Resistors, Capacitors, and Inductors 259
current for any capacitor is limited by the tempera-
ture within the capacitor and the rate of heat dissipa-
tion from the capacitor. Lower ESR and longer cans
or enclosures increase the ripple current rating.
- Reverse Voltage. Aluminum electrolytic capacitors
can withstand a reverse voltage of up to 1.5 V
without noticeable effect from its operating charac-
teristics. Higher reverse voltages, when applied over
extended periods, will lead to some loss of capaci-
tance. Excess reverse voltages applied for short
periods will cause some change in capacitance but
may not lead to capacitor failure during the reverse
voltage application or during subsequent operation in
the normal polarity direction.
A major use of large value capacitors is for
filtering in dc power supplies. After a capacitor is
fully charged, when the rectifier conduction
decreases, the capacitor discharges into the load until
the next half cycle, Fig. 10-20. Then on the next
cycle the capacitor recharges again to the peak
voltage. The 'e shown in the illustration is equal to
the total peak-to-peak ripple voltage. This is a
complex wave which contains many harmonics of the
fundamental ripple frequency and is the ripple that
causes the noticeable heating of the capacitor.
This can be mathematically determined or the
ripple current through the capacitor can be measured
by inserting a low impedance true rms ammeter in
series with the capacitor. It is very important that the
impedance of the meter be small compared with that
of the capacitor, otherwise, a large measurement error
will result.
- Standard Life Tests. Standard life tests at rated
voltage and maximum rated temperatures are usually
the criteria for determining the quality of an electro-
lytic capacitor. These two conditions rarely occur
simultaneously in practice. Capacitor life expectancy
is doubled for each 10°C (18°F) decrease in oper-
ating temperature, so a capacitor operating at room
temperature will have a life expectancy 64 times that
of the same capacitor operating at 85°C (185°F).
- Surge Voltage. The surge voltage specification of a
capacitor determines its ability to withstand the high
transient voltages that occur during the start up
period of equipment. Standard tests specify a short on
and long off period for an interval of 24 hours or
more; the allowable surge voltage levels are usually
10% above the rated voltage of the capacitor. Fig.
10-21 shows how temperature, frequency, time, and
applied voltage affect electrolytic capacitors.
- Tantalum Capacitors. Tantalum electrolytics have
become the preferred type where high reliability and
long service life are paramount considerations.
Most metals form crystalline oxides that are
nonprotecting, such as rust on iron or black oxide on
Figure 10-20. Capacitor charge and discharge on a
full-wave rectifier output.
E
$e
Figure 10-21. Variations in aluminum electrolytic character-
istics caused by temperature, frequency, time, and applied
voltage. Courtesy of Sprague Electric Company.
Cap
Frequency
DF
Frequency
ESR
Frequency
Z
Frequency
Cap
Time
DF
Time
= Increased or higher
or later
Z, ESR, C(normalized)
Z
ESR
C
Temperature
25°C
Ripple currentcapabilityTemperature
Failure rate
Temperature
Leakage currentVoltage
Failure rate
% Rated voltage
