A metal film resistor is a passive resistor. It uses a thin film of metal alloy deposited around a ceramic cylinder to provide the current path of the resistor. The ceramic cylinder provides mechanical strength to the resistor. The tolerance of the metal film resistor is quite lower than that of the carbon composition resistor and carbon film resistor.
Manufacturing Process of a Metal Film Resistor
Firstly, we take a ceramic rod or cylinder. It is a solid ceramic cylinder. We use high-quality aluminum oxide for this purpose.
Next comes the metal film. Then we deposit a very thin nickel chromium alloy onto that ceramic cylinder. The deposition process happens in a vacuum chamber. Actually, the metal vaporizes in a vacuum and coats the ceramic rod eventually.
Then we use a laser to cut a spiral groove in the metal coating on the cylinder. Therefore, this groove winds around the resistor like a spring. Obviously, this spiral structure of metal film increases the length of the current path in the resistor. By adjusting the number of rounds of spiral and the dimension of the groove, we can fine-tune the exact resistance value of the resistor.
After that, we attach metal caps to the two ends of the assembly. Then we weld tinned copper wire at both ends of the assembly on the cap.
Lastly, we apply epoxy coating to seal the entire resistor to protect it from the outside. Then we paint the color coding on the resistor body.
Advantages of Metal Film Resistors
Accuracy: These resistors offer perfect accuracy. Most resistors come with a tolerance of ±1% to ±2%. Obviously, this is much better than that of the carbon composition resistor and carbon film resistor. Sometimes we can achieve ±0.1% precision in this type of resistor.
Temperature Stability: Heat can change the resistivity of the metal film slightly. It can drift around 50 to 100 ppm per degree Celsius. It makes this resistor incredibly stable.
Noise Level: This type of resistor offers quite a low noise level compared to other types of resistors of the same category.
Common Sizes: Handle 0.25 Watts to 2 Watts. Since larger bodies dissipate more heat, we can use bigger forms of this resistor to handle more power.
Resistance Range: The resistors available in the market offer a wide range of resistance values. It is from 1 ohm to 10 megohms. That range covers almost all applications of resistors.
Drawbacks of a Metal Film Resistor
As we have already told, it has a certain power handling limit. However, a wire-wound resistor can handle more power than a metal film resistor.
As the thickness of the film is very small, a surge can make a permanent defect in that film. Therefore, the resistor becomes defective and useless.
These resistors cost more than carbon film resistors.
Application of Metal Film Resistors
Because of the low noise level, we mainly use this type of resistor in equipment and devices related to audio and music. Also, we use this type of resistor in measuring instruments because of its high accuracy and precision. Also, this type of resistor is used in medical devices, computers, industrial control circuits, and in all other modern electronic devices.
How to Identify a Metal Film Resistor?
As per normal practice, metal film resistors are always colored blue. In other words, the blue body color of the resistor helps us to identify a metal film resistor.