The circuit below uses a switch and a transistor to switch a

The circuit below uses a switch and a transistor to switch a light bulb on or off. Which setting of the switch turns off the light? Justify your answer. Which setting of the switch turns on the light? Justify your answer. In the \"on\" configuration, why would the transistor probably be ruined if the resistor R_b is taken out?

Solution

(a) If the switch is open then the base current is 0 and so the collector current is Ic=beta*Ib = 0.

So the bulb does not glow.

(b)If the switch is ON the transistor has some collector current due to which the bulb glows.

(c)In \"on\" configuration if Rb is taken out then the voltage at base would be 5V and the base current would be high. The transistor cannot bear such high base current. So in this case the transistor will be ruined if Rb is taken out.