What is a Resistor?
An electric resistor is a two-terminal passive component specifically used to oppose and limit current. A resistor works on the principle of Ohm’s Law which states that voltage across the terminals of a resistor is directly proportional to the current flowing through it.
Ohm’s Law: V = IR
where V is the voltage applied across resistor,
I is the current flowing through it,
and R is the constant called resistance.
The unit of resistance is ohms.
Types of Resistors:
Resistors can be broadly classified based on the following criteria: the type of material used, the power rating and resistance value.
1. Fixed resistors.
In some scenarios, an electrical circuit may need a lesser amount of current to flow through it than the input value. Fixed resistors are used in these situations to limit the flow of current.
1.1 Carbon Composition Resistors:
These resistors are cylindrical rods which are a mixture of carbon granules and powdered ceramic. The resistor value depends on the composition of the ceramic material. A higher quantity of ceramic content will result in more resistance. Since the rod is coated with an insulated material, there are chances of damage due to excessive heat caused by soldering.
High current and voltage can also damage the resistor. These factors bring irreversible changes in the resistance power of these resistors. This type of resistor is rarely used nowadays due to their high cost and are only preferred in power supply and welding circuits.
1.2 Carbon film resistors:
This resistor is formed by depositing a carbon film layer on an insulating substrate. Helical cuts are then made through the carbon film to trace a long and helical resistive path. The resistance can be varied by using different resistivity carbon material and modifying the shape of the resistor. The helical resistive path make these resistors highly inductive and of little use for RF applications.
They exhibit a temperature coefficient between -100 and -900 ppm/ °C. The carbon film is protected either by a conformal epoxy coating or a ceramic tube. The operation of these resistors requires high pulse stability.