TECHNICAL TERMS
Capacitance:
A measure of the energy storage
ability of a capacitor, given as C = K A/D, where A is the area
of the electrodes, D is their separation, and K is a function of
the dielectric between the electrodes. The formula yields a result in farads (F
), but a farad is so large that the most commonly used values are expressed
in microfarads ( µf = 10-6F ) or picofarads ( pf = 10-12F
).
Working voltage (Wvdc,
Wvac):
The maximum continuous voltage that
should be applied to a capacitor. Rated voltages for DC and AC operation are
usually not the same.
Temperature Coefficient
(TC):
The change in capacitance with
temperature expressed linearly as parts per million per degree centigrade
(PPM/°C), or as a percent change over a specified temperature range. Most film
capacitors are not linear and TC is expressed in percent.
Dissipation
Factor (DF):
A measure of the power factor (or
losses) of a capacitor, given as DF = 2 P fRC X 100%, where R
is the equivalent series resistance of the capacitor, f is the
frequency, and C is capacitance. Dissipation factor varies with
frequency and temperature.
Equivalent Series
Resistance (ESR):
A measure of the total lossiness of a
capacitor which includes the leads, electrodes, dielectric losses, leakage (IR)
and most important, the end spray connecting the leads to the metallized film.
The lower the ESR the higher the current carrying ability the capacitor will
have.
Insulation Resistance
(IR):
A measure of the resistance to a DC current
flow through the capacitor under steady state conditions. Values for film and
ceramic capacitors are usually expressed in megohm-microfarads for a given
design and dielectric. The actual resistance of the capacitor is obtained by
dividing the megohm-microfarads by the capacitance.
Dielectric
Absorption (DA):
An apparent "recovery
voltage" measured after the capacitor is discharged and expressed as a
percent of the initial charge voltage. DA is due largely to the dipole moment
of the dielectric and to lesser degree the migration of free electrons to the
surface of the dielectric.
Volumetric
efficiency:
Energy density in µf-volts per cubic
inch, from: (capacitance) X (working voltage) ÷ (volume).
Longer capacitors are more efficient
than shorter units, because of volume used by encapsulation and unused
dielectric at the capacitor ends (The margins). Cylindrical units have a
smaller volume than rectangular units, although rectangular units can be stacked
more compactly.
Any electrically detectable, field
intensified ionization that does not result immediately in complete breakdown
of the insulation and electrode system in which it occurs. Its incidence can be
reduced or avoided through special designs.
Pulse
Operation:
Capacitors
subjected to DC pulses or non-sinusoidal voltages with fast rise or drop times
(High DV/DT) will be exposed to high current. This current must be limited to
within the maximum peak current allowed. These peak currents refer to an
unlimited number of pulses charging or discharging the capacitors.
AC
VOLTAGE:
The sum of the DC and Peak AC voltage
applied to the capacitor should not exceed the rated DC voltage, nor should the
RMS voltage exceed the Corona Start Voltage.
I RMS:
The maximum RMS ripple current in amps
at a given frequency.
I PEAK:
The maximum peak current in amps @
+25° C for non-repetitive pulses or where the pulse time off is sufficient to
allow cooling so overheating will not result.
DV/DT:
Is the maximum allowed change in volts
per microsecond at the rated voltage.
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