How to check the 'Value' of a Resistor

A resistor is the most basic component used in most of electronic circuits. These are use to control the flow of current and voltage in different portions of a circuit. The resistor resists the flow of electrons from it and hence, the flow of current is controlled. The value of a resistor (or resistance) is determined in 'ohms'. The unit ohm is named after George Simon Ohm who purposed the Ohm's Law giving the fundamental relation between resistance(R), voltage(V) and current(I).

V=I.R

The symbol of resistance is the Greek letter omega (Ω).

The symbol for resistor can be represented in an electronic circuit in one of the following ways:-

The resistors may be classified into 4 categories according to their composition:-

• Carbon Composition Resistor - Made of carbon dust or graphite paste, low wattage values
• Film or Cermet Resistor - Made from conductive metal oxide paste, very low wattage values
• Wire-wound Resistor - Metallic bodies for heatsink mounting, very high wattage ratings
• Semiconductor Resistor - High frequency/precision surface mount thin film technology

The most of the resistor used are in axial form with a pattern of color stripes to indicate value of resistance. And the surface mount or SMD resistors are marked with numerals to determine the value.

The axial resistors are coded either with 4 band or 5 band resistors. 

Color Coding:-

The following table shows colors used to identify the value:-

The marking on different types resistors is as shown below:-

Four-band resistor, Five-band resistor, Cylindrical SMD resistor, Flat SMD resistor

Some SMD resistors are made in the shape of small cylinder while the most common type is flat. Cylindrical SMD resistors are marked with six bands - the first five are "read" as with common five-band resistors, while the sixth band determines the Temperature Coefficient (TC), which gives us a value of resistance change upon 1-degree temperature change.

The resistance of flat SMD resistors is marked with digits printed on their upper side. First two digits are the resistance value, while the third digit represents the number of zeros. For example, the printed number 683 stands for 68000Ω, that is 68kΩ.

The most popular and easy way to remember the color coding scheme for resistor is to remember the phrase:- 

"B. B. Roy of Great Britain has a Very Good Wife"

B-Black, B-Brown, R-Red, G-Green, B-Blue, V-Voilet, G-Grey, W-White.

Wattage Rating:-

When current is passed through resistors, heat is produced due to resistivity and if the temperature rises beyond certain level the resistor may get damaged. The ability to dissipate this heat is know as wattage rating. The wattage rating of a resistor depends upon its size and dimensions, i.e., bigger the size of resistor greater is the wattage rating. The following figure gives an idea about size and wattage rating:-