Circuit diagram – Circuit components and their functionsCopy

Learning Objectives

• Draw and interpret circuit diagrams containing sources, switches, resistors (fixed and variable), heaters, thermistors, light-dependent resistors, lamps, ammeters, voltmeters, galvanometers, magnetising coils, transformers, bells, fuses and relays
• Draw and interpret circuit diagrams containing diodes

Contents to learn

Electric circuit

An electrical device that provides a path for electrical current to flow. In electric circuit we use different electrical components like resistor, capacitor, diode, transistors etc. to get different output.

Circuit symbols

D.C. Source	A.C. Source
Resistor	Variable Resistor (Rheostate)
Light Dependent Resistor	Thermistor
Transistor	AND Gate
OR Gate	NOT Gate
NOR Gate	NAND Gate

Circuit diagrams
A circuit diagram (also known as an electrical diagram, elementary diagram, or electronic schematic) is a simplified conventional graphical representation of an electrical circuit, in which we use standard symbols of electrical components.

transistors
A transistor is a semiconductor device invented in 1947. Transistors have three electrodes-known as the base, the emitter and the collector. They are of two types PNP and NPN as shown below.

NPN-Transistor

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PNP-Transistor

Transistors can be used as current amplifiers or as switces. In this syllabus we are foucusing the function of transistor as a switch only.

Transistor as switch
A light operated switch: It is often useful to have a light switch on when the light level decreases. Consider the following circuit.

When the intensity of light is high, the resistance of LDR decreases, so the inout voltage to the base is low. At the time of drakness, the inetensity level decreases so the resistance of LDR increases. So the voltage to the base also increases, turning the light on. Instead of using 10 K Ω resistance, a variable resistor can be connected. The variable resistor can be used to set the precise levels at which the light switches on.

The advantage of using the transistor, rather than connecting the potential divider straight to the lamp is that the light switches fully on for a very small change in input voltage, rather than being partly on over a range of input voltages or light levels.

Capacitors
Capacitor is a device for storing electric charge. A simple capacitor consists of two metal plates separted by some insulaor like air, paper, mica, oil, etc. A circuit symbol for capacitor is shown below.

When a potential difference is applied between the two plates of a capacitor, the plates acquire certain amount of charge on them. The plate which was at high potential (connected with positive terminal of battery) acquires positive charge and the plate at lower potential (connected with negative terminal of battery) acquires an equal amount of negative charge. the charge remains on them even after the removal of applied potential difference.

Capacitance of the capacitor
The charge ‘Q’ given to these plates remains on their inner surfacesdue to attraction. The charge ‘Q’ stored by the capacitor depends upon the potential difference, V between the plates. Greater is the potential difference larger will be the charge stored by the capacitor i.e.

Q V

Q = CV

Where C is constant of proportionality and is called capacitance of capacitor. The capacitance of capacitor depends upon the area of platesand th einsulating material or dielectric used between the plates of the capacitor.

S.I. unit of capacitance is called “Farad”. It is denoted by ‘F’. Farad is a large unit, its smaller units are

1 micro Farad = 1uF= 10-6 F

1 nano Farad = 1nF = 10-9 F

1 pico Farad (1uuF)= 1pF = 10-12 F

One Farad
The capacitance of the capacitor is said to be one farad, if a charge of one coulomb on a capacitor results in a potential difference of 1 volt across it.

Different types of capacitors
Capacitors can be classified as

(i) Fixed Capaciors: Those capacitors whose components are fixed by design are called fixed capacitor

(ii) Variable Capacitor: These are the capacitors whose capacity can be changed when desired. There are different methods by which capacitance of a capacitor can be changed. For example, by changing the separation between the plates of a capacitor, or by changing the exposed area of plates, or changing the dielectric material. Variable capacitors are used in the tuning circuits of a Radio, T.V. etc.

Time delay circuits
As it takes a finite time for a capacitor to charge or discharge, they can be used in time delay circuits. A circuit such as shown below can be used as a prt of burglar alarm or temperature sensor.