How To Use Transistor As A Switch

Use Transistor As A Switch

In this artical we have explained the Use Transistor As A Switch in different applications.  A transistor is a three-terminal semiconductor device which can be used for switching applications, amplification of weak signals and quantities of thousands and millions of transistors are interconnected and embedded into a tiny integrated circuit/chip. A Transistor switch, which is used for ON or OFF of a circuit, so its mean that  the transistor is commonly used as a switch in the electronic devices .Transistor work as a switch when it is in cutoff and saturation regions. In this article, we will Explain how to use a transistor as a switch.

Use Transistor As A Switch

Bipolar junction transistors has two junctions base emitter junction, base collector junction. Accordingly there are four different regions of operation in which either of the two junctions are forward biased reverse biased or both. But the BJT can be effectively operated in there different modes according to the external bias voltage applied at each junction. i.e. Transistor in active region, saturation and cutoff. The other region of operation of BJT is called as inverse active region.The operation of transistor in these modes is explained below.

Transistor in Cut off region

In this region both junctions of the transistor are reverse biased. Hence transistor in cut off does not conduct any currents expect for small reverse saturation currents that flow across junctions and can be used as a practical applications in pce san diego repair shops. In cutoff condition emitter current is zero and the collector current consists of small reverse saturation currents. The transistor when used as switch is operated in cutoff on condition and saturation regions which corresponds to switch off an on condition respectively.
Fully Of Region
Then we can define the “cut-off region” or “OFF mode” when using a bipolar transistor as a switch as being, both junctions reverse biased, IC = 0 and VB < 0.7v. For a PNP transistor, the Emitter potential must be -ve with respect to the Base.

Transistor in Saturation region

Saturation region is one in which both Emitter Base and Base Collector junctions of the transistor are forward biased. In this region high currents flows through the transistor, as both junctions of the transistor are forward biased and bulk resistance offered is very much less.Transistor in saturation region is considered as on state in digital logic.
A transistor is said to be in saturation if and only if
 β > Ic/Ib
Transistor Saturation Region
This is due to the fact that as both junctions of transistor are forward biased along with electron current flowing from emitter to base in active region there will be additional component of electron current flowing from collector to base.Small changes in Collector to base forward voltage leads to large variations in collector currents.

Inverse active region of transistor

In inverse active region is just inverse or complementary to active region. In inverse active region the Base emitter junction is forward biased and Base Collector junction will be reverse biased.

use of transistor as a switch

Use Of Transistor As Switch

The diagram shows the two current paths through a transistor.
The small base current controls the larger collector current.
When the switch is closed a small current flows into the base (B) of the transistor. It is just enough to make LED B glow dimly. The transistor amplifies this small current to allow a larger current to flow through from its collector (C) to its emitter (E). This collector current is large enough to make LED C light brightly.
When the switch is open no base current flows, so the transistor switches off the collector current. Both LEDs are off.
You can build this circuit with two standard 5mm red LEDs and any general purpose low power NPN transistor (BC108, BC182 or BC548 for example). It is a good way to test a transistor and confirm it is working.
A transistor amplifies current and can be used as a switch, as explained on this page.



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  1. Tem muito tempo que eu me formei em eletronica, e nao me lembro mais das formulas. se eu tiver uma fonte de 12v ou 5v e quiser exitar um trasistor de germanio ou silicio cada um com sua tensao especifica de base 0,3V 0,7V, e tendo uma corrente de 1A. o calculo? outra duvida alem de usar o resistor, como poderia usar um zener em uma configuraçao para por exemplo pegar uma fonte de 15V/2A e trasformar em 12v ou 6V? Desculpe tantas duvidas, mas eu tenho meus 48 anos de idade e me formei aos 20 anos, nao lembro mais disso. Preciso reciclar

    1. It’s been a long time since I graduated in electronics, and I do not remember the formulas anymore. if I have a source of 12v or 5v and want to exact a germanium or silicon transistor each with its specific base voltage 0.3V 0.7V and having a current of 1A. the calculation other doubt besides using the resistor, how could use a zener in a configuration for example pick up a source of 15V / 2A and transform into 12v or 6V? Sorry for all the doubts, but I’m 48 years old and I graduated at age 20, I do not remember that anymore. I need to recycle

  2. Tem muito tempo que eu me formei em eletrônica, e não me lembro mais das formulas. se eu tiver uma fonte de 12v ou 5v e quiser exitar um transistor de germânio ou silício cada um com sua tensão especifica de base 0,3V 0,7V, e tendo uma corrente de 1A. o calculo? outra duvida alem de usar o resistor, como poderia usar um zener em uma configuração para por exemplo pegar uma fonte de 15V/2A e transformar em 12v ou 6V? Desculpe tantas duvidas, mas eu tenho meus 48 anos de idade e me formei aos 20 anos, não lembro mais disso. Preciso reciclar

    It’s been a long time since I graduated in electronics, and I do not remember the formulas anymore. if I have a source of 12v or 5v and want to exact a germanium or silicon transistor each with its specific base voltage 0.3V 0.7V and having a current of 1A. the calculation other doubt besides using the resistor, how could use a zener in a configuration for example pick up a source of 15V / 2A and transform into 12v or 6V? Sorry for all the doubts, but I’m 48 years old and I graduated at age 20, I do not remember that anymore. I need to recycle

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