The resistors function is to oppose the flow of electric current.
The most common schematic symbol for a resistor is a zigzag line shown in above diagram.
An alternative schematic symbol for a resistor is a small and rectangular in shape
Resistors are also classified according to the material from which they are made.
The typical resistor is made of either carbon film or metal film.
The value of resistor is determined by its colour.
While selecting a resistor it is important to take value of tolerance and electric power rating of the resistor into consideration along with resistance value of the resistor. The tolerance of a resistor denotes how close it is to the actual rated resistance value. For example, a ±1% tolerance would indicate a resistor that is within ±1% of the specified resistance value, and the power rating indicates how much power the resistor can safely tolerate.
The maximum rated power of the resistor is specified in Watts.
There are two kinds of resistors namely fixed resistors and the variable resistors.
- Fixed Resistors
A fixed resistor is one in which the value of its resistance cannot change.
- Carbon film Resistors
This is the most general purpose, cheap resistor. Usually the tolerance of the resistance value is ±5%. Power ratings of 1/8W, 1/4W and 1/2W are frequently used.
Carbon film resistors have a disadvantage; they tend to be electrically noisy. Metal film resistors are recommended for use in analog circuits.
- Metal film Resistors
Metal film resistors are used when a higher tolerance (more accurate value) is needed. They are much more accurate in value than carbon film resistors. They have about ±0.05% tolerance. They have about ±0.05% tolerance. Resistors that are about ±1% are more than sufficient. Ni-Cr (Nichrome) seems to be used for the material of resistor. The metal film resistor is used for bridge circuits, filter circuits, and low-noise analog signal circuits.
- Variable Resistors
There are two general ways in which variable resistors are used. Adjustment of one of the resistor is such that its value can be easily changed, like the volume adjustment of Radio while adjustment of Semi-fixed resistors are done to compensate for the inaccuracies in the resistors, and to fine-tune a circuit. The rotation angle of the variable resistor is usually about 300 degrees. At certain situation, variable resistors must be rotated according to get the whole range of resistance they offer. This allows for very precise adjustments of their value. These are called "Potentiometers" or "Trimmer Potentiometers."
- Other Resistors
There is another type of resistor besides the carbon-film type and the metal film resistors. It is called wirewound resistor.
A wirewound resistor is made of metal resistance wire, which helps to give them precise values. Also, using a thick wire material can make high-wattage resistors. Wirewound resistors cannot be used for high-frequency circuits as Coils are used in high frequency circuits. Another type of resistor is the Ceramic resistor. These are wirewound resistors in a ceramic case, strengthened with special cement having very high power ratings, from 1 or 2 watts to dozens of watts. While designing or making these resistors, it is necessary to take its heat withstanding ability into account as these devices can easily get hot enough to burn.