[Elecraft] Capacitors...simple yet complex question

[Lyle Johnson]

Hello Tim!

If you look up capacitors in the Handbook, or any basic reference, you'll
see that the capacitance is related to the area of the "plates", the
dielectric constant of the insulator between them, and the thickness of the
dielectric.

What we're generally seeing is improving technology providing higher
dielectric constants and thinner insulation. This leads to smaller parts
with essentially the same ratings as earlier, larger parts.

But often the chemistry that makes this work has other issues.  For example,
early tantalum caps could fail spectacularly.  And many times the material
inside "electrolytic" caps had to be kept wet.  As it dried out, the part
would lose capacitance. (For example, in man of the electrolytic caps in
AO-40, we sealed the rubber seal on the cap with a drop of high-vacuum-rated
silicone to prevent drying out the electrolyte in a hard vacuum.)

Temperature ratings play a big part here, and parts rated to work at higher
temps often are better sealed, or the electrolyte has a higher "boiling
point."  Often, really small parts have limited lifetime ratings.  Look
through your digi-key catalog (or the on-line PDF version) and note the
differences in sizes and price and temps and lifetime specs for the various
capacitors.

As pointed out in an earlier post, there are other factors to consider, such
as "equivalent series resistance" (ESR) which plays an important role in
switching supplies or high current supplies.  Leakage current is another
factor that may play a part in a particular circuit.  It turns out for PLL
loop filters you often have to use a polystyrene or polyester cap rather
than a ceramic cap to keep noise down.

Finally, like any real world part, a capacitor is also a set of resistors
(each lead is a series resistor, and there is a parallel resistor -
leakage - across the plates), a pair of inductors (each lead, even in a tiny
surface mount part) as well as the capacitor itself.  Sometimes you can use
these parasitic elements to your advantage; usually they sneak up behind you
and catch you when you aren't looking (think of Golem in the Two Towers).

73,

Lyle KK7P