermal shock, or whatever it is that kills them.

I think linear modulation with a tetrode is a good idea, and will be pursuing it as a Plan B

Nicko

Sat Nov 26 2011, 03:29AM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

Steve McConner wrote ...

...

...

"May contain traces of nuts" ??? (look closely)

Well, it is made in Scotland...

Steve Conner

Sun Nov 27 2011, 03:32PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

All my Tesla coil boards have some sort of nut reference in them!

Now publishing a schematic of the driver, since people are asking me for it.

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