b. The markings on a diode case often become illegible, and it is impossible
to tell from visual inspection which lead is the cathode and which is the anode.
In a situation such as this, it is possible to determine the cathode and anode of a
diode by using an ohmmeter such as the PSM-6. To forward bias a diode, you must
place a negative potential on the cathode.
Therefore, when checking a diode for
resistance ratio, if you know which lead is negative and which is positive, you can
determine which lead is the cathode and which is the anode.
Learning Event 11
IDENTIFY SOME COMMON PROCEDURES FOR CHECKING TRANSISTORS
1.
Transistors may be thought of essentially as two diodes mounted back to back.
In this project you will use the PSM-6 to check the condition of transistors and
determine their types. A transistor may become open or shorted. In either case,
it is useless.
The PSM-6 may be used to check the condition of the transistor.
The following discussion will acquaint you with some of the common procedures for
checking a transistor.
2.
While using an ohmmeter to check the condition of a transistor, it is also
possible to determine its type (PNP or NPN). Since a transistor is essentially two
diodes mounted back to back, to check its condition you must measure the resistance
ratio (about 10 to 1) of each junction emitter to base and collector to base, then
measure the resistance between the emitter and the collector. Remember, when using
the PSM-6 as an ohmmeter, the meter leads reverse polarity according to the way the
internal power source is connected.
The red lead becomes the negative lead, and
the black lead becomes the positive lead.
a. Also, a transistor must be isolated from its related circuit before you
can check its type or condition with an ohmmeter. In many cases this requires the
transistor to be desoldered from the circuit and it is possible to obtain an
erroneous ohmmeter indication. One example is shown in Figure 1-27.
Figure 1-27.
False ohmmeter reading
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