Chapter 22 Basic Electrical Tests 357
Testing Diodes
An ohmmeter is normally used to check diodes. A
bad diode may have an internal open (PN junction
burned and separated), or it may be shorted (PN
junction burned and fused). The diode should have
infi nite resistance in one direction and low resis-
tance in the other. If it does not, the diode should be
replaced.
Testing Transistors
A bad transistor will no longer amplify its base
signal because one of the internal junctions has been
damaged. Special transistor testers are available, but
an ohmmeter can also be used to test transistors.
To check a transistor with an ohmmeter, you must
fi rst fi nd the base lead. To do this, connect an ohm-
meter test lead to any of the transistor leads. You can
tell which lead is the base because the ohmmeter
reading will be the same when you measure resis-
tance between the base lead and either of the other
two transistor leads. The ohmmeter reading will
vary when you connect the meter across the emitter-
collector junction.
After fi nding the base, you must make four mea-
surements. With one test lead on the base, measure
resistance across the base-emitter junction and then
the base-collector junction. Remove the test lead
from the base and check the resistance of the emitter-
collector junction Reverse the ohmmeter lead and
check resistance across the emitter-collector junc-
tion again. In one direction, the resistance reading
should be low (about 2–10 ohms). When the meter
leads are reversed, the reading should be slightly
higher (about 60–150 ohms). These readings show
that the transistor junctions are functioning nor-
mally. A transistor is bad and needs to be replaced if
it shows infi nite resistance or zero resistance across
one or more of the junctions.
Figure 22-36. Using a test light is the easiest way to check for
a blown fuse. A blown fuse will light the tester on the feed side,
but it will not light the tester on the output side.
Figure 22-37. Before using an ohmmeter to check a capacitor, be sure to ground the capacitor lead to discharge any electricity
stored in the capacitor.
Test light
ground
Test probe
At first, meter
shows low
resistance
Bar graph increases
toward infinity as
capacitor charges
Capacitor
~V
OFF
V
Ω
mA
mA
A
~
1.262
Ω