Transistor NPN – BD139

Transistors amplify current. For example, they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay, motor or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.

The difference between NPN and PNP transistors:

  • NPN. If the base is at a higher voltage than the emitter, current flows from collector to emitter.
  • NPN. Small amount of current also flows from base to emitter.
  • NPN. Voltage at base controls amount of current flow through transistor (collector to emitter).

  • PNP. If the base is at a lower voltage than the emitter, current flows from emitter to collector.
  • PNP. Small amount of current also flows from emitter to base.
  • PNP. Voltage at base controls amount of current flow through transistor (emitter to collector).

NPN transistors are more common and in a sense easier to use so in most cases you should stick with them.

BD139

The circuit below turns an LED on when the base pin is HIGH. This circuit is really just for testing purposes. Normally you would use a transistor to turn on something that requires a lot more current or voltage than an LED.