Capacitor quirks
As with inductors, the ideal capacitor is a
purely reactive device, containing absolutely zero resistive
(power dissipative) effects. In the real world, of course,
nothing is so perfect. However, capacitors have the virtue
of generally being purer reactive components than
inductors. It is a lot easier to design and construct a
capacitor with low internal series resistance than it is to
do the same with an inductor. The practical result of this
is that real capacitors typically have impedance phase
angles more closely approaching 90o (actually,
-90o) than inductors. Consequently, they will
tend to dissipate less power than an equivalent inductor.
Capacitors also tend to be smaller and
lighter weight than their equivalent inductor counterparts,
and since their electric fields are almost totally contained
between their plates (unlike inductors, whose magnetic
fields naturally tend to extend beyond the dimensions of the
core), they are less prone to transmitting or receiving
electromagnetic "noise" to/from other components. For these
reasons, circuit designers tend to favor capacitors over
inductors wherever a design permits either alternative.
Capacitors with significant resistive
effects are said to be lossy, in reference to their
tendency to dissipate ("lose") power like a resistor. The
source of capacitor loss is usually the dielectric material
rather than any wire resistance, as wire length in a
capacitor is very minimal.
Dielectric materials tend to react to
changing electric fields by producing heat. This heating
effect represents a loss in power, and is equivalent to
resistance in the circuit. The effect is more pronounced at
higher frequencies and in fact can be so extreme that it is
sometimes exploited in manufacturing processes to heat
insulating materials like plastic! The plastic object to be
heated is placed between two metal plates, connected to a
source of high-frequency AC voltage. Temperature is
controlled by varying the voltage or frequency of the
source, and the plates never have to contact the object
being heated.
This effect is undesirable for capacitors
where we expect the component to behave as a purely
reactive circuit element. One of the ways to mitigate
the effect of dielectric "loss" is to choose a dielectric
material less susceptible to the effect. Not all dielectric
materials are equally "lossy." A relative scale of
dielectric loss from least to greatest is given here:
Vacuum --------------- (Low Loss)
Air
Polystyrene
Mica
Glass
Low-K ceramic
Plastic film (Mylar)
Paper
High-K ceramic
Aluminum oxide
Tantalum pentoxide --- (High Loss)
Dielectric resistivity manifests itself both
as a series and a parallel resistance with the pure
capacitance:
Fortunately, these stray resistances are
usually of modest impact (low series resistance and high
parallel resistance), much less significant than the stray
resistances present in an average inductor.
Electrolytic capacitors, known for their
relatively high capacitance and low working voltage, are
also known for their notorious lossiness, due to both the
characteristics of the microscopically thin dielectric film
and the electrolyte paste. Unless specially made for AC
service, electrolytic capacitors should never be used with
AC unless it is mixed (biased) with a constant DC voltage
preventing the capacitor from ever being subjected to
reverse voltage. Even then, their resistive characteristics
may be too severe a shortcoming for the application anyway. |