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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
Transistor+Switch Closed Base Transistor act as a switch  Working and transistor switching circuit
Base terminal is open
Transistor+Switch Open Base Transistor act as a switch  Working and transistor switching circuit

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
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