■ CAPACITOR FUNDAMENTALS
electrodes and leads
Leaks some charge
of the stored energy
FIGURE 1. The model for a real capacitor includes inductance
and loss that affect its performance.
Acapacitor is just a pair of conducting plates (
electrodes) that are separated by an insulator (dielectric). When
voltage is applied, electrons are forced onto one plate and
removed from the other, charging the capacitor and creating an electric field between the electrodes. The electric
If your application requires a capacitor to connect
directly to the AC power line (such as filtering), select a
capacitor rated specifically for that use. These capacitors
are rated by a safety agency such as Underwriter Labs
(UL), CSA (Canada), or VDE (Europe) for “across the line”
use. They are designed to be “fail safe” and not cause hazards to equipment users. The additional cost is minimal.
field’s energy is stored in the capacitor’s dielectric. All
capacitor types are just variations on this general theme.
Capacitance is the measure of the amount of energy
that the capacitor stores for a given amount of charge
and voltage. The area of the electrodes and the material
used for the dielectric determine the capacitor’s ability to
store energy. More area or a thinner dielectric increases
capacitance. Like resistors, a capacitor’s value has a
certain precision that shows how close the capacitance
must be to the labeled or nominal value.
DC will not flow between the two electrodes as long as
the dielectric material can withstand the applied voltage.
AC is considered to flow between the plates as the electrons flow onto and off of the electrodes. Energy is stored
and removed from the dielectric with each half-cycle. Like
juggling two balls from hand to hand, there is a lot of energy transferred, but no net change. As the capacitor
charges, the building voltage opposes the flow of additional charge. This opposition to current flow is called reactance and is abbreviated XC. (XL is inductive reactance.)
The materials used to make the capacitor and their
physical configuration cause some variation from the ideal
capacitor. The size of the electrodes and the leads used to
connect them to circuits introduce a small amount of parasitic inductance. Dielectric materials dissipate a small
amount of the stored energy. There is also a little leakage
current between the electrodes whenever voltage is present.
These extra and unwanted effects are shown in Figure
1 as the model of a real capacitor. The parasitic inductance
(called ESL for Equivalent Series Inductance) is represented by LS, leakage by resistor RP, and dissipation by resistor
RS. (The ‘s’ and ‘p’ stand for series and parallel.) This model
works well enough to represent capacitors over most frequencies and for all but the most demanding applications.
Parasitic inductance is quite small, from picohenries
to a few nanohenries. At DC and low frequencies, LS can
NUTS & VOLTS
Make a Perfect Capacitor
There is no one perfect capacitor type for all applications,
but you can make several types share their best characteristics. For example, power supplies for digital electronics
need filters that take out everything from low-frequency
ripple all the way up to switching transients at hundreds of
MHz. Since no single capacitor is suitable, the “perfect”
capacitor is made as shown in Figure 2. The electrolytic
takes care of low-frequency ripple, the tantalum medium
frequencies, and finally the ceramic cap takes care of everything up into the VHF range.
FIGURE 2. The “perfect capacitor” is made by combining the best
characteristics of three different types of capacitor.
cera m ic