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Carbon Film Resistor VS Metal Film – How to Tell the Difference

Resistors are electronic components that decrease electric current flow in circuits, thereby regulating electric current.  Film resistors come in three main types: metal oxide resistors, carbon film resistor vs metal film resistors.

Carbon film and metal film resistors often have the same shape and appearance. But how do you differentiate between the two?

Today, our tutorial will discuss a carbon film resistor vs. a metal film resistor. 

Carbon Film Resistor VS Metal Film: Definition

A carbon film resistor is a type of resistor that slows the flow of electricity by using a thin layer of carbon. Unfortunately, carbon film resistors are more expensive to make than other types of thin film resistors. They also generate a lot of thermal noise and can portray low accuracy. 

carbon film resistor

(carbon film resistor)

Despite its challenges, it can withstand high-energy pulses and thus are suitable for applications like surge protection electronic circuits, computers, test equipment, DC power supplies, and high-frequency applications. 

A metal film resistor limits the flow of electricity by using a thin metal film. It is also the most popular type of thin film resistor. Moreover, metal resistors are cost-effective, compact, have low tolerance, generate less noise, and operate widely compared to carbon resistors. 

metal film resistors

(metal film resistors)

Therefore, you’ll find them in bridge circuits, active filters, and high-frequency applications. 

Carbon Film Resistor VS Metal Film: Construction

The two resistor types have different processes.

Carbon film resistor construction

Manufacturers make carbon composition resistors from a solid, cylinder-shaped resistive element with wire leads and metal caps on the ends. Further on, the materials in the resistive element are a mixture of clay-based ceramic and graphite or carbon powder. Generally, the carbon powder lets electricity flow through it well.

Internal part of a carbon film resistor

(Internal part of a carbon film resistor) 

So, during the deposition process, manufacturers start by heating the ceramic substrate in a hydrocarbon like methane. Next, at high temperatures of approximately 1000°C, cracking reactions will occur in the gas. The pure graphite’s crystalline carbon then sticks to the ceramic. 

Afterward, the resistive material gets a plastic covering to keep it safe from the heat generated from outside. Manufacturers finally apply copper to make the wire leads at both ends of the resistive element. 

Metal film resistor construction

Contrarily, manufacturers use a special method for metal film resistors, where they artificially age the deposited metal in low temperatures for an extended time. 

Here, the material employed as the resistance material is Nickel chromium (NiCr). However, in other applications, you’ll find other materials like tantalum nitride, gold with platinum, and tin plus antimony.

Then, the resistor’s stability and resistance often depend on the metal film thickness. Thus, a larger thickness will mean a lower resistance value and greater stability on both films ends. 

Manufacturers use connection leads to press a metal cover for the final touches. They can now achieve the desired resistance via a spiral-shaped slot cut in the thin metal thickness.

Carbon Film Resistor VS Metal Film: Availability

Carbon Film Resistor

You’ll find carbon resistors in 1ohm to 25megaohms with a power rating ranging between one-quarter watt and five watts. 

The three factors affecting the resistance of carbon film resistors are;

  • The solid cylindrical rod’s cross-sectional area is inversely proportional to the resistance of the resistor.
  • Then, we have the rod’s length directly proportional to the resistance.
  • Lastly, the amount of carbon you add to the resistor is inversely proportional to the resistor’s resistance.

Metal Film

Usually, the available metal film resistors in the market have tolerances of 2%, 1%, 0.5%, 0.25%, and 0.1%. Additionally, the resistors are likely to have temperature coefficients ranging from 10 to 100ppm/K and some resistance. 

The two main factors affecting the resistance of metal film resistors include:

  • The width of the spiral metal film cut is inversely proportional to the metal film resistor resistance.
  • Secondly, the layer of the metal film is also inversely proportional to the resistance. 

Carbon Film Resistor VS Metal Film: Similarities and Differences

  • Similarities

The film resistors are similar in the following ways;

  • First, you can regulate both by controlling their thin film thickness.
  • Then, their non-conducting material is ceramic.
  • They also have the same size.
  • Lastly, their shape and appearance are similar. 
Film resistors in a circuit board

(Film resistors in a circuit board)

  • Differences

We have summarized the differences between the metal film and carbon film resistors in the table below.

Conclusion

Both carbon film and metal film resistors limit current flow through circuits. However, carbon film resistors use carbon, while metal film resistors use metal to reduce current flow.

For more information on the resistors, kindly reach out to us

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Hommer Zhao
Hi, I am Hommer, the founder of WellPCB. So far, we have more than 4,000 customers worldwide. If you have any questions, you can feel free to contact me. I really appreciate any help you can provide.

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