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Current, Voltage and Resistance
According to Ohm's Law, the relationship between resistance, current and voltage is given by R = V/I, where R is the resistance, V is the voltage and I is the current. This relationship means that you can reduce the amount of current flowing into a part of the circuit by placing a resistor in the path of the current. To find the value of the resistor you need to bring the current down to the desired level, you must use the formula for Ohm's Law. Resistance is measured in ohms.
Resistor Types
Resistors can be discrete -- separate components -- or they can be part of an integrated circuit (IC). Resistors have two terminals; these can be leads or flat metalized areas if the resistor is of the surface-mount type. Resistors can be made from a coil of wire (wire-wound resistors), metal film or metal oxide. Some are made from carbon -- either a carbon composite material or a rolled sheet of carbon film.
Terminology
Abbreviations are often used to describe both the composition of a resistor and the role it plays in the circuit. Carbon film resistors are often called CF resistors. The current limiting resistor in a circuit is often labeled the CL resistor. This can cause some confusion, as "CL" may be mistaken for a term relating to a resistor's composition.
Example of a Current Limiting Resistor
One example of a current limiting resistor is the resistor typically used with an LED (light emitting diode). LEDs have a very low internal resistance, meaning that they allow so much current to pass through them that they will quickly burn out and stop working. For this reason a current limiting resistor must be placed in series with an LED. This reduces the amount of current flowing through the LED and stops it from burning out.