Re: Microwave caps.

["Tesla list" <tesla@xxxxxxxxxx>]

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

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, considering
> that 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