Meter Usage - Continued
The two-diode concept.
It is common knowledge that a semiconductor diode can be checked by measuring its forward and
reverse resistances and comparing these two readings. It is also reasonably well known that a
transistor may be considered as a sort of two-diode device as shown in figure 8-4. To illustrate
them, let us assume we wish to check a low-power p-n-p transistor, represented by figure 8-4. One
of the effective diodes, as shown in figure 8-4 (b) exists between base and collector connections;
the other exists between base and emitter connection. Quite simply, if either of these diodes is
defective (open or shorted), the transistor cannot function properly.
For our first test, we connect the negative lead of the meter to the base of our p-n-p transistor and
the positive lead to the collector. Since we are trying to force a current through the base-collector
diode in the forward direction, we will get a low resistance reading if this diode is in good condition.
The exact reading may vary depending on the characteristics of the ohmmeter as well as of the
transistor, but a low-power type should generally read quite low, something like 120 ohms.
In the second step, the base-emitter diode is checked by disconnecting the positive lead from the
collector and connecting it to the emitter. Note that the forward resistance of the base-emitter diode
is slightly higher than that of the base-collector diode, 140 ohms. This is normal for the low-power
In the third step, we connect the positive lead of the meter to the base of the p-n-p transistor and the
negative lead to the collector. Now we are trying to force a current through the base-collector diode
in the reverse direction and we will get a high resistance reading if this diode is in good condition,
generally in excess of 50,000 ohms. Noteworthy here is the high ratio of reverse to forward
resistance -- in the order of 500:1 or greater. The two readings just referred to are test steps 1 and
3 of table 8-1 which is applicable to low-power p-n-p transistors.
In the fourth step, the base-emitter diode is checked by disconnecting the negative lead from the
collector and connecting it to the emitter. The resistance reading will be quite high if this diode is in
good condition, generally in excess of 50,000 ohms. Note the high ratio of reverse to forward
resistance -- in the order of 500:1 or greater. The two readings just referred to are test steps 2 and
4 of table 8-1.
So far, the readings we have taken apply only to a minority of the transistors that would ordinarily be
encountered. For one thing, nothing has been said of n-p-n types. For low-power units in this
group, the base-collector and base-emitter diodes effectively have reversed polarity as compared to
their counterparts in p-n-p transistors. We can check them by reversing the polarity of connections.
When this is done (the first four lines of table 8-2 can be used as a guide to correct connections),
the readings correspond to those obtained with the p-n-p transistors we are using for illustration.