FABLE OF A FARAD
Of all the various
electronic components, capacitors seem to come in the greatest variety of sizes
and shapes, perhaps because of the shortcomings of each type. Correct capacitor selection, both in original
circuit construction and for repair replacements, can be vital for proper
device performance.
Aside from proper voltage
and current ratings, and physical size and shape, capacitors can be
distinguished by their dielectric material.
It’s the dielectric that largely determines the capacitor’s characteristics.
Let me show you what I
mean: if you need a filter capacitor for
a power supply, you might need a unit of a few hundred microfarads, with good
current carrying capacity. Most likely,
just because of issues of size and weight, you’ll end up with an aluminum
electrolytic capacitor. Be sure to pay
attention to the amount of current cycling in and out of the capacitor, as well
as providing for some voltage margin. As
far as packaging is concerned, be aware that manufacturers are stressing radial
capacitor production, so axial lead units are getting harder to locate, and
their prices are running up quickly.
Now your electrolytic cap
packs a lot of capacity in a small space, but performance is generally
optimized for 120 Hz or so, the internal resistance and inductance may be quite
high, and the device is generally unipolar. So-called computer-grade electrolytics
allow higher circulating ripple currents and offer lower equivalent series
resistance (ESR). An electrolytic cap is
fine for a DC power supply filter; not so great if RF or transient performance
is important. And its value isn’t
stable, so forget about using it in a tuned circuit of any kind. Dipped tantalum and solid tantalum caps are
similar to aluminum electrolytics, but offer some
performance improvements in density and ESR.
The oil-filled capacitor
uses oil-impregnated paper for a dielectric, often in a metal can outfitted
with leads. These are used where you’d
like to use an electrolytic cap, but can’t because the voltage is too high, or
you need an AC device (phase delay for AC motor windings, and power factor
correction for AC motor loads, to offer two examples). You should assume any of these manufactured
before the mid-80’s is impregnated with PCB-bearing
oil, and so must be tagged and disposed of properly upon failure to avoid legal
and environmental issues. Their modern
replacements look similar but use mineral oil or mylar in their dielectrics to protect the environment
from dioxins
and furans.
Ceramic or monolithic caps
are small and inexpensive, and good for non-critical bypass and filter
applications. Often you’ll find them in
parallel with electrolytics in power supplies, to
smooth out transients that are too quick for treatment by electrolytics. But their stability can be even worse than
the electrolytics, so you can’t use them in any
precision applications. An exception is
the so-called plate ceramic cap, which is quite stable, but 99.9% of ceramic
caps shouldn’t be used when you’re timing or tuning.
Polystyrene and polyethylene
caps do offer precision and stability, but they can be bulky and are, by their
nature, inductive. Good for tuning, but
not at RF frequencies. Mylar and
polyester caps can be used for moderate precision, and they offer good
stability and fairly low inductance. The
granddaddy of stability for capacitors is the silvered mica cap, which is great
for RF and precision applications, but you may find it quite expensive and hard
to obtain.
Of course, in broadcasting,
we often run into high power RF applications, and so we use a few types of
capacitors that aren’t seen much elsewhere:
there are high power versions of the ceramic and mica caps, and then
there are the vacuum and vacuum ceramic types: the only types even more expensive than
silvered mica.
Often an RF capacitor can be
improvised out of available materials for a particular use: the so-called plate blocker in a tube
transmitter is often a Teflon or Mylar sheet, used as a bypass capacitor at the
plate of a PA tube to shunt parasitics, harmonics and
other electronic miscreants and troublemakers to ground right at their source.