What is a Carbon Film Resistor?

G.W. Poulos

A carbon film resistor is an electronic component that provides resistance to electrical current. While there are many types of resistors made of many different materials, a carbon film resistor has a thin film of carbon formed around a cylinder. As electrical current flows from one end of the resistor to the other, the carbon film provides a measured amount of resistance to the flow of the electrical current.

A resistor is an electronic component that can lower a circuit’s voltage and its flow of electrical current.
A resistor is an electronic component that can lower a circuit’s voltage and its flow of electrical current.

Construction of carbon film resistors is a straightforward process. The carbon used in the resistors undergoes processing to establish its resistive value. A non-conductive cylinder, typically made of ceramic or epoxy, then has the carbon deposited on its surface to create a thin carbon film. Metal caps, each with a metallic lead, then attach to either end of the cylinder. Finally, the resistor is coated with a non-conductive paint and marked with a color-code that identifies its resistive value.

When the finished resistor is in an electronic circuit, electrical current will arrive at one end cap, travel through the resistor’s carbon film, and then go out the other end cap to the rest of the circuit. Therefore, resistive value of a carbon film resistor depends upon the distance electrical current travels through the resistor’s carbon film. Often, however, a resistor must have a value higher than the carbon alone can provide in a straight line.

In order to create carbon film resistors with higher resistive values, the path through the carbon film must increase. To accomplish this, a cut is made in the carbon film of the resistor to create an extended pathway. While the shape of this cut can be of many different patterns, the most common is a spiral cut. This in essence creates a long ribbon of carbon film wrapped around the cylinder.

Depending on the width of the ribbon, this path can be many times longer than a straight shot from one end of the resistor to the other. Additionally, by varying the width of the cut, many different resistors, with different resistive values, can be manufactured from the same basic carbon film resistor design. The use of this cutting method allows for the economic production of carbon film resistors, making them the least expensive of all standard resistor types.

Carbon film resistors have other desirable attributes besides their low cost. Their simple yet highly effective and durable design lends them to many uses, which include nearly every consumer electronic device imaginable. They can have resistive values from 1 to 10,000,000 ohms, offer a power rating from 1/8 to 5 watts, handle up to 600 volts, and endure temperatures from -67°F to over 300°F (about -55°C to over 149°C).

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