capacitor, as in Figure 2. This is actually a tuning capacitor
often found in old tube radios. This one has four sections
ganged together yielding, in effect, four capacitors that
you tune in unison.
A dielectric is a nonconductor of electricity, especially
a substance with electrical conductivity of less than a
millionth ( 10-6) of a Siemens. A Siemens describes electrical
conductance and is equal to one ampere per volt.
Dielectric constant and relative permittivity are synonyms
and measure a material’s ability to resist the formation of an
electric field within it.
FIGURE 2. An air core capacitor that allows a
better ‘visualization” of factors constituting a
Charge in Coulombs represents stored energy and is
proportional to the applied voltage and the capacitor’s
Charge is: Q = C x V Formula 1
where Q = charge in Coulombs, C = capacitance in
Farads, and V = voltage in volts.
Energy is: W = E2 x C / 2 Formula 2
where W = energy in Joules, C = capacitance in Farads,
and E = voltage in volts.
The capacitor’s dielectric largely determines its
behavior. Capacitor dielectrics fall into two broad
categories: bulk insulators and metal-oxide films; the
latter is synonymous with electrolytic capacitors. That is
why you will often see capacitors described by their
dielectrics, such as ceramic, mica, mylar, solid tantulum,
or electrolytics. You’ll find vast differences in such things
as capacitance for a given size, leakage (the conductivity
of the dielectric), capacitance versus frequency, voltage
and current ratings, variations with temperature, and
polarized and non-polarized capacitors.
Leakage current through the dielectric is usually very
minute and you can usually ignore it. Since the resistance
of the dielectric is extremely high, you can safely assume
the dielectric effectively prevents current flow through the
capacitor. However, if the leakage current appreciably
increases, a rapid loss of charge and overheating occur.
Equivalent Series Resistance (ESR)
This important capacitor parameter is the effective
resistance that is not part of the capacitor’s pure
capacitance. Its principal sources are the capacitor’s
internal lead and plate resistances. A low capacitor ESR of
0.01 ohms is ideal for high current pulse applications that
deliver huge currents into very low impedance circuits
(which can be dangerous). A photo flash is a good
example. Figure 3 shows a circuit at turn-on driving
a capacitor; the irregularity seen on the scope’s trace
represents the capacitor’s ESR.
The effect of a capacitor’s ESR as viewed
on a scope, with a
circuit just being
on showing the
You can measure an aluminum electrolytic capacitor’s
ESR as the resistance of the equivalent series circuit in
a measuring bridge supplied by a 120 Hz source free
of harmonics, with a maximum AC signal voltage of 1V
RMS and no forward-bias voltage (see Figure 4). Since
a capacitor passes AC only, the bridge will only detect
its resistance or ESR. A full wave rectified bridge yields
this 120 Hz.
Equivalent Series Inductance (ESL)
The causes of capacitor ESL are the same culprits as
in ESR. For any capacitor, there is a frequency at which
it ceases to behave as pure capacitance. As we will soon
discover, inductors purposely oppose changes in current.
Decoupling capacitors are a must for high speed logic
circuits. Without them, the IC demands current faster than
the power supply can supply it under rapidly switching