The Capacitor
Corona – Corona is the ionization of air or other vaporswhich 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. Insulation Resistance – Insulation Resistance is theresistance measured across the terminals of a capacitor
and consists principally of the parallel resistance R Dielectric Strength – Dielectric Absorption – Dielectric Strength is an expressionof 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. A capacitor does not dischargeinstantaneously 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.
I (Ideal)I (Actual)PhaseAngleLossAngleVIR P shownin 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).
f
s In practice the current leads the voltage by some otherphase angle due to the series resistance R
S . The comple-ment of this angle is called the loss angle and:Power Factor (P.F.) = Cos Dissipation Factor (D.F.) = tan
f
or Sine for small values of the tan and sine are essentially equalwhich has led to the common interchangeability of the two
terms in the industry. Equivalent Series Resistance – The term E.S.R. orEquivalent 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. Dissipation Factor The DF/PF of a capacitor tells what percent of the apparent power input will turn to heat in the capacitor.
Multilayer ceramic
supplier of 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.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.Multilayer ceramic capacitors are available in a wide range ofcharacteristics, Electronic Industries Association (EIA) and
the military have established categories to help divide the
E.S.R.C Dissipation Factor = E.S.R. = (2 π fC) (E.S.R.)X
C The watts loss are: Watts loss = (2 π fCV
2 ) (D.F.) Very low values of dissipation factor are expressed as theirreciprocal for convenience. These are called the “Q” or
Quality factor of capacitors.
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