In this guide, we look at the types of resistors and the respective use case for each.
Carbon Composition Resistors
Carbon composition resistors used to be the most common type of resistor because of the low cost and 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. It 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 these 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 improved stability. However, the frequency response is worse due to the inductance and capacitance caused by the resistive path cut into the film.
Metal Film Resistors
One of the 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 percent. Metal film resistors are available up to about 35 watts. However, resistance options begin to diminish above 1 or 2 watts. Metal film resistors are low noise. These resistors are 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. These 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. Today these are available with tolerances as low as 0.1 percent in packages that can handle up to 250 watts. Thick film resistors 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. Sputtering is where a thin layer of conductive material is deposited on an insulating substrate. This thin layer is 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 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 stable resistors. There is a price for the precision of thin film resistors, which are generally twice the price of thick film resistors.
Wirewound Resistors
The highest power and most precise resistors are wirewound resistors, which are rarely 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 wrap pattern, the properties of the wirewound resistor can be tailored to the application. Resistance tolerances are as tight as 0.005 percent 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 percent or 10 percent but have power ratings in the kilowatt range. Wirewound resistors suffer from high inductance and capacitance due to the nature of the construction, limiting these to low-frequency applications.
Potentiometers
Varying a signal or tuning a circuit is a common requirement for sensitive electronic applications. One easy way 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 may be precise, multi-turn variable resistors, but are often 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 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 prototyping.
Other Resistor Types
As with most components, several specialty resistor variants serve niche needs. Several are quite common, including the resistive element in the incandescent light bulb. Other specialty resistor variants include heating elements, metal foil, oxide, shunts, cermet, and grid resistors.
