Practical considerations
Capacitors, like all electrical components,
have limitations which must be respected for the sake of
reliability and proper circuit operation.
Working voltage: Since capacitors are
nothing more than two conductors separated by an insulator
(the dielectric), you must pay attention to the maximum
voltage allowed across it. If too much voltage is applied,
the "breakdown" rating of the dielectric material may be
exceeded, resulting in the capacitor internally
short-circuiting.
Polarity: Some capacitors are
manufactured so they can only tolerate applied voltage in
one polarity but not the other. This is due to their
construction: the dielectric is a microscopically thin layer
if insulation deposited on one of the plates by a DC voltage
during manufacture. These are called electrolytic
capacitors, and their polarity is clearly marked.
Reversing voltage polarity to an
electrolytic capacitor may result in the destruction of that
super-thin dielectric layer, thus ruining the device.
However, the thinness of that dielectric permits extremely
high values of capacitance in a relatively small package
size. For the same reason, electrolytic capacitors tend to
be low in voltage rating as compared with other types of
capacitor construction.
Equivalent circuit: Since the plates
in a capacitors have some resistance, and since no
dielectric is a perfect insulator, there is no such thing as
a "perfect" capacitor. In real life, a capacitor has both a
series resistance and a parallel (leakage) resistance
interacting with its purely capacitive characteristics:
Fortunately, it is relatively easy to
manufacture capacitors with very small series resistances
and very high leakage resistances!
Physical Size: For most applications
in electronics, minimum size is the goal for component
engineering. The smaller components can be made, the more
circuitry can be built into a smaller package, and usually
weight is saved as well. With capacitors, there are two
major limiting factors to the minimum size of a unit:
working voltage and capacitance. And these two factors tend
to be in opposition to each other. For any given choice in
dielectric materials, the only way to increase the voltage
rating of a capacitor is to increase the thickness of the
dielectric. However, as we have seen, this has the effect of
decreasing capacitance. Capacitance can be brought back up
by increasing plate area. but this makes for a larger unit.
This is why you cannot judge a capacitor's rating in Farads
simply by size. A capacitor of any given size may be
relatively high in capacitance and low in working voltage,
visa-versa, or some compromise between the two extremes.
Take the following two photographs for example:
This is a fairly large capacitor in physical
size, but it has quite a low capacitance value: only 2 �F.
However, its working voltage is quite high: 2000 volts! If
this capacitor were re-engineered to have a thinner layer of
dielectric between its plates, at least a hundredfold
increase in capacitance might be achievable, but at a cost
of significantly lowering its working voltage. Compare the
above photograph with the one below. The capacitor shown in
the lower picture is an electrolytic unit, similar in size
to the one above, but with very different values of
capacitance and working voltage:
The thinner dielectric layer gives it a much
greater capacitance (20,000 �F) and a drastically reduced
working voltage (35 volts continuous, 45 volts
intermittent).
Here are some samples of different capacitor
types, all smaller than the units shown previously:
The electrolytic and tantalum capacitors are
polarized (polarity sensitive), and are always
labeled as such. The electrolytic units have their negative
(-) leads distinguished by arrow symbols on their cases.
Some polarized capacitors have their polarity designated by
marking the positive terminal. The large, 20,000 �F
electrolytic unit shown in the upright position has its
positive (+) terminal labeled with a "plus" mark. Ceramic,
mylar, plastic film, and air capacitors do not have polarity
markings, because those types are nonpolarized (they
are not polarity sensitive).
Capacitors are very common components in
electronic circuits. Take a close look at the following
photograph -- every component marked with a "C" designation
on the printed circuit board is a capacitor:
Some of the capacitors shown on this circuit
board are standard electrolytic: C30 (top of
board, center) and C36 (left side, 1/3 from the
top). Some others are a special kind of electrolytic
capacitor called tantalum, because this is the type
of metal used to make the plates. Tantalum capacitors have
relatively high capacitance for their physical size. The
following capacitors on the circuit board shown above are
tantalum: C14 (just to the lower-left of C30),
C19 (directly below R10, which is
below C30), C24 (lower-left corner of
board), and C22 (lower-right).
Examples of even smaller capacitors can be
seen in this photograph:
The capacitors on this circuit board are
"surface mount devices" as are all the resistors, for
reasons of saving space. Following component labeling
convention, the capacitors can be identified by labels
beginning with the letter "C". |