it offers resistance to the flow of current. The two main characteristics of resistors are its Resistance (R) in ohm and Power rating (W) in watts. In electronic circuits, the resistors with higher 'R' values have lower wattage rating because they have less current.
All electrical devices that consume energy must have a resistor (also called a resistance) in their circuit model. Inductors and capacitors may store energy but over time return that energy to the source or to another circuit element. Power in the resistor, given by p = vi = i2R = v 2/R, is always positive as illustrated.
Because the relationship between voltage, current, and resistance in any circuit is so regular, we can reliably control any variable in a circuit simply by controlling the other two. Perhaps the easiest variable in any circuit to control is its resistance. This can be done by changing the material, size, and shape of its conductive components (remember how the thin metal filament of a lamp created more electrical resistance than a thick wire?).
Special components called resistors are made for the express purpose of creating a precise quantity of resistance for insertion into a circuit. Theyare typically constructed of metal wire or carbon, and engineered to maintain a stable resistance value over a wide range of environmental conditions. Unlike lamps, they do not produce light, but they do produce heat as electric power is dissipated by them in a working circuit. Typically, though, the purpose of a resistor is not to produce usable heat, but simply to provide a precise quantity of electrical resistance.
The most common schematic symbol for a resistor is zig-zag line:
Resistor values in ohms are usually shown as an adjacent number, and if several resistors are present in a circuit, they will be labeled with a unique identifier number such as Rı, R2, R3, etc. As you can see, resistor symbols can be shown either horizontally or vertically:
This is resistor "R1" with a resistance value of 150 ohms.
This is resistor "R2" with a resistance value of 25 ohms.
Real resistors look nothing like the zig-zag symbol Instead, they look like small tubes or cylinders with two wires protruding for connection to a circuit. Here is a sampling of several kinds of resistors:
In keeping more with their physical appearance, an alternative schematic symbol for aresisto! looks !ike a smal1, rectangular box:
Resistors can also be shown to have varying rather than fixed resistances. This might be for the purpose of describing an actual physical device designed for the purpose of providing an adjustable resistance, or it could be to show some component that just happens to have an unstable resistance.