A carbon film resistor is first made by depositing a thin layer of carbon onto a ceramic (usually alumina) cylindrical substrate through a process called pyrolysis or vacuum deposition. Then, during construction, the carbon film is carefully trimmed or spiraled to achieve the desired resistance value. After that, metal end caps and lead wires are attached to ensure good electrical contact. Finally, the entire assembly is coated with an insulating layer to protect it from moisture and mechanical damage.
Construction of Carbon Film Resistors
There is a central ceramic rod. Obviously, this central rod of high-grade ceramic material provides mechanical strength.
Then, a very thin film of crystalline carbon is deposited uniformly on the ceramic surface. After that, a precise spiral groove is cut into the carbon film to create the desired resistance value and increase the resistive path length.
Then we fit metal caps on both ends for electrical contact.
Then we weld or solder tinned copper wires to the end caps
Lastly, we provide an epoxy or phenolic resin coating on the structure to protect the carbon film resistors against moisture and mechanical damage. After that, we paint the color coding on the resistor.
Manufacturing Process of Carbon Film Resistor
Film Deposition
The manufacturer deposits carbon on the central ceramic rod in a controlled atmosphere at high temperatures. The thickness of the film determines the base resistance. The deposition process creates a uniform, stable carbon layer on the ceramic rod.
Resistance Adjustment
After that, the manufacturer cuts a helical groove into the carbon film using laser trimming or abrasive methods. Obviously, the pitch and depth of the spiral determine the final resistance value. This allows precise adjustment to achieve the desired resistance and tolerance,
Characteristics of Carbon Film Resistor
These resistors are typically available from 1Ω to 10MΩ. Although the most common range of carbon film resistors is from 10Ω to 1MΩ.
The tolerance of carbon film resistors is more precise than carbon composition resistors. The standard tolerance is ±5% (gold band). Although the better grades are also available, such as ±2% (red band) or ±1% (brown band).
Temperature Coefficient of Resistance
It generally has a small negative temperature coefficient. The resistance of a carbon film resistor decreases slightly as the temperature increases. It has better stability than a carbon composition resistor.
Power Rating
Common ratings of these resistors are 1/8W, 1/4W, 1/2W, 1W, 2W. These wattage ratings are determined by physical size and heat dissipation capability.
Noise Performance
It performs with low to moderate noise levels. Obviously, it performs better than carbon composition, but it is noisier than a metal film resistor. Thermal noise follows the Johnson-Nyquist formula.
Advantages of Carbon Film Resistor
A carbon film resistor is less expensive than a metal film resistor. The stability, which means the performance of this type of resistor is better than a carbon composition resistor. Because of its low inductance, it performs better at high-frequency applications. The resistance range of a carbon film resistor is quite wide. In other words, this type of resistor covers most common circuit requirements. The tolerance offered by this resistor is reasonable for most general-purpose applications. Its size is quite compact. It can perform satisfactorily over a wide range of temperatures, normally from -55 to +155\(^oC\).
Disadvantages
It provides more noise than a metal film resistor of the same rating. It is not as stable as a metal film resistor over time. Since its tolerance is normally more than 1%. It is not suitable for precision applications. Again, it is more temperature sensitive than a metal film resistor. That means it has a higher temperature coefficient of resistance than a metal film resistor. It has lower power handling capability than a wire-wound resistor. A carbon film resistor may drift slightly over an extended period of its lifespan.