If the switch is open as in the figure above (a), the base wire of the transistor will be left “floating” (not connected to anything) and there will be no current through it.

Transistor: (a) cutoff, lamp off (b) saturated, lamp on.

Perhaps the simplest thing to do would be to connect a switch between the base and collector wires of the transistor as in the figure below (a). Remember that for an NPN transistor, base current must consist of electrons flowing from emitter to base (against the emitter arrow symbol, just like the lamp current). Without a connection to the base wire of the transistor, base current will be zero, and the transistor cannot turn on, resulting in a lamp that is always off. Going back to the NPN transistor in our example circuit, we are faced with the need to add something more so that we can have base current. All that matters is that the proper current directions are maintained for the sake of correct junction biasing (electron flow going against the transistor symbol’s arrow). 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.

Transistor as a switch download