Colour code for resistors

Introduction

A resistor is an electrical component with two terminals that performs the critical function of managing electrical resistance as a circuit element. Resistors perform the vital function of reducing the current flow. They are also crucial in adjusting the signal levels and dividing voltages. They bias active elements and assist in the termination of transmission lines. Owing to the tiny size of resistors, it is a challenge to print the resistance values on them. This challenge is resolved using a colour code for resistors with colour bands representing their resistance value. The Radio Manufacturers Association (RMA) invented and implemented the colour code for resistors in the year 1920.

Resistors

A resistor is a two-terminal passive electrical component used to limit or regulate the flow of electric current in electrical circuits. It allows the introduction of a controlled amount of resistance into an electrical circuit. 

The main job of a resistor is the reduction of current flow and the lowering of voltage in a specific section of the circuit. It is composed of copper wires wrapped around a ceramic rod and coated with insulating paint.

They are universally accepted elements of electrical networks and electronic circuits, and their existence is ubiquitous. Practical resistors are discrete components whose composition involves various compounds and forms. They are also an integral part of integrated circuits.

The resistance value of resistors indicates their electrical function. The commonly used resistors have an exclusive range of over nine orders of magnitude. The nominal value of the resistance falls within the manufacturing tolerance framework indicated by colour bands on the component.

The resistor’s value is measured in ohms, and the symbol of the resistor is represented by the Greek letter, capital omega (Ω). The standard symbol of the resistance is denoted by either R or Ω.

Types of resistors

Generally resistors are divided into the following two categories:

• Linear resistors

Linear resistors have a linear relation between current and voltage drop. These types of resistor are dependent on applied temperature and voltage. Linear resistors also can be divided in two types as fixed and variable resistors.

• Fixed resistors

Fixed resistors have a specific value and are most widely used in circuits. Their usage in electronic circuits results in the setting up of the correct conditions in that particular circuit.

Fixed resistors have following types:

1. Wire wound resistors

2. Thin film resistors

3. Carbon composition resistors

• Variable resistors

In many circuits we need different resistance values to understand current-voltage relation so that we need variable resistors. Variable resistor can be achieved by slider taps on the resistive element. Potentiometer is an example of Variable resistor.

Example of variable resistor are following:

1. Potentiometer

2. Rheostat

3. Trimmer resistor

• Non-linear resistors

Unlike linear resistors, some resistors show non-linear relations in between current and voltage. Current flow in non-linear resistors also depends on applied temperature and voltage. Non-linear resistance can be seen in diodes.

Some example of non-linear resistors are following:

• Thermistor resistors

Thermistor resistors are devices that are highly sensitive and quickly detect a variation in temperature. They are two-terminal resistors that are highly sensitive to temperature and its variations. Thermistor’s resistance is inversely proportional to the temperature.

• Varistor resistors

A varistor comprises a semiconductor and current with which it non-linearly depends on the applied voltage across the resistor. Metal oxide varistor is the most commonly used variant of varistors.

• Photoresistor or light dependent resistor

As the name suggests, a photoresistor or light-dependent resistor is a variable resistor that is controlled by light. The resistance of the photoresistor is inversely proportional to the incident light intensity. Increased light intensity decreases its resistance. It is also known as a photoconductive cell.

Colour code for resistors

Printing of resistance values on resistors is critical as an incorrect selection of resistors may damage the other components. The tiny size of resistors makes it challenging for the resistance values to be printed on them. This challenge is resolved using a colour code for resistors with colour bands representing their resistance value. The colour code for resistors was invented and implemented in 1920 by The Radio Manufacturers Association (RMA).

Calculation of resistance using colour code for resistors

The following steps need to be followed to calculate the resistance for resistors:

1. The first step is to hold the resistor in a manner that the tolerance band is facing your right. Tolerance bands are gold or silver in colour, and they are placed slightly further away from other bands.
2. Start the reading from the left side of the colour bands and sequentially make a note of them.
3. Next, use the colour code table and note down the digits they represent.
4. The band next to the tolerance band is the multiplier band. If the colour on the multiplier band, for example, is red, then its representation is 2, and the value given is 102.

Let us find the resistance in the above case using the colour codes.

The resistance is 56k Ω ± 5%

Conclusion

A resistor is an electrical component with two passive terminals. It performs the critical function of executing electrical resistance as a circuit element, thereby regulating the flow of electrical current. Resistors are the most important and commonly used components in an electronic circuit.

Printing of resistance values on resistors is critical as an incorrect selection of resistors may damage the other components. A designated colour code for resistors is used to represent their resistance value with the help of colour bands. Learning to recognise the colour bands is vital to determine their resistance value.