A typical record is shown in Table I, from which we find that the transistor is npn-type and lead 2 is the base. But, lead 1 or lead 3 could be the emitter or vice versa.
To detect the collector and the emitter, proceed as follows:
7. If the transistor is npn-type, set the pole of switch S1 towards ‘npn’ marking. If the transistor is pnp-type, set the pole of switch S1 towards ‘pnp’ marking.
8. Next, insert the base lead in socket C, any one of the remaining two leads in socket A and the rest in socket B. Observe as before the status of the two LEDs.
For an npn-type transistor, if the green LED glows the lead inserted in socket A will be collector, but if the red LED glows it will be emitter.
On the other hand, for a pnp-type transistor, if the green LED glows the lead inserted in socket A will be emitter, but if the red LED glows the reverse will be the case, i.e., the lead will be collector.
The status of the LEDs for the above-mentioned npn-type transistor having lead 2 as base is shown in Table II. From this table, we find that lead 1 is collector and lead 3 is emitter of the transistor in the example.