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The Transistor as a Switch
In the circuit below we use a rheostat as a variable potential divider to apply a variable voltage across the base and emitter of the transistor to see how this affects the voltage across the collector and emitter.
Adjust the variable potential divider so that Vbe = zero.
Slowly increase Vbe. Notice that the LED lights when Vbe = (about) 0∑6v and that Vbe does NOT increase much above this figure no matter what we do with the rheostat.
Vbe < 0∑6v, transistor is OFF and Vce = the voltage of the battery
Vbe > 0∑6v, transistor is ON and Vce = about 0∑2v
When we say that the transistor is ON, we mean that it allows current to flow easily into its collector and out of its emitter.
Transistors used as switches are found in nearly all modern electronic equipment, e.g. computers, calculators, T.V.ís.