The Different Types of Resistors

Resisters are designed for specific use cases and aren't interchangeable

Resistors
Don Farrall/Photographer's Choice RF/Getty Images

Like many electronic components, resistors come in a variety of shapes, sizes, capacities, and types, and each presents significant differences in typical values for resistor noise, tolerances, wattage rating, temperature coefficient, voltage coefficient, frequency response, size, and reliability. Some resistors are 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 because of their relatively 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 affected 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 percent. 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 viable for high-frequency applications.

Carbon Film Resistors

Carbon film resistors use a thin layer of carbon on top of an insulating rod that 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 percent. 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 into the film.

Metal Film Resistors

One of the more common axial resistor types used today are metal film resistors. Similar in construction to carbon film resistors, the main difference follows from the use of a metal alloy as the resistive material rather than carbon.

The metal alloy, typically a nickel-chromium alloy, provides 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

Thick film resistors became popular in the 1970s and are common surface mount resistors even today. They are made by a screen printing process using a conductive ceramic-and-glass-mixture composite suspended in a liquid. After 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 percent in packages that can handle up to 250 watts. Thick film resistors do have a high-temperature coefficient, with a 100-degree Celsius temperature change resulting in up to a 2.5 percent change in resistance.

Thin-Film Resistors

Borrowing from semiconductor processes, thin-film resistors are made 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 photoetched to create a resistive pattern.

By precisely controlling the amount of material deposited and the resistive pattern, tolerances as tight as 0.01 percent 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.

Wirewound 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, the alloy of the wire, and the wrap pattern, the properties of the wire-wound resistor tailored to the application.

Resistance tolerances are as tight as 0.005 percent for precision wire-wound resistors and can be found with power ratings up to around 50 watts. Power wire-wound resistors typically have tolerances of either 5 percent or 10 percent 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.

Potentiometers

Hand turning stereo dial
Kirby Hamilton

Varying a signal or tuning a circuit is a common requirement for sensitive electronic applications. 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 tune or calibrate a circuit on a PCB before being sealed and shipped to customers.

Potentiometers can be very precise, multi-turn variable resistors, but often they are simple single-turn devices that move a wiper along a conductive carbon path to change 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 serve niche needs. 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.