1-16
Figure 1-11.—Safety shorting probe.
When using the general-purpose safety shorting probe, always be sure first to connect the grounding
clip to a good ground connection (if necessary, scrape the paint off the grounding metal to make a good
contact — paint can be replaced, lives can't). Then, while holding the safety shorting probe by the handle
behind the protective shield, touch the end of the metal rod to the points to be discharged. Touch each
point several times to ensure that the circuit is completely discharged. Be extremely careful that you do
not touch any of the metal parts of the safety shorting probe while touching the probe to the exposed "hot"
terminal. Don't develop a nonchalant or routine attitude about these procedures. It pays to be safe; use the
safety shorting probe with care.
Large capacitors, dormant in storage, can also develop a large static charge. This charge is caused by
environmental conditions such as a close proximity to an rf field. An easy way to avoid this condition is
to short the stored capacitor's terminals with a piece of wire before putting it in storage. Remember to
remove the wire before installing the capacitor. If you receive a large capacitor that is not shorted, short
the terminals together. Remember, CHARGED CAPACITORS CAN KILL.
Q-12.
Charged capacitors can kill. True or false?
BRIDGE-TYPE MEASUREMENTS
Capacitor tests involving quality and value must be made in the course of everyday troubleshooting.
You must make the important decision of whether to reject or continue to use a certain capacitor after it
has been tested. Capacitance measurements are usually accomplished by either a bridge-type or a
reactance-type capacitance meter. The bridge-type capacitance meter is much more accurate than the
reactance-type meter. You may want to review rcl bridges in chapter 1 of NEETS, module 16, before
reading further. Capacitance tolerances vary more widely than resistance tolerances and are dependent
upon the type of capacitor, the capacitance value, and the voltage rating. The results of capacitance tests
must be evaluated to determine whether a particular capacitor will fulfill the requirements of the circuit in
which it is used.
The power factor of a capacitor is important because it is an indication of the various losses
attributable to the dielectric, such as current leakage and dielectric absorption. Current leakage is of
considerable importance, especially in electrolytic capacitors.
Figure 1-12 is a simplified schematic of a capacitance bridge. As you can see, a capacitance bridge is
very similar in construction to a resistance bridge with the exception of the standard capacitor (CS) and
the unknown capacitor (C
X). Because current varies inversely with resistance and directly with
capacitance, an inverse proportion exists between the four arms of the bridge. The following expression
shows the inverse proportion between resistors A and B and capacitors CS and CX:
