resistors Components of electric circuit with known value of resistance*. Various types of resistors are in use.
(I) Carbon composite resistors: Stability of this type of resistors is poor. Heating of these resistors causes permanent damage.
(ii) Carbon film resistors: These are prepared by heating ceramic rods in methane gas. This produces a thin film of carbon on the surface of the ceramic. The resistance of the carbon film depends on its thickness. and width. Grooves are cut on the surface of the film to a desired width of the film.
(iii) Wire wound resistors: These are made of nicrome alloy (alloy of nickel, copper and chromium). These are used as standard resistors. These must be non inductively wound to avoid self inductance. Such coils are used in resistance boxes.
The value of resistance on a resistor is marked using color codes shown in fig. , or resistance code discussed later.
Table-RI Color code for resistors
|
black |
0 |
green |
5 |
|
brown |
1 |
blue |
6 |
|
red |
2 |
violet |
7 |
|
orange |
3 |
gray |
8 |
|
yellow |
4 |
white |
9 |
The fourth band of color shows precision of rating, called tolerance. Red signifies that the resistance is within 2%, gold 5%, and silver 10%. No fourth band signifies that tolerance is 20%.
In the resistance code the value of resistance is printed on the resistor in the way shown below.
1.6W is written as 1R6
1.6kW is written as 1k6
160kW is written as 160k or M16
The extra letter at the end gives the tolerance.
F 1%; G 2%; J 10%; M 20%.
Variable resistor: The resistance of such resistors can be altered by turning a spindle. Three terminal variable resistors are used as voltage divider or potentiometer. The rheostat is also a type of variable resistor.