Switch as a transistor, Schemes and Mind Maps of Electronics

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Transistor as a switch

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Eelectronic Devices and Circuit

Active Learning Assignment

What is transistor?

• When an n-type or p- type semiconductor is Connect between two p-type or n-type semiconductor respectively then two PN junction formed. Such a two junction device is called as TRANSISTOR
• Thus transistor is a two junction , three layered, three terminal semiconductor device.
• The transistor made by Silicon , Germanium material.
• Transistor has mainly two type.
  1. PNP Transistor
  2. NPN Transistor

1. Cut-off Region

• The areas of operation for a Transistor Switch are known as the Saturation Region and the Cut-off Region. This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its “fully-OFF” (cut-off) and “fully-ON” (saturation) regions as shown in figure.
• The pink shaded area at the bottom of the curves represents the “Cut-off” region while the blue area to the left represents the “Saturation” region of the transistor. Both these transistor regions are defined as:
• 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, VB < 0.7v and IC = 0. For a PNP transistor, the Emitter potential must be negative with respect to the Base.

• Here the operating conditions of the transistor are zero input base current ( IB ), zero output collector current ( IC ) and maximum collector voltage ( VCE ) which results in a large depletion layer and no current flowing through the device. Therefore the transistor is switched “Fully- OFF”.
• transistor switch in cut-off

Digital Logic Transistor Switch

• Then we can define the “saturation region” or “ON mode” when using a bipolar transistor as a switch as being, both junctions forward biased, VB > 0.7v and IC = Maximum. For a PNP transistor, the Emitter potential must be positive with respect to the Base.
• Transistor switches are used for a wide variety of applications such as interfacing large current or high voltage devices like motors, relays or lamps to low voltage digital logic IC’s or gates like AND gates or OR gates. Here, the output from a digital logic gate is only +5v but the device to be controlled may require a 12 or even 24 volts supply. Or the load such as a DC Motor may need to have its speed controlled using a series of pulses (Pulse Width Modulation). transistor switches will allow us to do this faster and more easily than with conventional mechanical switches.

Digital Logic Transistor Switch

Application

• Transistor switches can be used to switch and control lamps, relays or even motors.
• When using the bipolar transistor as a switch they must be either “fully- OFF” or “fully-ON”.
• Transistors that are fully “ON” are said to be in their Saturation region.
• Transistors that are fully “OFF” are said to be in their Cut-off region.
• When using the transistor as a switch, a small Base current controls a much larger Collector load current
• When using transistors to switch inductive loads such as relays and solenoids, a “Flywheel Diode” is used.
• When large currents or voltages need to be controlled, Darlington Transistors can be used

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