Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx> Hi,The inserted tab metal is also aluminum... Aluminum quickly (hours) forms a thick aluminum oxide layer which is an excellent insulator. So if it has been sitting around the factory for awhile, it is basically "insulated". Aluminum oxide is also rather porous and can absorb oil to help it insulate as well. It works fine still at 20 amps, but hit it with 2000 amps of RF and watch it blow like a fountain firework into a beautiful spray of sparks >;) If they really do a bad job, the connecting aluminum strip will also see oil slosh or mechanical movement. So after a while the connections just fatigue and break off anyway... Aluminum has NO fatigue limit!! If it bends, it WILL fail... Of course, the casual contact may have never been good enough to carry high RF currents in the first place...
I have seen a few blown up caps and the interconnects have been really poor... They try so hard to make the cap so well, and then they tuck an aluminum strip in it and call it connected... They only saved like 50 cents in a $1000 part.... Of course, sometimes the caps fail the first time and then they have to try and "explain" it to a room full of mad engineers.... The company fixes it on the new batch and has to take back all the old defective batch. Guess were the defects get sold ;-)) You are much better off getting a "used" cap that did work for a long time, rather than a brand new (never tried) in that case... If they are selling brand new, never used $1000 caps for $5, be very afraid...
These aluminum problems also burnt down houses due to the infamous aluminum house wiring thing...
Of course, you can get some of these caps very cheap on E-bay since they are periodically replaced and sold as high class scrap... They may work fine, but as Bert says, be ready for them to blow!! If they were still great caps, they would not sell so cheap....
Note that microwave caps and such are usually in a metal can that can really give it a "BANG" when it blows up too!!! The oils all burn as oil does too, so fire is a concern. If the old cap blows and the oil is NOT flammable, then your REALLY screwed =:O
This is why so many just make nice MMCs now-a-days :-)))
Cheers
Terry
At 07:27 AM 1/18/2006, you wrote:
Dave and Ed,You make a good point! Inexpensively constructed capacitors often use inserted tabs. These tabs are simply mechanically pressed against the foil instead of being firmly soldered to the ends of the roll as in extended foil caps. The electrical connection to the plates is definitely a weak link in these caps, even if the PP dielectric system is sound.Inserted tab capacitors have had a dismal history in pulsed power use for both high current and fast pulse repetition applications. Due to overheating, gas formation, and arcing at the tab-plate connections, they have a reduced lifetime in high current and fast pulse repetition applications.But these capacitor are often cheap, or free, for the beginning Tesla Coiler. The tank current on smaller coils may not be sufficient to induce failure, or the caps may fail after 10's or 100's of hours. I certainly wouldn't suggest they be used on a higher power system. And, since the failure mode may be spectacular, putting them into a container to contain the pieces and oil might be a good idea. :^)Bert Tesla list wrote:Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx> Hi Bert, Do you think that the newer PP dielectric MOT caps would still be suitable for Tesla coil use, consideringthat the internal connection may still be quite fragile and not of the extended end foil type connections,since they are still basically desinged for filtering and not pulse discharge? David Rieben ----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx> To: <tesla@xxxxxxxxxx> Sent: Tuesday, January 17, 2006 5:57 PM Subject: Re: Microwave caps.Original poster: Bert Hickman <bert.hickman@xxxxxxxxxx> Hi Christopher,It really depends on the capacitor. It turns out that some newer microwave caps use a polypropylene (PP) dielectric system (GOOD!). However, many older ones use Polyester (Mylar - BAD!). While Mylar caps will rapidly overheat when used in a tank circuit and possibly explode, polypropylene caps should work just fine. Unfortunately, there's no way to tell which is which without testing, dissection, or getting the information from the manufacturer.Best regards, Bert Tesla list wrote:Original poster: "Christoph Bohr" <cb@xxxxxxxxxxxxxxx> Hallo Frosty > is is possible to use a whole lot of microwave capacitors in series yes > to bring up the voltage rating and bring down the capacitance to a > useful value? depends on what you call usefull. You get a HV cap for sure, but it will most likely not be useable as tank cap in a TC. The dielectric can't deal with the high frequencies and the caps might blow up from heating. Additionally the connections inside the cap might be to weak for the large currents involved. However, a fellow coiler ,Alex Boeckeler, proved me wrong, he used such a construction and got away with it. But I doubt this is the general case.... > if so this would be a really cheap alternative for > capacitors. where i live i can get just about as many old microwaves > as i want (i suppose i would need lots if each capacitor is about 1-2 > uF, supposing all 1 uF you wold need at least 32) Ideally they would all be of the same value the spread the voltage stress evenly. > has anyone done this before, wouuld just like to know before i go > out and find 32 old microwaves! sound like free workout.... I guess even a 32-MOT-Stack would sound like a better idea ;-) good luck Christoph Bohr