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Michael W.

Wed Aug 02 2006, 04:23AM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

Well folks i'm ready to make my first MMC, casting off the salt caps. I'm using a rectified flyback driven with a 2n3055 at 12V. I've been reading around and the problem is that I'm not sure of the exact wattage (voltage and amperage) of my system. Is there anyway to safely estimate what size of MMC I need? Can I over compensate the voltage and capacitance? I'm looking for a more professional opinion than I can offer myself. Thanks.

dan

Wed Aug 02 2006, 04:45AM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

I think before you toss away those salt caps you need to think about getting a better power supply. Like a NST or at least an OBIT. With a rectified power supply (like your flyback) you can have as big of a capacitor as you want. However BPS will suffer if its too big. I wouldn't go above 0.01uF with that flyback just as a ball park figure. Voltage wise shoot for at least 1.5x peak voltage but 2-2.5x is highly recommended.

Michael W.

Wed Aug 02 2006, 02:01PM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

What would my target peak voltage be aprox. ?

Terry Fritz

Thu Aug 03 2006, 07:10AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

All these calculations are real computer intensive!! Good data in, good data out...

I suggeest you try this thing to find them out:

It is a little scary looking

but it works!!!

ScanTesla is also out there, but it is even more geeky

But it all needs lots of "cold hard facts" about your coil system to come up with cold hard answers...

Cheers,

Terry

Michael W.

Sat Aug 19 2006, 12:43AM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

Ok, I've managed to get my hands on a 15KV, 60mA NST. Now I need to build an MMC for it. Any Capacitor recommendations (model number etc) or does anyone have any surplus they'd like to sell? Thanks.

Marko

Sat Aug 19 2006, 01:08AM

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

900W NST is indeed nice, some big sparks I smell

You are going to need a bit moe than 10nF, 30kV MMC for that..

Michael W.

Sat Aug 19 2006, 01:43AM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

I know, I love the NST (I got it free of charge) I could just go an build an MMC on my own but i'm not sure on the best capacitor, capacitor configuration for my money....

Coronafix

Sat Aug 19 2006, 03:01AM

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

I've got an NST of same specs.
The MMC I'm going with is 3 rows of 8
3kv 0.047uf polyproplene
this gives me 17.6nf @ 24kv
this is for a LTR static gap of course.
Don't know about brands and such as they probably wouldn't translate to American.
Total cost is about $100 bucks U.S.
Check out what Chris Hoops has got for his MMC on his 900W coil.

Part Scavenger

Sat Aug 19 2006, 03:01AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

The worst thing about MMCs (especially in SGTC's) is that they cost more than salt and aluminum foil. Here's what you need though...

CDE 942 series 2000V capacitors.

For the NST you will need at least 17kV volt rating. Peak Voltage is calculated by V*1.414, if I remember correctly. If possible, use 20kV. WinTesla suggests a tank capacitance of 13.3 nF for your PSU, TeslaCAD suggests 15.9nF. So, this gives you a good range to play with. I'd suggest getting enough caps to try up to 16nF.

The most popular MMC capacitor is the .15uF cap in that series. However, this one would need 10 in series, and a single strand MMC is not supposed to be very stable IIRC. (they tend to overheat, someone back me up on this?) I would find something with a smaller capacitance so that you can parallel and series them. This will make for a more stable MMC.

The best MMC caps are film/foil caps (that's not film or foil, BTW). According to one CDE datasheet, this is because in an overvoltage situation, the capacitor is designed to burn away the foil so it does not short allowing the capacitor to still function nicely "even after repeated overvoltage." There are also some caps form General Electric that EVR likes. I don't recall the part #...

BTW, the reason you don't see many MMC's with flybacks is because the voltage is really high. On average a flyback can put out like 25kV, which means you need about 36kV minimum volt rating. Usually that many caps isn't worth it for the low power.

And... Be careful with your NST! They're fragile. You may already know this, but:

1. Primary strikes are really bad for the transformer (and you BTW). Use a strike rail.
2. To properly set the spark gap, set it so that the NST will just jump it when disconnected from everything. (ie, the spark gap is the only thing connected to the NST) Then, never open the gap farther than that. You can close it all you want, but backing it out too far will overvolt and kill the NST. That's what happened to mine.
3. Safety gaps are handy for catching overvolt on an NST.

Hope that helps.

Michael W.

Sat Aug 19 2006, 03:52AM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

I have the identical Xformer that Chris H. has but he used many polypropolene caps see (

)
I was thinking something more like this...

I just want as cheap as possible. Now looking at the costs of mmc's i'm having second thoughts. I just hate bottles caps as I can't measure thier capacitance, so I can't calculate the amount I need and therefore can't achieve resonance, which gives me 2 inch arcs...

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