Carbon and metal film resistors often look similar in size and shape. However, clues exist to identify them from each other. The color of the resistor, number of colored bands, tolerance and temperature coefficient point toward a resistor’s carbon or metal film identification. Use these guides as a general rule, noting that manufacturers do vary in marking resistors. However, these insights combine to provide a good indication as to whether a resistor is of the carbon or metal film variety.
Look at the color of the resistor, a potential indication to its makeup. Tan tends to signal a carbon film resistor, whereas a light blue resistor is likely made of metal film.
Count the color bands, the number of which provides a good hint toward which type of resistor you have. A carbon film resistor normally sports four color bands. Five and six color bands almost exclusively indicate a metal film resistor.
Find the tolerance. A carbon film resistor’s performance usually fluctuates more than a metal-film resistor’s. The tolerance band, which stands farthest to the right on resistors printed with four and five bands, is the fifth band on resistors with six color bands. The colors brown and red represent a more stable resistor value and indicate a metal film resistor. Gold signifies a +/- 5 percent rating, the most common rating to carbon-film resistors. However, if the resistor exhibits five bands and a gold tolerance code, it is likely of the metal-film variety. The rare silver tolerance band signals a +/- 10 percent rating, noting an older-model carbon film capacitor.
Take the resistor's temperature coefficient (TC), into account in the event you know it; if so, you can likely differentiate between a carbon and metal film resistor. Lower TC values such as 20, 10 and 5 denote a metal film resistor. Higher TC ratings in the 100s or above indicate a carbon film capacitor.
Combine your answers. Resistor color codes may occasionally differ between companies, but many concur with standard markings. If you own a light blue resistor with five color bands, the fifth being red, you can safely guess it’s a metal film resistor. A tan resistor displaying four color bands would likely be of the carbon film variety.
Carbon film resistors often add noise to a circuit. With resistors already installed into a piece of audio equipment, noise can serve as an added clue to the type of resistor used.
Always unplug any electrical appliance and drain capacitor charge before performing any repair to a circuit.