Resistors are two terminal electrical or electronic devices that can restrict a flow of voltage by a specific amount. They are used to bring down the amount of voltage that flows in a circuit and are also used to protect circuits from excess voltage. They are designed to be used with any voltage and current values as long as their rated voltage/current value is not exceeded. The electrical resistance is equal to the voltage drop across the resistor divided by the current that is flowing through the resistor. Resistance is measured in ohms and is directly proportional to the length and the diameter of the resistor. Resistors are found in electrical networks and electronic circuits.
A resistor is generally used to produce a known voltage-to-current ratio in an electric circuit. If the current in the circuit is known, then a resistor can be used to produce a known potential difference proportional to the current. If the potential difference between two points in a circuit is known, a resistor can be used to produce a known current proportional to that difference.
An attenuator is a network that consists of two or more resistors (a voltage divider) used to reduce the voltage of a signal. A line terminator is a resistor at the end of a transmission line or daisy chain bus (such as in SCSI), that is designed to match impedance and minimize reflections of the signal. All resistors dissipate heat and this is the principle used for electric heaters.
There are two common types of electrical resistors; fixed and variable. Some resistors are cylindrical and have the actual resistive material in the centre or on the surface of the cylinder and a conducting metal lead projecting along the axis of the cylinder at each end. There are carbon film and metal film resistors. Resistors that are made with semiconductor material are usually more difficult to fabricate and take up too much valuable chip area.
A variable resistor can have its value adjusted by turning a shaft or sliding a control. These are types of resistors are also known as potentiometers or rheostats and they enable you to change the resistance by hand. Variable resistors can be low cost single-turn types or multi-turn types with a helical element. Some variable resistors come with a mechanical display to count the number of turns.
Other types of resistors include a metal oxide varistor (MOV), a thermistor, a positive temperature coefficient (PTC) resistor, a negative temperature coefficient (NTC) resistor, and a sensistor.
Resistors are color coded to identify their resistance values. Resistors can be configured as chip arrays, resistor networks or standalone resistors. While selecting a resistor you must consider certain things such as: its power rating, the rated power supported, the resistance range; the tolerance (which is the tolerance value of the resistor) and mounting details, etc.