52
Fig. 44: The Equivalent Series Resistance (ESR) is
composed of all the combined internal resistances
in the capacitor.
Dielectric Absorption
One of the most common types of failures of electrolytic
capacitors is dielectric absorption. Dielectric absorption
is the result of a capacitor remembering a charge that is
placed on it. The capacitor cannot be completely
discharged and a voltage will reappear after the capacitor
has been discharged. Another name for dielectric
absorption is battery effect. As this name implies, a
capacitor with excessive dielectric absorption will act
like a battery in the circuit. This will upset the circuit by
changing bias levels. A capacitor with excessive
dielectric absorption will also have a different effective
capacitance when it is operating in a circuit. Dielectric
absorption will not normally show up in film or ceramic
capacitors, but if the ReZolver test does indicate
dielectric absorption the capacitor is likely to fail in use.
Dielectric absorption in these capacitors will generally
be associated with a high leakage as well.
Leakage
One of the most common capacitor failures is caused by
current leaking through the capacitor. Some capacitors
will show a gradual increase in leakage, while others
will change rapidly and even short out entirely. In order
to effectively test a capacitor for leakage, it is necessary
to test the capacitor at its rated voltage.
When a DC voltage is applied to a capacitor, a certain
amount of current will flow through the capacitor. This
current is called the leakage current and is the result of
imperfections in the dielectric. Whenever this leakage
current flows through an electrolytic capacitor, normal
chemical processes take place to repair the damage done
by the current flow. Heat will be generated from the
leakage current flowing through the capacitor and will
speed up the chemical repair processes.
Fig. 45: The Equivalent Series Resistance has the
result of isolating the capacitor from the power
supply line, reducing its filtering capabilities.
As the capacitor ages, the amount of water remaining in
the electrolyte will decrease, and the capacitor will be
less capable of healing the damage done by the various
leakage paths through the dielectric. Thus, as the amount
of water in the electrolyte decreases, the capacitor will
be less capable of healing the leakage paths and the
overall leakage current in the capacitor will ultimately
increase. The increase in leakage current will generate
additional heat, which will speed up the chemical
processes in the capacitor. This process, of course, will
use up more water and the capacitor will eventually go
into a run-away mode. At some point, the leakage
current will finally get large enough to adversely affect
the circuit the capacitor is used in.
Cathode
Lead
Resistance
Cathode
Lead-To-Plate
Resistance
Resistance Of
Cathode Plate
Resistance
Due To
Electrolyte
Resistance Of
Anode Plate
Anode
Lead-To-Plate
Resistance
Anode
Lead
Resistance
R1
R2
R3
R4
Rs
R5
R6
R7
=
Capacitor
Isolated
From Power
Supply Line
Fluctuating
AC
Rs
Rp
C
