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MMC vs. Rolled Poly vs. Everything else

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cduma

Mon Jul 20 2009, 04:27PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

I am trying to decide which to build. I am building two TCs at the same time one is 12kv 60mA(needs .0133uF) and the other is 15kv 60mA(needs .0107uF). I am on a serious budget. What approach would you take?

GeordieBoy

Mon Jul 20 2009, 05:05PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

In my opinion an MMC made with decent metal-foil/Polypropylene-film capacitors beats a DIY "rolled polythene in oil" capacitor hands down. The voltage stresses are spread among many individual capacitor's dielectrics in the MMC. This is contrasted to the single dielectric that is apt to fail if there is any weak spot or over-voltage in a rolled poly capacitor. I've never had an MMC capacitor bank fail even when used on coils producing 10ft sparks, but I blew up loads of rolled poly caps on dinky table top coils.

Soldering up an MMC is also a lot less hassle than cutting foil and poly sheet and messing about with vacuums and oil. Yuk!

-Richie,

Alexandre

Mon Jul 20 2009, 05:31PM

Registered Member #1228 Joined: Tue Jan 15 2008, 01:08AM
Location: Curitiba, Brazil
Posts: 162

MMC is the Best!!!

cduma

Mon Jul 20 2009, 06:02PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

OK
It makes no sense to use rolled poly at this point. One small failure and its all done.
What sort of capacitors can I use? Where can I get them for cheap? Is it true that Ctotal=1/(C1+C2+C3+C4)?

cduma

Mon Jul 20 2009, 06:04PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

Are salt water caps a bad idea?

magnetomotive

Mon Jul 20 2009, 07:54PM

Registered Member #267 Joined: Mon Feb 27 2006, 09:44PM
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cduma wrote ...

Is it true that Ctotal=1/(C1+C2+C3+C4)?

Total capacitance is determined by this:

When connecting capacitors in parallel:
Ctotal = C1 + C2 + C3 + ... + Cn

When connecting capacitors in series:
Ctotal = 1/(1/C1 + 1/C2 + 1/C3 + ... + 1/Cn)

Same information here: http://www.allaboutcircuits.com/vol_1/chpt_13/4.html

You may have to series AND parallel the capacitors to get your desired capacitance and voltage rating.

Also, as GeordieBoy said, the best capacitors are the metal-foil/Polypropylene-film type like this:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=338-1175-ND

When you design the MMC, it is good to have twice the voltage rating of your transformer, so you would need 24 kV and 30 kV MMCs. I did this with my SGTC, and I haven't had any capacitors fail on me yet. Better safe than sorry when you are using such expensive capacitors.

GeordieBoy

Mon Jul 20 2009, 08:32PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

I used these Arcotronics capacitors from RS here in the UK:

I would also rate the capacitor for twice the RMS output voltage of the transformer. A 12kV NST can easily charge the tank capacitor to a peak of 24kV with an RSG.

So that means 16 capacitors in series to get 24kV voltage rating. Then connect as many strings as required in parallel to get the desired capacitance / current handling ability.

-Richie,

Herr Zapp

Mon Jul 20 2009, 10:22PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

cduma -

You didn't provide any information on what type of Tesla coils you intend to build, but if you are talking about a conventional static-spark-gap design, then you definitely DON'T want to use the capacitor values you stated.

The capacitor values you cited are "resonant" with your NSTs, and can cause serious voltage spikes that can instantly destroy both your hard-to-find 60ma NSTs, AND the capacitors.

What you actually want is a "larger-than-resonant" tank capacitor value ("LTR"). Do some research on "LTR" capacitors, and also read the excellent write-up at Richie Burnett's website (

) covering the dangers of "resonant" capacitor values.

After you understand the difference between resonant and LTR cap values, download the "MMC CAP CHART" from

, and find the correct cap value for whatever vtype of spark gap you intend to use.

If this/these are your first Tesla coils, I certainly would NOT recomment that you try to design and build two different coils simultaneously. All that will do is allow you to make the same mistakes on both coils, simultaneously. Build one, get it tuned and running properly, and then use everything you learned during construction of the first coil to make improvements on the second coil.

The potential performance difference between 12/60 and a 15/60 transformers is rather small, so it probably doesn't matter which one you start with.

Regards,
Herr Zapp

Sulaiman

Mon Jul 20 2009, 11:06PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

An MMC is definately best,
as you can see - initial designs aren't always final designs.
Then there's the new & improved Mark XXXX

MMC's main benefit is reuseability and reconfigurability.

Maybe once a simple SGTC is running a quick 'play' with saltwater caps is good,
just to understand the difference and the technology of Tesla's time.

The only reason I can see to try rolled and plate glass caps etc. is to be able to say
'been there, done that, don't bother'

cduma

Tue Jul 21 2009, 02:34AM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300

My first 'Coil was a 7500v 30mA static spark gap coil with a .00576uF home made cap. The cap dialectric failed near a sharp point. These will be my second and third coils

I have heard that some people use the DC voltage rating instead of the AC voltage rating. Which should I use?

[Edit: Double post]

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