However, the collector current is not zero since a little leakage current always flows through the collector. When switch S is OFF, the base current is zero.The most important point to note here is that there are no moving contacts in a transistor and the conduction is by transistor action.Here, the base circuit corresponds to relay coil and the transistor to the contacts of the relay. This operation of the transistor is similar to relay where with a small relay coil current, we can control much heavier load current.Since the base current is much smaller than the load current (collector current), therefore, we can switch ON a much large current with a small current.When switch S is closed, the base-emitter junction is forward biased and the transistor conducts i.e.Z is the load in the collector circuit and the controlling switch S is in the base circuit. Consider an NPN transistor as shown in Figure.The action of the transistor, while using a transistor as a switch is explained below: A transistor can also act as a switch and can turn ON and OFF power in an electrical load.
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