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Determing MMC value.

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Ken M.

Sat Jun 09 2007, 01:28AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

I'm planning on making a new TC, with a 6in Dia and 24in tall made of 24AWG. I haven't ordered anything as of yet, because first I wanna see if I can't fix up the old 2in SSTC to use Steve Wards Mini coil schematic over the micro schematic. The issue that I have though is, with secondary and primary coils differing in # of turns and different sizes plus material differences (mostly tolerances) I'm a little unsure of what value I should use for an MMC. If I should just buy about 20 CDE or GE caps and just try out different series parallel setups to get good tuning, or if there is either a program or a way to figure out a good value for the MMC.

Coronafix

Sat Jun 09 2007, 03:02AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

What sort of TC are you planning on building?
Spark gap or DRSSTC?

...

Sat Jun 09 2007, 03:29AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

I am a little unclear on what you are trying to do... Steve's mini coil doesn't need a MMC, just a few caps to form the other half of the halfbridge... The value of these is not critical as long as you have enough C.

Unless you are trying to make a coil that will work both as a SSTC and a SGTC, in which case you would probably need to have 2 separate primaries (since sstcs want very tight coupling while sgtc's want it pretty loose)

Ken M.

Sat Jun 09 2007, 03:22PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

My mistake. I was thinking of two different TC and 2 different SSTC designs. So I guess the real question i should be asking is, should I just follow his design and use .68uF caps even though I have a 2" secondary, or should i use a different value.
Also Since I'm really looking forward to building a DRSSTC with the near future, I guess a second question would be, How do I determine the value of an MMC for it?

sparky

Sat Jun 09 2007, 11:53PM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

I'd might be aiming for a taller secondary using smaller diameter wire - like 27AWG. That way you'll get greater inductance on the secondary which can ...overall increase output voltage and spark size.

Ken M.

Sun Jun 10 2007, 04:06AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Well since someone has posted, which won't get me in trouble for double posting. I hit up the local ACE hardware and the LARGEST diameter PVC tubing they had was 4" and luckily sorta they had an 18" piece of it in the junk bin, so needless to say I had to change my secondary design ideas. With that said, the secondary is now going to be 4" diameter, 16" tall coil with ~1" spacers at the ends, using 30AWG wire. Which coincidentally is EXTREMELY similar to Steve's mini coil...

sparky

Sun Jun 10 2007, 07:14AM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

That will certainly raise the inductance... hey, what wattage are you planning on running through the coil?

Ken M.

Sun Jun 10 2007, 03:21PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Not sure....Like I said in the earlier posts I'll be using Steve Ward's mini coil design so it'll be just a regular SSTC, then I think in a month or so I'll try my luck with DR's.

Brett Miller

Sun Jun 10 2007, 07:28PM

Registered Member #593 Joined: Tue Mar 20 2007, 12:32AM
Location:
Posts: 50

WhiteArc,

That halfbridge self resonant SSTC design of Steve's (inspired by a design by Justin and Aron) is one of the most enjoyable projects I've ever worked on. Mine still works great, even though I've tried changing just about everything on it, I've ran it CW, even shorted the toroid to ground, tried about 5 different resonators, many different toroids, used it to measure Fo of secondaries, CW powerarcs, half wave ac, staccato, you name it.

When I first phased the GDT outputs and got mine working with the interrupter, I was using an 8" x 3.5" PVC secondary with AWG 30 and no toroid. Sparks were about 3 inches at 30pps and 2ms on, at 120v input (filtered DC to the bus) to the IRFP260N's. I added a John Freau 6 x 1.75" toroid and spark length doubled, plus got thicker, pink-pale white.

Later, I tried a secondary very similar to yours. 12.5" x 4.5" pvc wound with AWG31. With that I got 12" of spark easily with the same interrupter characteristics mentioned above....in fact I could get 12-13 inches no problem at around 30ms on time for super thick vttc type sparks. I never tried half wave rectified ac with that secondary, but I bet it would have allowed even larger than the max 14" I got using the interrupter with a filtered DC input to the half-bridge.

I believe the success of the larger secondary in SSTC duty is more due to the lower Fo than any inductance gains. This is because the FETs don't have to be switched on and off for as many cycles each time the drive is enabled by the interrupter. Consequently, the dies don't heat up as much and drive the system loss as high. If this is a wrong or incomplete picture I am sure someone will correct me or have something to add.

Good luck with your SSTC, make sure you try a breadboarded interrupter you can mod on the fly to get all the great spark effects you can enjoy with this coil. In my case I had that, plus a metal enclosed, grounded interrupter that I could rely on to be robust and give the sparks I wanted.

Oh, another thing I found was that with the low Fo secondaries, I could disconnect the interrupter and kick start it into CW operation. With a 12 x 3" toroid and breakout point...I got an 8 inch cw flame that rivals that of my 833A system in CW mode with very little to no heating of the mosfets. Many people have reported no advantage to large toroids in a self resonant conventional SSTC, but in my case it have a definate spark length advantage.

-Brett

Ken M.

Mon Jun 11 2007, 03:20AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Hmmm, this is tempting me so much I'm going insane! Anyways...yea, the way I'll be trying it is with a remote interrupter with a rotary switch and the 2 pots, another good idea is if you think the idea of half wave rectified would be good, would be to try all three types Filtered, half wave and phase shifted, Just have a 2nd remote say and a high current relay or something along that line and just switch between the 3 types along with the adjustable interrupter. As for a toroid I think I'll go with the 12' x 3" idea, just hit up ace and get some flex ducting or whatever its called and a disc of some kinda, which will work nicely since I already have some AL tape siting around. Hmm another though just accord to me...anyone know if a bought GDT work?

The GDT I plan on using:

It's a 1:1.5:1.5 and what ever wave form goes in comes out just the same on the other side.

Couldn't get the image resize thing to work.

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