Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>
Hi Dan, all,
I'd like to offer a warning about _bead_ tantalums in
particular:
On 6 Dec 2004, at 21:41, Tesla list wrote:
> Original poster: "Daniel McCauley"
> <dhmccauley@xxxxxxxxxxxxxxxxxxxxxxxxxx>
>
> > Tantalum capacitors are not supposed to be RF coupling
> > capacitors. They are made for DC and are polarised. I am sure that
> I put > them in the right way, but even if you do put them in right,
> they will be > in backwards 100,000 times a second (or whatever your
> resonance happens to > be)!
>
> If you look at the components most of us use for tesla coiling, many
> components are being used for applications above and beyond of what
> they were designed for. Take those IGBTs for instance. They were
> never meant to take the peak currents we are pushing through them.
> The tantalums were used simply for convenience and size. The parts
> list is simply a guideline. Use whatever parts you feel is necessary
> in your particular design. I used tantalums because that is what i
> had available and through many hours of testing, they have neither
> felt warm, nor failed - so i kept them as is.
>
> Dan
I have lost count of the number of times I have replaced these
because they have failed, usually low-R or shorted for no obvious
reason. That is, they have not been subjected to reverse polarities
or operated at anything like their rated working voltages. The
majority of replacements have been in places where they are or have
been commonly used such as across 3-T regulators. I simply cannot
explain it but green dipped and blue dipped ones in particular seem
to be the least reliable although I have replaced the odd yellowish
ones on occasions and from memory, at least two red dipped ones.
These days, the only designs I would contemplate using them in
are ones where there is no simple substitute - e.g. long time
constant circuitry where high resistances are used as the other part
of the timing element and low leakage is a must AND lives and other
equpiment was not at risk. If I was in the business of designing
medical equipment, spacecraft or avionics, their use would not even
be contemplated. FWIW, mini-electrolytics and/or mini ceramic types
work just as well in the regulator applications.
Malcolm