The Capacitor
Insulation Resistance – Insulation Resistance is the resis-
tance measured across the terminals of a capacitor and
consists principally of the parallel resistance RP shown in
the equivalent circuit. As capacitance values and hence the
area of dielectric increases, the I.R. decreases and hence
the product (C x IR or RC) is often specified in ohm farads
or more commonly megohm microfarads. Leakage current
is determined by dividing the rated voltage by IR (Ohm’s
Law).
I (Ideal)
I (Actual)
Loss
Phase
Angle
Angle
␦
f
Dielectric Strength – Dielectric Strength is an expression
of the ability of a material to withstand an electrical stress.
Although dielectric strength is ordinarily expressed in volts,
it is actually dependent on the thickness of the dielectric
and thus is also more generically a function of volts/mil.
V
IRs
In practice the current leads the voltage by some other
phase angle due to the series resistance RS. The comple-
ment of this angle is called the loss angle and:
Dielectric Absorption – A capacitor does not discharge
instantaneously upon application of a short circuit, but
drains gradually after the capacitance proper has been dis-
charged. It is common practice to measure the dielectric
absorption by determining the “reappearing voltage” which
appears across a capacitor at some point in time after it
has been fully discharged under short circuit conditions.
Power Factor (P.F.) = Cos or Sine ␦
Dissipation Factor (D.F.) = tan ␦
f
for small values of ␦ the tan and sine are essentially equal
which has led to the common interchangeability of the two
terms in the industry.
Corona – Corona is the ionization of air or other vapors
which causes them to conduct current. It is especially
prevalent in high voltage units but can occur with low
voltages as well where high voltage gradients occur. The
energy discharged degrades the performance of the
capacitor and can in time cause catastrophic failures.
Equivalent Series Resistance – The term E.S.R. or
Equivalent Series Resistance combines all losses both
series and parallel in a capacitor at a given frequency so
that the equivalent circuit is reduced to a simple R-C series
connection.
CERAMIC CAPACITORS
E.S.R.
C
Multilayer ceramic capacitors are manufactured by mixing
the ceramic powder in an organic binder (slurry) and cast-
ing it by one technique or another into thin layers typically
ranging from about 3 mils in thickness down to 1 mil or
thinner.
Dissipation Factor
The DF/PF of a capacitor tells what percent of the
apparent power input will turn to heat in the capacitor.
Metal electrodes are deposited onto the green ceramic
layers which are then stacked to form a laminated
structure. The metal electrodes are arranged so that their
terminations alternate from one edge of the capacitor to
another. Upon sintering at high temperature the part
becomes a monolithic block which can provide extremely
high capacitance values in small mechanical volumes.
Figure 1 shows a pictorial view of a multilayer ceramic
capacitor.
E.S.R.
Dissipation Factor =
= (2 π fC) (E.S.R.)
XC
The watts loss are:
2
Watts loss = (2 π fCV ) (D.F.)
Very low values of dissipation factor are expressed as their
reciprocal for convenience. These are called the “Q” or
Quality factor of capacitors.
Multilayer ceramic capacitors are available in a wide range of
characteristics, Electronic Industries Association (EIA) and
the military have established categories to help divide the
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