Resistor

Types of resistor

Resistors may be fixed or variable.

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Fixed resistors

Some resistors are cylindrical, with the actual resistive material in the centre (composition resistors) or on the surface of the cylinder film resistors, and a conducting metal lead projecting along the axis of the cylinder at each end. This is called an axial package resistor. The photo above right shows a row of common resistors. Power resistors come in larger packages designed to dissipate heat efficiently. At high power levels, resistors tend to be wire wound types. Resistors used in computers and other devices are typically much smaller, often in surface-mount packages without wire leads. Resistors are built into integrated circuits as part of the fabrication process. A single IC may contain millions of resistors.

Related Topics:
Wire wound - Surface-mount - Integrated circuit

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Variable resistors

The variable resistor is a resistor whose value can be adjusted by turning a shaft or sliding a control. These are also called potentiometers or rheostats and allow the resistance of the device to be altered by hand.

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Variable resistors can be inexpensive single-turn types or multi-turn types with a helical element. Some variable resistors can be fitted with a mechanical display to count the turns.

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Variable resistors can sometimes be unreliable, because the wire or metal can corrode or wear. Some modern variable resistors use plastic materials that do not corrode and have better wear characteristics.

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Some examples include:

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• a rheostat: a variable resistor with two terminals, one fixed and one sliding. It is often used with high currents.
• a potentiometer: a common type of variable resistor. One common use is as volume controls on audio amplifiers and other forms of amplifier.

Other types of resistor

Note that some of the following resistors do not obey Ohms law

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• A metal oxide varistor (MOV) is a special type of resistor that has two very different resistance values: a very high resistance at low voltage (below the trigger voltage) and very low resistance at high voltage (above the trigger voltage). It is usually used for short circuit protection in power strips or lightning bolt "arrestors" on street power poles, or as a "snubber" in inductive circuits.
• A thermistor is a temperature-dependent resistor. There are two kinds, classified according to the sign of their temperature coefficients:
• A Positive Temperature Coefficient (PTC) resistor is a resistor with a positive temperature coefficient. When the temperature rises the resistance of the PTC increases. PTCs are often found in televisions in series with the demagnetizing coil where they are used to provide a short-duration current burst through the coil when the TV is turned on. One specialized version of a PTC is the polyswitch which acts as a self-repairing fuse.
• A Negative Temperature Coefficient (NTC) resistor is also a temperature-dependent resistor, but with a negative temperature coefficient. When the temperature rises the resistance of the NTC drops. NTCs are often used in simple temperature detectors and measuring instruments.
• A sensistor is a semicondutor-based resistor with a negative temperature coefficient, useful in compensating for temperature-induced effects in electronic circuits.
• Light-sensitive resistors are discussed in the photoresistor article.
• All wire, except for superconductors, exhibits some resistance, based on its cross section area and the conductivity of the material it is made of.