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.
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