BASIC ELECTRONICS TUTORIAL1- RESISTORS

FIGURE AND SYMBOL

                                                                      

                                                        

INTRODUCTION

Resistor is most basic and very important electronic component used in almost every electronic circuit so its very important for beginners to have a good

Knowledge of handling resistors efficiently. Resistors are used to control the  electric current in a circuit, for example a resistor  in series with a LED  limits the current passing through the LED.in this article we will discuss about resistors and their uses.

Features

Resistors are non polar may be connected either way round. Resistors are highly heat resistive elemrnts can withstand with very high temperature so generally these are not damaged by high temperature when soldering.

Colour coding

Unit of resistance is ohms, the symbolized an omega Ω.1Ω  is quite small so resistor values are often given in k Ω and M Ω. 
1 k Ω = 1000 Ω 

1 M Ω = 1000000 Ω.

Resistance value is calculated by the colour band of resistor each colour represents a specific number as shown and here is the technique to calculate exact value  by colour coding.

Generally resistors have 4 bands and these bands represent-      

The Resistor    Colour Code
Colour	Number
Black	0
Brown	1
Red	2
Orange	3
Yellow	4
Green	5
Blue	6
Violet	7
Grey	8
White	9

 

• First band= First digit.                                                  
• Second band = Second digit.
• Third band = Number of zeros.
• Fourth shows tolerance (precision) of the resistor.

Example-

This resistor has red (2), violet (7), yellow (4 zeros) and gold bands. 
So its value is 270000 Ω = 270 k Ω. 
On circuit diagrams the Ω is usually omitted and the value is written 270K.

Small value resistors (less than 10 ohm)

The standard colour code cannot show values of less than 10 Ω. To show these small values two special colours are used for the third band: gold which means × 0.1 and silver which means × 0.01. The first and second bands represent the digits as normal.

For example:
red, violet, gold bands represent 27 × 0.1 = 2.7 Ω
green, blue, silver bands represent 56 × 0.01 = 0.56 Ω

Tolerance of resistors (fourth band of colour code)

The tolerance of a resistor is shown by the fourth band of the colour code. Tolerance is the precision of the resistor and it is given as a percentage. For example a 390 Ω resistor with a tolerance of ±10% will have a value within 10% of 390 Ω, between 390 - 39 = 351 Ω and 390 + 39 = 429 Ω (39 is 10% of 390).

A special colour code is used for the fourth band tolerance:
silver ±10%,   gold ±5%,   red ±2%,   brown ±1%. 
If no fourth band is shown the tolerance is ±20%.

Tolerance may be ignored for almost all circuits because precise resistor values are rarely required.

Resistor value writing terminology

·        R is used forohms.

560R means 560 Ω 

·        K is used for kilo ohms but when it comes between two numbers it works as a dot as dot can be missed easily so instead of dot K is used

    2K7  means 2.7 k Ω = 2700 Ω 
   39K  means 39 k Ω 

·        M is used for mega ohms.

          1M0  means 1.0 M Ω = 1000 k Ω

Special resistors (the E6 and E12 series)

The E6 series (6 values for each multiple of ten, for resistors with 20% tolerance)  10E, 15E, 22E, 33E, 47E, 68E

 
10E =100ohms

15E=150 ohms         and so on  

here  the step size increases as the value increases. For this series the step (to the next value) is roughly half the value.

The E12 series (12 values for each multiple of ten, for resistors with 10% tolerance) 
10E, 12E, 15E, 18E, 22E, 27E, 33E, 39E, 47E, 56E, 68E, 82E

27E=270ohms

68E=680 ohms       and so on

Power rating

Power ratings of resistors are rarely quoted in parts lists because for most circuits the standard power ratings of 0.25W or 0.5W are suitable. For the rare cases where a higher power is required it should be clearly specified in the parts list, these will be circuits using low value resistors (less than about 300 Ω) or high voltages (more than 15V).

The power, P, developed in a resistor is given by:

P = I² × R or P = V² / R

where:

P = power developed in the resistor in watts (W) I  = current through the resistor in amps (A) R = resistance of the resistor in ohms (Ω) V = voltage across the resistor in volts (V)

Examples:

• A 470 Ω  resistor with 10V across it, needs a power rating P = V²/R = 10²/470 = 0.21W. 
  In this case a standard 0.25W resistor would be suitable.
• A 27 Ω resistor with 10V across it, needs a power rating P = V²/R = 10²/27 = 3.7W. 
  A high power resistor with a rating of 5W would be suitable.