The most common types of resistors include carbon-composition, carbonfilm, metal-film, wire-wound, and surface-mount or chip resistors. Carbon-film and metal-film resistors are better than carbon-composition resistors because they have tighter tolerances, are less affected by temperature and aging, and generate less noise internally.
A thermistor is a thermally sensitive resistor whose resistance value changes with temperature. If the resistance of a thermistor increases with temperature, it is said to have a positive temperature coefficient (PTC). If the resistance of a thermistor decreases with temperature, it is said to have a negative temperature coefficient (NTC).
Wire-wound resistors are typically used in high-current applications. Wire-wound resistors are available with wattage ratings of about 1 to 100 W.
Resistors are usually color-coded to indicate their resistance value in ohms. Either a four-band or a fiveband code is used. The five-band code is used for more precise R values. Chip resistors use a three- or four-digit code to indicate their resistance value.
Zero-ohm resistors are used with automatic insertion machines when it is desired to short two points on a printed-circuit board. Zero-ohm resistors are available in 1/8- or ¾-W ratings.
A potentiometer is a variable resistor with three terminals. It is used to vary the voltage in a circuit. A rheostat is a variable resistor with two terminals. It is used to vary the current in a circuit.
The physical size of a resistor determines its wattage rating: the larger the physical size, the larger the wattage rating. There is no correlation between a resistor's physical size and its resistance value.
The most common trouble in resistors is an open. An ohmmeter across the leads of an open resistor will read infinite, assuming there is no other parallel path across the resistor.
