A resistor is a two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in the accordance with ohm’s law.
R = V/I
The electrical resistance is equivalent to the voltage drop across the resistor divided by the current specifically flowing through the resistor. Resistors are integral part of all electronic circuits. There are basically two types of resistors (1) General purpose (2) high stability. General purpose resistors are of carbon composition. They are inexpensive. They are not so stable and their tolerance is 5 to 20% in value. High stability resistors include metal-film, cermets, metal foil, and wire would resistors. Their tolerances are low and range from 0.1 to 5%. While selecting a resistor, the following points are to be considered:
(1) Value, size and shape, leads
(2) Stability
(3) Tolerance
(4) Temperature coefficient of resistance
(5) Power rating
(6) Maximum operating voltage
(7) Reliability
(8) Change in resistance value with frequency
(9) Load life, environmental condition and age.
The SI unit of electrical resistance is the ohm (O). In an ideal resistor, the resistance remains steady regardless of the applied voltage or current flowing with the device or the rate of change of the current. While real restrictors cannot attain this goal; they are designed to present little variation in electrical resistance when subjected to these changes, or to changing temperature and other environmental factors.
CHARACTERISTICS OF RESISTORS: The following characteristics are inherent in all resistors and can be controlled by design considerations and choice of material.