Resist is a word that means “to oppose”. Resistance is the property of opposing the flow of electrons in a conductor or semiconductor. A resistor is an electronic component that has the property of resistance.
“A passive electronic component with two terminals is used to limit or regulate the flow of electric current in electrical circuits“.
A resistor is a passive two-terminal electrical component that applies electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, divide voltage, divide active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate several watts of electrical power as heat can be used as part of motor control, in power distribution systems, or as test loads for generators. Static resistors have resistances that only change slightly with temperature, time, or operating voltage. Variable resistors can be used to adjust circuit elements, or as sensing devices for heat, light, humidity, force, or chemical activity.
The main purpose of the resistor is to reduce the current flow and to reduce the voltage across a particular part of the circuit. It is made of copper wires that are coiled around a ceramic rod and the exterior of the resistor is coated with insulating paint.
The SI unit, Symbol, and Formula of the Resistor
The SI unit of resistance is the ohm, which is represented by the omega.
Symbols of Resistor
The symbol for a resistor is shown below.
The Formula of Resistance
The formula for resistance is,
R = V/I
Where V is voltage and I is current. It would be really hard to build up resistors with each value. Therefore, some values are chosen and resistors of such values are simply manufactured. These are called “preferred values”. In practice, resistors with close values are chosen to match the required applications.
Resistor Color Code
Resistor color codes were invented by the Radio Manufacturers Association (RMA) in the 1920s. A process called color coding is used to determine the value of the resistance of a resistor. A resistor is coated with a color band where each color determines a particular value.
The table below shows a list of the values that each color indicates.
How to calculate the value of the resistor?
Tolerance: Tolerance is the range of value up to which a resistor can withstand without being destroyed. This is an important factor. The following figure shows how the value of a resistor is determined by the color code.
According to Resistor Color Code with Resistor Bands Table:
For a 4-band resistor: The first two colored bands indicate the first and second digits of the value and the third color band represents the multiple zeros added(Multiplier). The fourth color band indicates the tolerance value.
For a 5-band resistor: The five color band resistors are manufactured with tolerances of 2% and 1% and are also manufactured for other high accuracy resistors. The first three colored bands indicate the first, second, and third digits of the value and the Fourth color band represents the multiple zeroes added(Multiplier). The fifth color band indicates the tolerance value.
For a 6-band resistor: The first three colored bands indicate the first, second, and third digits of the value, and the Fourth color band represents the multiple zeroes added(Multiplier). The fifth color band indicates the tolerance value and the sixth color band represents the Temperature Coefficient value.
Example: Set the value of the resistor with the color codes of yellow, blue, orange, and silver.
Solve: The value of yellow is 4, blue is 6, and orange is 3 which represents the multiplier. Silver is ±10 which is the tolerance value.
Hence the value of the resistor is,
46×1000 = 46000 = 46kΩ ( as 1000 = 1K)
The maximum resistance value for this resistor is,
46kΩ or 46000Ω + 10% = 46000 + 4600 = 50600Ω = 50.6kΩ
The minimum resistance value for this resistor is,
46kΩ or 46000Ω – 10% = 46000 – 4600 = 41400Ω = 41.4kΩ
Types of Resistor
Several types of resistors are available and can be used in electronic circuits. These different types of resistors have different properties depending on the type and construction. Different types of resistors are available for different applications. Resistors are available in various shapes, sizes, and materials. Generally, resistors can be classified into two types namely linear resistors and non-linear resistors.
Linear Resistors
The resistors whose value changes with the applied temperature and voltage are called linear resistors. Most types of resistors are linear devices that produce a voltage drop when current flows through them. There are two basic types of resistors with linear properties namely fixed resistors and variable resistors.
Fixed Resistors
Fixed resistors are resistors with a specific value. Fixed resistors are one of the most widely used resistors. Fixed resistors are used in electronics circuits to determine the correct position in a circuit.
Types of Fixed Resistors
- Wire Wound Resistors.
- Carbon Composition Resistors.
- Thin Film Resistors.
- Thick Film Resistors.
Types of Thick Film Resistors
- Fusible Resistors.
- Cermet Film Resistors.
- Metal Oxide Resistors.
Types of Thin Film Resistors
- Metal Film Resistor.
- Carbon Film Resistor.
Variable Resistors
Variable resistors have a slider that taps on the main resistive element and a fixed resistive element. Simply we can say that a variable resistor is a potentiometer that has only 2 connecting wires instead of 3.
Types of Variable Resistors
- Potentiometer.
- Rheostat.
- Trimmer Resistor.
Non-Linear Resistor
Non-linear resistors are those types of resistors in which the current flowing through it changes with the exchange of applied voltage or temperature and does not change according to Ohm’s law. There are many types of non-linear resistors, but the most commonly used resistors are mentioned below.
Thermistors
Thermistors are a type of variable resistor that detects changes in temperature. In other words, it is a 2 terminal device that is very sensitive to temperature. The resistance of a thermistor is inversely proportional to temperature.
Varistor Resistors
A varistor is a non-linear resistor composed of a semiconductor and current that depends non-linearly on the voltage applied across it. The MOV or metal oxide varistor is the most commonly used form of varistor resistor.
Photo Resistor or LDR (Light Dependent Resistors)
A photoresistor or LDR (Light Dependent Resistors) or photoconductive cell is a light-controlled variable resistor. The photoresistor resistance decreases with an increase in the intensity of the incident light.
Surface Mount Resistors
Surface mount resistors also known as SMD resistors are rectangular in shape. SMD is an electronic component designed for use with SMT or Surface Mount technology.
Some Important Terms
Resistance
Resistance is the property of a resistor that opposes the flow of current. When an alternating current passes through a resistance, a voltage drop is produced that is in phase with the current.
- Indication − R
- Units − Ohms
- Symbol − Ω
There are other important terms along with resistance, called reactance and impedance.
Reactance
The resistance given to alternating current can be understood as reactance due to capacitance and inductance present in the circuit. When the alternating current passes through a pure reactance, a voltage drop is produced that is 90° out of phase with the current
Depending on the phase, i.e. +90° or -90° reactance may be called inductive reactance or capacitive reactance.
- Indication − X
- Units − Ohms
- Symbol − Ω
Impedance
Impedance is the effective resistance to alternating current arising from the combined effects of ohmic resistance and reactance. When an alternating current passes through an impedance, a voltage drop is produced that ranges from 0° to 90° with the current.
- Indication − I
- Units − Ohms
- Symbol − Ω
Conductance
It is the ability of a material to conduct electricity. It is the reciprocal of resistance.
- Indication − G
- Units − Mhos
- Symbol − ℧