Resistor

From ArticleWorld

Resistance is the opposition that a conductor offers to the flow of current. The current flowing through a conductor depends on the electric field and the resistivity of the conductor. The component which resists the flow of current is known as a resistor. Resistance offered by a resistor is measured in ohm. Resistors are placed very commonly in electronic circuits.

Types of resistors

Resistors are mainly classified into two types - fixed value resistors and variable resistors.

1.Fixed value resistors: A fixed resistor is a two terminal resistor whose resistance has been fixed to a standard value. Fixed value resistors are further classified as:

• Carbon composition resistors or carbon film resistors: these are manufactured using carbon and silica.
• Wire wound resistors: here a nickel chromium wire is wound on a ceramic base.
• Metal film resistors: It is manufactured using different types of metals like nickel, chromium, copper, etc.

2.Variable resistors: A variable resistor is a three terminal resistor whose resistance can be changed between zero to a certain maximum value. Variable resistors are of the following types:

• Potentiometers: these are variable resistors either of carbon or wire wound type used for controlling voltage and current.
• Rheostats: a rheostat is a variable resistor with two terminals connected in series with load.
• Presets (trimmers): the value of preset can be fixed to a certain value by using a threaded lead screw or a worm gear.

Resistor colour coding

Printing the numerical value on the resistor is not possible or at least very difficult as the carbon resistors are physically small. Thus, for marking the value of the resistor colour coding system is used. Colour bands are printed on the body of the resistor which indicates some numerical values. Usually there are four bands marked on the resistor, out of which three are placed at one end of the resistor. These three bands indicate the value of the resistor. The fourth band is at the other end and indicates the tolerance of the resistor. Some resistors have only three bands - the absence of the fourth band shows that the tolerance of the resistor is (+/-)20%. Other resistors have five bands, out of which four colour bands give resistance value with tolerance and fifth band gives reliability level or failure rate.

Applications

Resistors are used in most electric circuits. They can also be used to provide intense light or heat. Hence they are used in heating elements in household cooking ranges. Resistors are used as tungsten filaments in incandescent lamps. Rheostats and potentiometers are used in electrical appliances which require the current or resistance to be varied, as in adjustable generators or lights that dim.