[Elecraft] Capacitors...simple yet complex question

[Timothy A. Raymer]

Thanks to all who responded to my question.

Season's Greetings!

Tim Raymer
73 de KA0OUV

At 09:46 12/24/2002 -0800, you wrote:
>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
>
>
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73 de KA0OUV

Timothy A. Raymer       <traymer@mail.state.mo.us>
                         <raymet@dhss.state.mo.us>
                         <KA0OUV@arrl.net>