Transistor as a switch

When a transistor is used as a switch it must be either OFF or fully ON. In the fully ON state the voltage VCE across the transistor is almost zero and the transistor is said to be saturated because it cannot pass any more collector current Ic. The output device switched by the transistor is usually called the 'load'.The power developed in a switching transistor is very small:

• In the OFF state: power = Ic × VCE, but Ic = 0, so the power is zero.
• In the full ON state: power = Ic × VCE, but VCE = 0 (almost), so the power is very small.

This means that the transistor should not become hot in use and you do not need to consider its maximum power rating. The important ratings in switching circuits are the maximum collector current Ic(max) and the minimum current gain hFE(min). The transistor's voltage ratings may be ignored unless you are using a supply voltage of more than about 15V. 

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Transistor as a switch