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Re: How could a pulse cap operate in TC?

Original poster: "Mike" <induction@xxxxxxxxxxx>

Hi Chris,
You thought about placing them in sand to help make the area near the caps safer. We have a couple of companies that take in our old HV rectifier tubes, at work, for the bases. Such as the 872A, 575A, even the larger 6693. They get rid of the mercury at their end and using the bases, empty forms, they take strings of diodes to the proper PIV and current, fit them the best possible for reduced arc over, fill the empty space with sand, add a plate type cap on top, connect the diode string to one prior filament pin, pot it together and you have a solid state plug in, 575 family 25 KPIV 2 amps C.C.S, more for the 6693.
Same for the diode sticks. But, the largest problem is getting rid of the heat because of that sand. The usual limit is 2 to 4 amps in sand.
After that, we go for open diode stacks, 5 or 10 amps per section and air heat sinks. 6 needed for 3 phase. With so many clients switching to SCR control from thyratrons and doing a full solid state rectifier conversion, either sticks or plug in, it's worth the bother to get these stacks built. But always, we have to keep in mind 25 KW and lower induction heaters are OK (tube type) for the plug in rectifier mods, above that it is best to use the open diode stacks.
Regards,
Mike
----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, July 13, 2005 4:38 PM
Subject: Re: How could a pulse cap operate in TC?


Original poster: "Chris Rutherford" <chris1@xxxxxxxxxxxxxxxxxxx>

Hi,

I'm using the recommended RF LRC capacitors, apparently they're good and I've not had any problems with them yet (I am operating at 1/2 rated DC voltage to be on the safe side). The operating parameters can be seen in this data sheet.

http://www.farnell.com/datasheets/43511.pdf

I have got 10 microwave capacitors that I keep on thinking about using. Rated at 1.3uF and 2500V AC I could have a whopping 25000VAC 130nF (7 Joules at 11KV, some 13 times greater than what I'm using now). The problem is they're likely to explode because they are designed to work at 50Hz. But rated at 25000AC and used at 11KV who knows...Anyone?

I'm planning on putting them in a plastic box filled with sand, so if they do go, then the sand will disperse shock wave in theory. But then how well would the sand help keep them cool...?

Any info on MMOCs (Multi Microwave Oven Capacitors) would be greatly appreciated.

Good question by the way.

Thanks

Chris


----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, July 13, 2005 7:28 PM
Subject: How could a pulse cap operate in TC?


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?




--
Best regards,
 Grishka                          mailto:ghome@xxxxxxxxx