- A resistor is a most commonly used passive component or device used to oppose or limit the flow of electric current. The ability of a resistor to limit the flow of current is known as resistance.
- The value of resistance of a conductor depends upon the following,
- The type of material of the conductor,
- The length of the conductor,
- The thickness (cross-sectional area of the conductor),
- The temperature at which it operates (in general, the higher its operating temperature the greater its resistance).
- Resistance, R = ρ(l/A)
- Where, A is the cross-sectional area, l is length, and ρ is the resistivity of the material in ohm-meters
- According to Ohm’s law, Resistance R =V/I Where, V is voltage and I is current.
- The unit of resistance is ohm, indicated by the Greek letter Ω (omega).
Some may think why the resistor is used in a circuit?
The main function of resistors in a circuit is to control the flow of current to other components. Take an LED (light) as an example. If too much current flows through an LED, it is destroyed. So a resistor is connected to it, to limit the current flowing to it.
Resistors are available in different range of resistance values. In large power resistors, the value of resistance, tolerance, and wattage rating are printed on it. In small resistors, colour bands are provided. By reading these colour bands, the value of the resistor can be identified. Tolerance is the amount that the ohm value can vary from the actual ohm value. Resistors are available in four band, five band and even six band. Let us just see the four band and five band resistors. It is shown in fig (a). The values for each colour is shown in the table. fig(a)
The fig(b) shows clearly how the values are found in the resistor using its colour codes. Let us see some resistors, whose values are found using its colour codes.
There are two types of resistors, Fixed and Variable.
Fixed Resistor: Fixed resistors have a specific value which cannot be changed. The resistors shown above with colour codes are the fixed resistors. There are different types of fixed resistors such as carbon composition resistor, wire-wound resistor, thick film resistor, thin film resistors.
Variable Resistor: Variable resistors are those whose value can be changed. Potentiometer, rheostat, trimmer are the variable resistors, in which the value of the resistor can be adjusted. The figure shown below is the rheostat, a variable wire wound resistor.
Individual resistors can be connected together in series connection, parallel connection or combination of both series and parallel together to produce a complex resistor network.
When the resistors are connected in series, they have a Common Current flowing through them as the current that flows from one resistor to another can take only one path.
R Total = R1 + R2 + R3
In this, the resistors are connected in a parallel way and have a Common Voltage across them. The current is not the same in a parallel circuit since there are multiple paths for the current to flow.