Write an experiment on transistors as a switch PDF

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Experiment# 11 Objective: Write an experiment on transistors as a switch. Apparatus:    Digital Analog Training System A transistor AC Voltage Source Schematic Diagram: Digital Analog Training System Transistor Theory: Transistor: A transistor is a semiconductor device used to amplify or switch e...

Description

Experiment# 11 Objective: Write an experiment on transistors as a switch.

Apparatus:   

Digital Analog Training System A transistor AC Voltage Source

Schematic Diagram:

Digital Analog Training System

Transistor

Theory: Transistor: A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. These terminals are Emitter, Base & Collector. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. It acts like the series combination of two diodes (one diode is Base-Collector & other diode is Base-Emitter). The basecollector junction provides larger resistance than base-emitter junction.

Usually the semi conductor materials used to manufacture a transistor are Germanium and Silicon. Sometimes combination of Gallium and Arsenide (GaAs) is used for this purpose.

Types of Transistors: There are two types of transistors.  NPN transistors  PNP transistors

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1. NPN Transistors: NPN is the type of bipolar transistor consisting of a layer of P-doped semiconductor between two N-doped layers. This P-doped semiconductor is base of transistor. A small current entering the base is amplified to produce a large collector and emitter current. These transistors are used as switch or amplifier in electrical circuits. In an NPN transistor, the emitter–base junction is forward biased (electrons and holes recombine at the junction), and electrons are injected into the base region. Because the base is narrow, most of these electrons will diffuse into the reverse-biased base–collector junction (electrons and holes are formed which move away from the junction) and be swept into the collector The symbol used to NPN transistor is shown below.

In electrical equipments, NPN transistor is arranged in the following way.

2. PNP Transistors: PNP is the type of bipolar transistor consisting of a layer of N-doped semiconductor between two P-doped layers. This N-doped semiconductor is base of transistor. In this transistor, the current flows towards emitter. The symbol used to NPN transistor and its electrical connection in the Electrical equipments is shown below.

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Digital Analog Training System: The unit contains a DC power supply, which provides the user with +5V at 1A, -5V at 300mA and variable outputs of 0V to +15V and 0V to -15V each up to a maximum of 500mA. Also built in is a function generator which operates in five variable ranges from 1Hz to 10 KHz to provide sine, triangle and square waves. The large 2712 tie point breadboard is also complemented by eight toggle switches, two pulse switches, eight LED indicators, three sevensegment LED displays, two potentiometers, and a speaker.

Procedure:  First of all, find that given transistor is PNP or NPN in the sense of polarity.  I have determined that transistor is NPN type.  I have make a circuit on the bread board in which Collector junction is connected to 5V supply, Emitter is connected to negative terminal of load (LED operated at 3V) and base junction is opened.  When base junction is connected to the 5v supply, then circuit completes and LED switched ‘ON’.  When we cut off the supply of Base junction, then current does not follow and LED is not working.  The following circuit shows the whole working procedure:

Conclusion: I have concluded that the transistors play an important role in the branch of electricity especially in electronics. All the Computers, Cell phone and other communicational equipments are Designed on the basis of transistors because it provides a save path to flow the current as compared to resistors.

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