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Transistor act as a switch- Working and transistor switching circuit

Transistor is a semiconductor device used for switching and amplification of weak signals. This article explains how a transistor work and the working of transistor as a switch. At present, transistors are applied in most of the electronic equipments for switching purposes. Digital ICs, Microcontrollers, etc uses thousands of embedded transistor for switching. The huge electrical networks are also switched by simple transistor switching circuit
(Most of the Power electronics circuits). Here we are discussing about npn transistor switching circuit. To realize the exact working principle, an LED (Light Emitting Diode) is connected to the collector of npn transistor. It glows according to the base current. Don’t forget to connect the base resistor (Rb) to limit the base current other wise the transistor become damage. Transistor switching circuits are also employed in DC Motor driver circuit

Circuit diagram showing Transistor as a switch

Base terminal is closed
Base terminal is open

Working principle of transistor switch

  • Consider the first figure in which base terminal of the npn transistor is closed then the transistor is said to be in ON state (similar to a short circuit between Vcc and Ground) so the collector voltage is very low (0.02V approx)
  • Whole current from Vcc will flows through the transistor, no current flow through the LED because current chooses low resistance path.
  • Therefor  LED connected at the Collector is in OFF state since the voltage at the anode of LED is 0.02V.
  • Consider the second figure, When the voltage at the base terminal removed (open circuit), the transistor become OFF (means an open circuit between Vcc and Ground) then its collector voltage will be Vcc (Supply voltage)
  • Since the transistor is in OFF state, the whole current will flows trough the LED, Then the LED glows

11 thoughts on “Transistor act as a switch- Working and transistor switching circuit

  1. can u specify the exact path of current flow when ON and OFF. And can u be a little more specific bout the reason for different paths for current.

    • hey curiosity,
      when the switch is closed , you are providing the base current to the transistor which in turn, will make the transistor on and provide the path for the current to flow. This is because when transistor is ON it provide the low resistance path for the current flow than the resistance across the LED. Because of this all the current flows through the transistor to the ground and no current across LED and hence LED is OFF.
      when is swicth is open or when no base current is applied the transistor is OFF and has High Resistance path in terms of MOhms since the transistor does not turn on until u provide base current. Now the current tries to flow in the least resistance path that is LED. Resistance of the LED is less compared to that of the open transistor. Now the current flows through the LED and the LED is ON….

      Hope U got the cleat view of the circuit

  2. it is better if you show the whole mechanism on how the base current relate to the collector and emitter, this is best for the bigginers to understand more ,

  3. Even if transistor is removed, a single switch can work there to on or off the transistor. What is the advantage of using a transistor here??

  4. Dear curiousity,
    First and foremost,a transistor is a semiconductor material which has one input(base) and two outputs(collector and emitter). These are the resistance characteristics of the two outputs.
    1. Collector (low resistance)
    2.Emitter (high resistance)
    In this case the led will turn on by the low resistance path, And also through its cathode.

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