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Old russian and NOS RF power capacitors (teflon, mica)

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Marko

Mon Oct 17 2011, 09:04PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys,

I'm searching for caps to use in my Big Bad SSTC in CW and possibly QCW mode (series resonant drive). So far I know people have used large transmitting mica doorknobs, but I'm looking for cheaper and easier to get alternatives. I have a geek cap MMC that will be suitable for tests, but I wanted something less bulky as I don't need a 12kV cap there.

For a while I was interested into those russian teflon doorknob caps that appear around ebay. Has anyone had experience with those? (perhaps used in SGTC/VTTC/DRSSTC/other?)

They seem like very superior construction caps, very low loss and high temperature dielectric along with aluminum foil electrodes... They are not self healing but I'm not going to push their voltage ratings. I'd need a 100nF cap that can stand 100-200 amps and about 2.5kV peak, and I wondered if that is attainable by stacking a number of those caps in parallel?

Some examples are

or these

Perhaps it would be wiser to stack 10 of 10nf caps than four 25nf's.

And now, what about mica caps? Those that look like small brown boxes and similar shapes.

Would need a whole bunch of them to get up to required ratings.

Marko

Matt Edwards

Tue Oct 18 2011, 12:27AM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

CDE 940, 941, or 942 series...

FYI, I destroyed a 20Kv Russian Teflon cap in about 5 minutes with a 5kv NST SGTC. Just something to ponder.

Marko

Thu Oct 20 2011, 01:08AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hm, that sounds like bad news. Though I've found out Eric Goodchild apparently used a teflon cap succesfully in a QCW coil!

It seems like teflon tends to have microscopic holes and tends to fail from overvoltage easily, and due to solid foil construction the cap is ruined after one puncture - no self healing capability!

They also seem to get hot in high current use which I find odd considering their low loss dielectric and electrodes.

Anyone used them in a CW application perhaps? I'd make sure not to overrate them by using 8kV caps for 2kV or so peak.

Marko

Proud Mary

Thu Oct 20 2011, 10:26AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Perhaps you should think of using high power tubular ceramic RF capacitors with a higher KVA rating, for example only, as to 40KVA tubular type:

As heat is dissipated from both the inner and outer surfaces of the cylinder, thermal stress is reduced.

PS: I would not myself call this a "doorknob" capacitor, so it's always helpful to study pictures of products offered for sale from Russia, and her lost empire.

Marko

Thu Oct 20 2011, 11:06AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Proud Mary wrote ...

Perhaps you should think of using high power tubular ceramic RF capacitors with a higher KVA rating, for example only, as to 40KVA tubular type:

Hi,

those were exactly the very first caps I considered (along with normal RF doorknobs). The only problem is capacitance, they only go up to few nF and I'd need way too many for a 100nF bank.

At that time I intended to use a high inductance primary, the cost of which turned out to be huge and am not sure how to fit it into my current design. I'm thinking of keeping my original primary now and use it with a ferrite transformer between it and inverter, at least for now.

Proud Mary

Thu Oct 20 2011, 11:53AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Friend Gintaras in Lithuania has got some of these, but I wouldn't like to guess at their high frequency performance:

Marko

Thu Oct 20 2011, 01:03PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Proud Mary wrote ...

Friend Gintaras in Lithuania has got some of these, but I wouldn't like to guess at their high frequency performance:

Hm, PIO caps?
I didn't even consider that type for this use. I thought they would be kinda like MOC's or even worse, not sure how would this single cap fare with 100A+ current. Lots of much better made pulse caps can't fare in TC duty well compared to MMC's.

Hence I thought about a MMC of several smaller caps to spread out the current and power dissipation.

Apparently teflon caps suck in SGTC duty, but I'd want to find out more how they fare in CW and QCW duty. If Eric's cap performed without dying after a while it might be a worthy try.

PS. when we're about lossy caps already, these are quite cheap

I have no idea about their intended purpose, but I think they're for DC filtering and according to TCML they tend to boil and turn into glass shrapnel bombs... ok, not sure if I'd use those.

Marko

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