An Overview of Resistors

What are resistors?

Resistors are the major part of electrical networks and electronic circuits. Resistors are electrical devices that control the current flow through a circuit. They are ubiquitous in electronic equipments.

Resistors can be used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, in electronic circuits.

The electrical resistance in a resistor is measured in Ohms (Ω).

Ohm's law:
V = I. R.

Ohm's law states that the voltage (V) across a resistor is proportional to the current (I), where the constant of proportionality is the resistance (R).

For example: if a 300 ohm resistor is attached across the terminals of a 12 volt battery, then a current of 12 / 300 = 0.04 amperes flows through that resistor.

 

Schematic diagram symbols

Two typical schematic diagram symbols are as follows:

Types

Resistors are available in different sizes, shapes and materials. There are various types of resistors that are used for different applications.

There are two basic types of resistors.

•    Linear Resistors
•    Non Linear Resistors

How Do They Work?

Resistors work on the principle that energy can be neither created or destroyed, but it can be changed from one state to another.

In resistors, electrical energy is converted into heat or thermal energy. All resistors generate heat as a product of their function. Every resistor has different resistance based on the color coding in it. The varying resistance is obtained by different doping concentrations.

A resistor is made of a material that has a specific amount of resistance to current flow. By selecting the correct amount of desired resistance in a circuit the installer can control the amount of output that passes through the resistor to the electronic component.

Resistor Value Calculation

Resistors are labeled by a series of color coded bands wrapped around the resistors. These color bands identify several features of the resistor to the user.

In the below figure, five color bands are shown around the resistor. This is the maximum number; but there may be as few as three. These color bands are oriented towards one end of the resistor. They can be read from left to right starting with the extreme left band.

The first three (3) bands have to do with the value of the resistor in resistance (Ohms).

i.e. The “First Digit”, “Second Digit”, and “Multiplier” identify the value of the resistor.

The fourth (4) band (if present) identifies the Accuracy or Tolerance of the resistor. This specification is measured in percentage (%).

The fifth (5) and final band (if present) indicates the failure rate of the resistor. This number is rated as a percentage of failure per 1000 hours of operation.

It can be calculated as:

Digit, Digit, Multiplier = Colour, Colour x 10 colour  in Ohm’s (Ω)

Sample Resistor Value Calculation  Using the following example, we can calculate the value of a resistor. Note that each color band corresponds with a numerical value. The following values provide us with a specific picture of the resistors characteristics:

a) 1st Digit - Brown, corresponds with a value of 1.
b) 2nd Digit - Green, corresponds with a value of 5.
c) Multiplier - Black, corresponds with a value of 1.
Calculation 15 X 1 = 15 or 15 Ohms of resistance.
d) Tolerance - Gold, corresponds with a value of +/- 5%.
e) Failure Rate - Brown, corresponds with a value of 1%

Uses of a resistor:

Resistors can be used as heating elements, single-use fuses, and resistive sensors. Heating elements that feature resistors include electric stoves, ovens, water heaters and coffee makers. Resistors are used in fuses to prevent damage to electronics.

Resistor-based sensors include lie detectors and gas sensors, among others. These types of resistor sensors measure the change in resistance caused by liquids and gases.

Functions of Resistor

Some of the important functions of resistor in electronic circuits are:

a) Resistors play a key role in limiting the current in any electronic circuits
b) Resistors provide appropriate bias voltage to the active devices
c) They provide bias stabilization in combination with capacitors
d) Resistors give time delays in combination with capacitors
e) Resistors set up the proper value of voltage in circuits