The choice between NPN and PNP is really arbitrary. Its application is shown in the figure above (c). (a) mechanical switch, (b) NPN transistor switch, (c) PNP transistor switch.Ī PNP transistor could also have been chosen for the job.
We must also make sure that the lamp’s current will move against the direction of the emitter arrow symbol to ensure that the transistor’s junction bias will be correct as in the figure below (b). Since it is the current through the lamp that we want to control, we must position the collector and emitter of our transistor where the two contacts of the switch were. Remember that the controlled current through a transistor must go between collector and emitter.
Such a circuit would be extremely simple, as in the figure below (a).įor the sake of illustration, let’s insert a transistor in place of the switch to show how it can control the flow of electrons through the lamp. Suppose we had a lamp that we wanted to turn on and off with a switch.
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