Resistors

The resistors often have 4 or 5 colour bands printed on them to show you their value in Ohms(Ω)

Resistors also come in a wide range of power ratings measured in Watts(W), for most low powered circuits 1/8 (one eighth) Watt 1/4 (Quarter Watt) 1/2 (Half Watt) 1W, and 2W are standard. What are know as power resistors can go to liquid cooled Mega Watt's.

You need to use the correct power rating so as to not over heat the resistor for the voltage times the current flowing through them.

The following examples are for common resister type used by hobbyist made from Carbon or Metal Oxide and coated with colour bands added for their resistor values. (There are many other designs and power ranges, for example like 'surface mount' and 'wire wound' resistors etc)

4 Colour

Four colour banded resistors have two digits the third band is the multiplier the forth is Silver(±10%) or Gold(±5%) band

Five colour banded resistors have three digits the forth band is the multiplier as they have a tighter tolerance the last is Red(±2%) or Brown(±1%) and sometimes the most accurate Green ±0.5% tolerance.

A Sixth colour band can also be added for the resistors Temperature Coefficient

6 band resistor colour chart

Resistors oppose the flow of electrons (current I) limiting the electrons ability to move freely, they can also produce heat in the form of electromagnetic radiation(photons) when the electrons collide with atoms slowing down and imparting energy that is then released as a photon. Radiation heat transfer can occur by infrared, ultraviolet, microwave and radiowaves. Infrared (electromagnetic radiant infrared energy) is the transfer of heat energy via invisible electromagnetic energy waves.

Reisitance(R) is mesured in ohms

The conection between the flow of electrons (Current I) and the resistance is related to the Voltage(V) applyed across the resitance in a circuit was first worked out by a famos German physicist called Georg Ohm and was published in 1827.

Ohm's Law V = I x R or V=IR where V = volts, I = current in Amperes and R = resistance in ohms Ω

Note: However some materials do not obey Ohm's law, these are called non-ohmic.

Standard Resistor values

Standard abbreviations used in circuit diagrams:

R is used to represent Resistances with values less than 1000 Ohms( 1K Ω )

K used for one thousand (Kilo Ohm)

M used for one million (Mega Ohm).

Examples:

A 8.2 Ohm resistor is written 8R2 and for an 4,700 Ohm resistor is written 4K7 while 2,200,000 ohms is written 2M2.

There are Two ranges of values used in manufacturing resistors E12 using 12 basic values and the additional E24 range both that can be modified by a decimal multiplier such as 2R2 , 22R , 220R , 2K2 , 22K , 220K or 2M2

The E12 Range

The E12 range has 12 "preferred" or "basic" resistor values, and all of the others are simply decades of these values:

1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8 and 8.2

The E24 Range

The E24 range is a more precise version of the E12 Range of preferred values that includes all of the E12 values, plus a further 12 to enable the selection of more accurate resistances values.

The E24 range the preferred values are:

1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2 and 9.1

Resistor Colour Coding Explained

Reading Resistor Chart Values

This following video is very good, it goes into power, frequency, Noise and Temperature Coefficient etc

What is Resistor? Different Types of Resistors and Different Characteristics of Resistors