Re: How could a pulse cap operate in TC?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

At 03:49 AM 7/13/2005, you wrote:
> Hello everybody,
> The capacitor is the most critical part in a SGTC, because it should withstand
> great power levels at rather high frequencies. Many factors are involved 
> here -
> maximum speed of voltage increase - i. e. dV/dt, dielectric loses in a cap,
> level of its reactive power & some others. Many people doing rather nicy 
> TCs are
> completely careless in choosing right capacitor (MMC) and its parameters 
> in operation.
> I`ve seen many coilers using MMC and saying that "it`s working - so it`s 
> OK". But
> even using a MMC we should calculate some very important details.

These are the critical factors:

1.  Self Healing - MMC capacitors can be driven to twice and sometimes 3 
times their rated DC voltage before they arc over internally.  Due to their 
construction, MMC capacitors will self heal and continue to operate even 
after thousands of over voltage hits.  This is pretty important since many 
coilers have "accidents" and things that arc the caps.  Mica caps are 
destroyed after just "one" hit as are many other types of caps.  Although 
it has been done, it is very difficult to destroy an MMC from over voltage.

2.  Selectable values - MMC are made by putting together many small caps so 
a wide range of values can be made.  This is important for say LTR coils 
that need pretty precise capacitor values.

http://hot-streamer.com/temp/MMCcapSales.gif

3.  Peak current - In Tesla coils, the peak current is the firing voltage 
divided by the primary circuit impedance ( sqrt(L/C) ).  This is usually a 
few hundred to perhaps a few thousand amps.  We need to use full metal 
"foil" caps.  The foil bonds very well to the end plates in the caps so 
they don't burn out.  Fully "metalized" caps have very weak end plate 
connections and they can fail under the high currents (found out that the 
hard way..).  This is why we are pretty strict about exactly what type of 
caps we use.

http://www.cornell-dubilier.com/film/9422000.htm

The peak current for our favorite 0.15uF 2000V caps is 432 amps.

4.  RMS Current - Tesla coil primaries can have some substantial RMS 
currents in the 75 - 300 kHz range which causes heating.  Often this is 10 
- 20 Arms.  We need to add enough strings of caps to handle the RMS 
current.  The chart above has all this taken into account.  Now days, the 
CD caps can do 13.5 Arms.  The caps should not get hotter than 10C above 
the air temperature or the dielectric deep inside may start to melt.

5. Peak voltage - For Tesla coils, we can use the DC rated voltage value 
and ignore the AC voltage rating.  This is due to the short pulsed nature 
of Tesla coils and the very short clock time that the caps are 
run.  Dielectric ion inception damage is just not a problem for us in the 
case of Tesla coils

6.  Bleeder resistors - It is an extremely good idea to put resistors 
across each cap to bleed off the voltage when not in use.  With say 20 caps 
in an MMC, it is not worth trying to discharge them by hand or anything...

Over the last few years, we have gotten very good at figuring out all the 
factors and what we need for Tesla coils.  There are programs and charts 
now so we rarely mention all the details and work behind it all.

> I remember Terry Fritz wrote here:
>
> http://www.pupman.com/listarchives/2002/July/msg01473.html
>
> "MMC caps have a giant advantage in that they are cheap and easy to test 
> under "deadly" conditions to
> determine exactly how "much they can take". Expensive commercial caps 
> can't be tested with such wild
> abandon and we have to trust the typically "scarce" information the 
> manufactures give us."

Now days, with our help, the data the manufacturers give is very very 
good!!  They are better at testing and specing them than "we" are now ;-))


> What is this "information"? What exactly parameters are the most criticle 
> and important
> for us? Now I`ve got a question about types of caps you use - reading some 
> articles I found
> the Maxwell caps to be the most reliable - if I`m not mistaken they were 
> designed to operate
> at laser applications. In such applications capacitor works in a pulse way 
> with fast discharge
> into low-resistance load with frequencies not more than 100..200 Hz.
>
> Now questions:
>
> -How could a Maxwell cap work in a TC, where frequencies are hundreds of KHz?

MMCs have only come about in the last 5 years so any information older than 
that will not mention them.  "Maxwell" caps are often used and can be found 
on e-bay.  They are big tough pulse caps that should work well.  They do 
not tolerate over voltage.  They are very low loss and "tough" so they work 
even if they are not quite meant for Tesla coils.

> -Does it overheat a lot? What are the practical values for the dielectric 
> loses in your caps?

It is purely a function of the RMS current which usually has to be 
calculated with circuit simulators.  It used to be about 8 amps but there 
is a new dielectric out now that does far better so 13.5 amps is the number 
for CD caps.  Some of this is dated now but:

http://hot-streamer.com/TeslaCoils/MMCInfo/mmcinfo.htm

Be sure and note the tips for making MMCs!!

I do not know the RMS current rating of Maxwell caps, but it is out there 
somewhere on the net.


> Maybe people, who works with high-power RF applications can give some 
> advices about using capacitors
> in these devices?

A lot of work has gone into it.  We try to make it simple for everyone with 
charts and programs.

Cheers,

        Terry


> --
> Best regards,
>  Grishka                          mailto:ghome@xxxxxxxxx