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 TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
mance per published specifications. To test most
ratus or another capacitor located above. An effort
capacitors, rectifiers (diodes), resistors, potentiom-
should be made to determine the nature of the leak-
eters, SCR's, and bipolar junction transistors, the
ing fluid. If the capacitor is not specifically
stamped: "NON-PCB" or "NO PCB's", then the
volt-ohmmeter (VOM) is the recommended instru-
Hazardous Waste Coordinator should be contacted
ment (para 13-2). There are many types and models
available; however, the analog meter still serves
and the capacitor disposed of as recommended by
very well for static testing. It should be noted that
that Office.
meters with very low energy resistance test ranges
(2) Testing. Open or solid capacitors maybe
found by using an ohmmeter to test as follows:
will not produce the same results described in the
paragraphs below. Most all new digital models have
(a) Identify the polarity of the terminals
when electrolytic capacitors are to be tested. Always
the low energy resistance test feature. The manu-
facturers have recognized this problem and usually
test with the plus (+) lead of the meter connected to
the terminal marked plus (+) or the red dot. Re-
provide one additional function marked "diode test"
versed polarity, even at low voltages, causes high
or simply the ANSI symbol for a diode. This type of
dissipation in the electrolyte paste and gives poor
test is also different from those described below. The
diode test feature differs from the resistance test,
test readings on a possible good unit.
using an ohmmeter, in that the instrument gener-
(b) For values under one (1) microfarad, use
ates a freed current (about 10-100 milliamperes)
the "X1OO" scale. For higher values, use either the
"X100" or the "10X" scale.
and passes it though the device under test. The
corresponding voltage generated across the device
(c) Discharge the capacitor before testing.
terminals is usually the reading that appears on the
Use a 100-1000 ohm resistor to limit the discharge
instrument's display. The maximum voltage is
current. Remove the resistor connection.
(d) Connect the test probes and note the
never more than the battery voltage used to power
the meter. The ohmmeter applies a relatively con-
meter deflection. If the capacitor is open, the ohm-
stant voltage and allows the current to vary in pro-
meter will continually indicate infinity ohms. The
portion to the total circuit resistance (Ohm's Law).
meter needle will not move the moment the leads
Before testing, the ohmmeter must be calibrated for
are touched to the capacitor terminals. Replace ca-
zero ohms. This nulls out the test lead resistance;
pacitors that are open circuit. If the capacitor is
the test probes are touched together and the meter
shorted, the meter needle will immediately deflect
reading is adjusted to indicate zero. Then, the par-
to zero or some low value and remain there. Replace
ticular component to be tested must be isolated
capacitors that are shorted. A good capacitor will
from the rest of the circuit. This is done by discon-
cause the meter needle to deflect toward zero ohms
necting at least one lead of the component.
the moment the leads are touched to its terminals.
b. Capacitors. A capacitor stores electrical energy
However, the needle will begin to indicate ever-
for dissipation as needed in an electrical circuit. The
increasing resistance as the capacitor charges up.
amount of charge stored depends upon the value of
The amount of initial deflection and the rate of
the capacitor (expressed in pico-, nano-, or microfar-
return of the needle depend on the value of the
ads) and the applied voltage. There are many types
capacitor and the ohms scale multiplier selected.
of capacitors used in power electronic and control
Capacitors that are "leaking electrically" will cause
equipment (fig 7l). The more commonly used types
the meter needle to deflect as usual; however, the
are: oil-impregnated and non-polarized; polarized
final resistance value may be only several hundreds
aluminum electrolytic; polarized wet slug and
of ohms rather than the several thousands that can
dipped tantalum; non-polarized wet slug and dipped
be expected. Capacitors not properly isolated from
tantalum; and, non-polarized paper, plastic film,
the circuit during the test often give this kind of
mica, or ceramic capacitors. A capacitor is defective,
reading because of the other components connected
or will soon be defective if it has a damaged case, is
in parallel. If it is certain the capacitor alone is
leaking fluid or electrolyte paste, or testing shows it
reading this way, it should either be replaced or
to be nearly shorted or completely open.
retested with an analyzer. All of the test results
(1) Inspection for oil leaks. Leaking capacitors
described above will be more readily understood if
can be found by locating the oil or fluid that has
several values of capacitors that are known to be
seeped from a cracked case or relief plug. A leaking
good are tested first.
capacitor may be kept in service for brief emergency
c. Rectifiers and semiconductor-controlled rectifi-
periods but should be replaced before it fails alto-
ers (SCR). A rectifier (diode) is a solid-state device
gether, or the leaking fluid damages other equip-
that limits the flow of electrical current to one direc-
ment. Before rejecting a capacitor for leaking oil, be
tion. The semiconducting material within the device
sure the oil was not deposited by some other appa-
acts as an insulator in one direction (within certain
7-2
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