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Re: [TCML] MMC's

Hello Adam,

I agree with what Dan stated. I will confine my comments mostly to the large energy discharge capacitors such as made by Maxwell, Aerovox, or Sangamo. I use a bank of 6 Maxwell 37667 capacitors in series-parallel. The only large capacitors that I have experience with for Tesla coil service are made by Maxwell. These seem to be the most popular. Its only practical to buy them used because the cost for a new one would be prohibitive. It is becoming hard to find suitable used ones though. Even if you find a used one on E-Bay, you have to check the specifications carefully. You need to insure that the voltage rating, maximum pulse rate, and shot lifetime are suitable for a Tesla coil. For example, many energy discharge capacitors are only rated for 1 PPS (pulse per second) which is obviously unsuitable for a Tesla coil.

As far as cost is concerned, what used to be true is now not so much. MMC capacitors, such as made by CDE, used to be fairly cheap. Making an MMC array then was cheaper than using a single, or several, commercial pulse discharge capacitors such as made by Maxwell. CDE capacitors have gotten pretty expensive lately. I would venture to say that today a single suitably-rated large capacitor may be cheaper than building an equivalent MMC array from CDE capacitors. Its certainly much easier to build with a single capacitor, or a few, than possibly 100 or more small capacitors with associated resistors and wiring.

For the large single capacitors I would want at least a 2.5x voltage rating. This may require connecting several in series-parallel to get the voltage rating that you need as I did.

As stated previously, my SGTC uses 6 Maxwell 37667 energy discharge capacitors connected in series-parallel. This gives me a voltage rating of 70 KV which is a 3.5x margin for my SGTC. I did a lifetime calculation using a Maxwell application note. With my usage pattern, the projected lifetime of the capacitor bank is 43 years! Thus, properly de-rated, these types of capacitors can last a very long time indeed. I have been operating my coil at 6.5 KVA for about 3 years now with the same capacitors.

There is no simple answer. Each type has its advantages and disadvantages.

Steve White
Cedar Rapids, Iowa

----- Original Message -----
From: "Dan Kunkel" <dankunkel@xxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Monday, November 23, 2020 8:58:03 AM
Subject: Re: [TCML] MMC's

Here is my experience and opinion. As far as the Information Unlimited
capacitor, I have not heard any reviews or info from others on these caps,
so I do not know their quality. However, one word of warning, the cap you
posted seems to have a marginal voltage rating of 20KVAC. It might be OK,
just depends on your supply transformer, spark gap and safety gap set up.
Also keep in mind that a pulse rated cap has a determined number of "shots"
before it is expected to die (keep that in mind when you plunk your money
down for a TC cap).

Now to your question, there several nice advantages to an MMC over a
traditional capacitor:

1) They are reconfigurable. I bought 100 caps from a list buy back in 1999
or 2000. Over the years and many coils and configurations, they have served
me well. It is also nice to be able to add or remove a leg on an MMC to see
how the performance changes, you'd be surprised. Obviously with a
commercial cap you stuck with the voltage or capacitance rating.
2) They are tough! The di-electric is self healing, so they survive the
rigors on TC use with ease. A lot of Tesla Coilers can get away with using
them at their rated voltage (or even over-volt them) and they just keep
working. Each time a hole is punched through the capacitance is reduced
(just a little bit), but that is certainly better than a dead capacitor!
3) If a cap dies, you can simply swap in a new cap.  So if using a large
single capacitor usually is going to cost a lot more, is less durable, and
if it dies, there is no way to bring it back to life.

The disadvantages:

1) The small leads pose a high impedance
2) They can take up a lot more space

Other info:

1) MMC's were born during a time when they were cheaper than
commercially produced pulse rated capacitors (and performed just as good).
So if cost is a factor, you would have to calculate which MMC array and
capacitor combo and compare.

2) Looks? Maybe that is subjective though...having a lot of neat rows of
arranged capacitors and bleeder resistors can look technical and complex to
the layperson, but a large single capacitor yields a more clean looking set

Good luck with your build!
Kansas City area (USA)

On Sun, Nov 22, 2020 at 4:56 PM Adam James <adamjamesarmy60@xxxxxxxxx>

> Trying to understand why I should build a MMC When I can purchase a single
> Capacitor from Information unlimited that would work for my upcoming coil.
> A .014u/20KVAC is only $70 versus much more $ to buy and use
> comparable single Caps to build a mmc?
> Adam James
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