To make current flow through the base of the transistor, you have to make the voltage on the base about Volts higher than the emitter. NPN Transistor as a Switch. Based on the voltage applied at the base terminal of a transistor switching operation is performed. When a sufficient voltage (Vin > V) is applied between the base and emitter, collector to emitter voltage is approximately equal to 0. Therefore, the transistor acts as a short circuit. In this tutorial we will show you how to use a NPN and PNP transistor for switching, with example transistor switching circuit for both NPN and.


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Transistor as a Switch

Npn transistor as a switch a silicon transistor it usually 0. To keep transistor in saturation enough base current Ib must flow. Different Ways of Connecting Loads Load can be connected to transistor in different ways, some of them are shown below.

Transistor as a Switch Different ways of connecting Loads In the case of inductive loads such as Relay a freewheeling diode should be connected in parallel to it.

Current now flows from the base to the emitter. Since the base region is thin and lightly doped, electrons entering from the emitter are attracted to the collector because of the potential difference between it and the base.


Current thus flows from the collector through to the emitter, lighting the light bulb connected to the collector. To show how the bulb looks when it is connected directly across the power supply, npn transistor as a switch can touch the free end of the black jumper to the outside of the lamp socket, or to the bracket that holds it.

Then when you press the button to turn the transistor on, the class can see that the bulb glows just as brightly as if it were connected directly to the supply. The base current approximately mA in this particular circuit is much smaller than the collector current about 2 A in this circuit.

The voltage applied to the base which ends up at about 0. This amplification phenomenon, whereby one can control a large current with a small current, or where a small change in voltage on the npn transistor as a switch of the circuit gives rise to a large change in voltage on the output, is called the transistor effect, and is what makes the transistor such a useful device.

It enables one to control a circuit or device that dissipates much more power than is dissipated npn transistor as a switch the circuit that drives the base. One does not get an increase in power for nothing, of course.

This additional power comes from the power supply that provides the current that the transistor controls.

Transistor as a Switch | Discrete Semiconductor Circuits | Electronics Textbook

It is this property that makes the transistor an active device, as opposed to a passive device, such as a resistor or capacitor.

While a step-up npn transistor as a switch does also increase the amplitude of a signal placed on its primary, it is also a passive device; it cannot dissipate more power in the secondary than in the primary.


Depending on the circuit conditions, a transistor can be in any of three states. When one uses a transistor to amplify a signal, one designs the circuit so that the transistor stays within its active region, so as not to clip the maxima or minima of the signal.

Try using an ammeter to connect the loose wire to the positive side of the power source, like this: You may have to select the most sensitive current range on the meter to measure this small flow.

As you can see, the transistor is acting npn transistor as a switch a kind of electrically-controlled switch, switching current on and off to the LED at the command of a much smaller current signal conducted through its base terminal.