Like many electronic components, resistors come in a variety of shapes, sizes, capacities and types. All resistors are not created equal, however. Each type of resistor has significant differences in typical values for resistor noise, tolerances, wattage rating, temperature coefficient, voltage coefficient, frequency response, size, and reliability. These differences bring advantages and limitations making some resistors ideal in some applications and a source of troubleshooting nightmares in others.
Carbon Composition Resistors
Carbon composition resistors used to be the most common type of resistors used in electronics due to their relative low cost and their great reliability. Carbon composition resistors use a solid block of material made from carbon powder, an insulating ceramic, and a binder material. The resistance is controlled by varying the ratio of carbon to the filler materials. The carbon composition in the resistor is effected by environmental conditions, especially humidity, and tends to change in resistance over time. For this reason, carbon composition resistors have a poor resistance tolerance, typically only 5%. Carbon composition resistors are also limited to power ratings of up to 1 watt. In contrast to their poor tolerances and low power, carbon composition resistors have a good frequency response making them an option for high frequency applications.
Carbon Film Resistors
Carbon film resistors use a thin layer of carbon on top of an insulating rod which is cut to form a narrow, long resistive path. By controlling the length of the path and its width, the resistance can be precisely controlled with tolerances as tight as 1%. Overall, the capabilities of a carbon film resistor are better than a carbon composition resistor, with power ratings up to 5 watts and better stability. However, their frequency response is much worse due to the inductance and capacitance caused by the resistive path cut in to the film.
Metal Film Resistors
One of the more common axial resistor types used today are metal film resistors. They are very similar in construction to carbon film resistors, with the main difference being the use of a metal alloy as the resistive material rather than carbon. The metal alloy used, typically a nickel-chromium alloy, is able to provide tighter resistance tolerances than carbon film resistors with tolerances as tight as 0.01%. Metal film resistors are available up to about 35 watts, but resistance options begin to diminish above 1-2 watts. Metal film resistors are low noise, and stable with little resistance change due to temperature and applied voltage.
Thick Film Resistors
Becoming popular in the 1970s, thick film resistors are common surface mount resistors even today. They are made in a screen printing process using a conductive ceramic and glass mixture composite suspended in a liquid. Once the resistor has been screen printed, it is baked at high temperatures to remove the liquid and fuse the ceramic and glass composite. Initially, thick film resistors had poor tolerances, but today they are available with tolerances as low as 0.1% in packages that can handle up to 250 watts. Thick film resistors do have a high temperature coefficient, with a 100°C temperature change resulting in up to a 2.5% change in resistance.
Thin Film Resistors
Borrowing from semiconductor processes, thin film resistors are made by through a vacuum deposition process called sputtering where a thin layer of conductive material is deposited on an insulating substrate. This thin layer is then photo etched to create a resistive pattern. By precisely controlling the amount of material deposited and the resistive pattern, tolerances as tight as 0.01% can be achieved with thin film resistors. Thin film resistors are limited to about 2.5 watts and lower voltages than other resistor types but are very stable resistors. There is a price for the precision of thin film resistors which generally are twice the price of thick film resistors.
The highest power and most precise resistors are wirewound resistors, although rarely are they both high power and precise at once. Wirewound resistors are made by wrapping a high resistance wire, generally a nickel chromium alloy, around a ceramic bobbin. By varying the diameter, length, alloy of the wire and the wrap pattern the properties of the wirewound resistor can be tailored to the application. Resistance tolerances are as tight as 0.005% for precision wirewound resistors and can be found with power ratings up to around 50 watts. Power wirewound resistors typically have tolerances of either 5 or 10% but have power ratings in the kilowatt range. Wirewound resistors do suffer from high inductance and capacitance due to the nature of their construction, which limits them to low frequency applications.
Varying a signal or tuning a circuit is a common occurrence in electronics. One of the easiest ways to manually adjust a signal is through a variable resistor or potentiometer. Potentiometers are commonly used for analog user inputs such as volume controls. Smaller surface mount versions are used to tune or calibrate a circuit on a PCB before being sealed up and shipped to customers. Potentiometers can be very precise, multi-turn variable resistors, but often they are simple single turn devices which move a wiper along a conductive carbon path to change a resistance from near zero to the maximum value. Potentiometers generally have very low power ratings, poor noise characteristics, and mediocre stability. However, the ability to vary the resistance and adjust a signal makes potentiometers invaluable in many circuit designs and in prototyping.
Other Resistor Types
As with most components, several specialty resistor variants exist. In fact, several are quite common including the resistive element in the incandescent light bulb. Some other specialty resistor variants include heating elements, metal foil, oxide, shunts, cermet, and grid resistors to name a few.