[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: How could a pulse cap operate in TC?

Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx>

Hello Grishka,

I echo much of what Terry has posted, and will try to supplement it with some more information that may help.

The quick answer to your question is yes - a properly derated Maxwell, Condenser Products, NWL, Hipotronics, or Plastic Capacitors, etc. polypropylene (PP) pulse capacitor can give many long years of service in a Tesla Coil. Most pulse caps have a 20% voltage reversal spec since they were typically designed for use in pulse forming networks.

To derate a commercial pulse cap properly, you first need to estimate the worst case Peak-To-Peak RF voltage that will occur in your TC tank circuit. This is the MINIMUM DC voltage rating that your commercial pulse cap should have in order to avoid overvolting it. By properly derating the cap, you no longer need to be concerned whether it uses self-healing technology. Maxwell's plastic cased pulse caps typically have at least a 25 kA peak current rating and at least 25A RMS current rating. So, RMS current is usually not an issue for disruptive Tesla Coil applications, but it can be a limitation within the tank circuits of high power VTTC or solid state CW coils.

Maxwell (now General Atomics Energy Products - http://www.gaep.com) mostly uses Mylar (PET), PP, and Polyvinylidene Fluoride (PVDF) film in their plastic cased pulse and energy discharge capacitors. Their very high-energy metal cased (Type C) capacitors use castor oil impregnated Kraft paper, a combination of paper and PP or Mylar film, or PP only as the dielectric. However, these particular capacitors are best suited for rail guns and coin shrinking, not Tesla Coils.

Many of Maxwell's newer pulse capacitors do use self-healing and fusible link metal-film technologies. Unfortunately, many pulse capacitors you'll find on the surplus market tend NOT to use self-healing technology so some care must be used so as not to overvolt them. PVDF is primarily used where higher energy density is required and higher cost can be tolerated. The main application for PVDF capacitors is in defibrillator capacitors and certain specialized/military applications. Most Maxwell caps appearing on eBay tend to be either Mylar of PP.

The best low loss capacitor dielectric for Tesla coil tank circuits is PP film. Although Maxwell doesn't directly indicate which type of dielectric film is used in a given plastic cased capacitor (unless asked), you can usually tell by looking at the specified discharge Repetition Rate (PPS). Capacitors rated at ~1 PPS will be Mylar, while capacitors rated for 1000 PPS will be Polypropylene. Although you can operate a Tesla Coil using Mylar caps, they will heat up and they won't perform as well as PP counterparts.

How do you tell which is which?
Well, Maxwell's older 31XXX series capacitors were mostly 1 PPS Mylar caps, but there are some notable exceptions, such as 31393, 31981, and 31583 (all 1000 PPS). Their series 37XXX caps were mostly 500-1000 PPS, and are therefore PP film.

An overview of Maxwell's existing capacitor line can be seen here:
http://www.gaep.com/capacitors.html. Their current PD series capacitors appear to be extended foil PP with NO self healing, but I have not confirmed this. Their DM series is self-healing, but these are not appropriate for Tesla Coil tank capacitor applications. Their series S, SS, and DE caps can be either Mylar or PP. It's not clear from Maxwell's specs whether these caps use self healing technology. You can easily look at the PPS rating to tell which dielectric is being used:
http://www.gaep.com/series-s-ss-de-capacitors.html

In the final analysis, if you have a question about a given capacitor, you can usually call or email the manufacturer and specifically ask them for a copy of the data sheet and for other useful information (such as the dielectric system and whether the cap uses self-healing technology).

Hope this helped,

Bert
--
---------------------------------------------------------------------
We specialize in UNIQUE items! Coins shrunk by huge magnetic fields,
Lichtenberg Figures ("Captured Lightning" in acrylic), & Out-of-Print
technical Books. Stoneridge Engineering - http://www.teslamania.com
---------------------------------------------------------------------

Tesla list wrote:

Original poster: Grishka <ghome@xxxxxxxxx>
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.
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."
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?
-Does it overheat a lot? What are the practical values for the dielectric loses in your caps?
Maybe people, who works with high-power RF applications can give some advices about using capacitors
in these devices?