LearnTronics
NPN Transistor as a Switch
Recipe #6 — NPN transistor as a switch
A transistor can let a small control current operate a larger load current.
The basic idea
With too little base current, the transistor is off and the load is off. With sufficient base drive, the transistor is driven on and current flows through the load.
Where it appears: LEDs, buzzers, small lamps, relay coils, solenoids, logic interfaces, and many other load-driver circuits.
Why not connect every load directly to a logic output?
The controlling circuit may not be able to supply enough current. The transistor provides current gain and isolates the control signal from much of the load current.
Real design still needs ratings
The transistor must be rated for the load current, voltage, power dissipation, and switching conditions. The base resistor must be chosen so the transistor receives adequate drive without exceeding the source or base-current limits.