26 May
2003
26 May
'03
5:31 p.m.
If you could humor my electronic ignorance, what
exactly IS a tantalum
cap and how does it differ from a normal capacitor? I was never before
aware that there could be more to a capacitor than plates (or foil) and
an electrolyte. How can a capacitor have a polarity?
A capactior is actually 2 plates separated by an insulator, called the
dielectric. In the case of something like a mylar capacitor (or one of
the other 'plastic' ones), the dielectric is a thin piece of mylar with
the metal films (for the plates) deposited on the 2 sides. The whole
thing is then rolled up to get it into a small enough package.
In an electrolytic capacitor (there are 2 common types, aluminium and
tantalum), the 2 plates are a metal plate and the electrolyte. The
dielectric is an oxide film on the surface of the plate. This film is
very thin, which meance you can get a large capacitance in a small
package _but_ it will also redisolve into the electrolyte, especially if
the capacitor is wired up backwards. That's why these units are
polarised. Connect them backwards and the dielectric disolves, the
capacitor becomes a short circuit. If enough current can then flow, the
electrolyte might even boil with the consequent explosion of the can!
Electrolytic caps fail in 2 main ways. Firstly the dielectric might
redisolve in the electrolyte, causing a short. Secondly the electrolyte
can evapourate ('dry up'), inceeasing the effective series resistance
(ESR) of the capacitor.
-tony