Carbon vs Metal Film Resistors: Differences

Carbon Film Resistor VS Metal Film: Definition

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A carbon film resistor refers to a device that restricts electrical flow utilizing a thin layer of carbon. Despite their utility, the production cost for carbon film resistors is relatively high compared to other thin film resistors. Furthermore, they produce a notable amount of thermal noise and may exhibit lower accuracy.

(carbon film resistor)

However, they excel in handling high-energy pulses, making them particularly favorable for applications in surge protection circuits, computers, testing devices, DC power supplies, and high-frequency scenarios.

A metal film resistor, on the other hand, constrains electrical flow by means of a thin metal film. This type is notably the most popular variant of thin film resistors. Metal resistors are not only cost-effective and compact, but they also possess low tolerance, generate minimal noise, and demonstrate broader operational versatility than their carbon counterparts.

(metal film resistors)

Consequently, these resistors find their applications in bridge circuits, active filters, and high-frequency environments.

Carbon Film Resistor vs Metal Film Resistor: Side-by-Side Comparison

Having defined each resistor type, we can now proceed to a comparative analysis of both. This assessment focuses on aspects like voltage coefficient of resistance, stability, and temperature coefficient of resistance, thus aiding in identifying optimal use cases for each type.

Temperature Coefficient of Resistance (TCR)

Similar to the effects voltage has on resistance, temperature changes also affect resistance. This characteristic is not unique to resistors; all conductors experience resistance variations with temperature changes.

The resistance variation is quantified in parts per million per degree Centigrade (ppm/°C) or Kelvin (ppm/K). When compared, metal film resistors exhibit lesser sensitivity to temperature fluctuations than carbon film resistors.

Metal film resistors typically have a TCR ranging from ±20 to ±200 ppm/K, whereas carbon film resistors have a TCR in the range of -200 to -30 ppm/K. This significant difference indicates that metal film resistors are more reliable under extreme temperature conditions.

Additionally, metal film resistors generate less thermal noise, making them suitable for applications in radios, intercoms, and radars.

Carbon Film Resistor Green Color

Voltage Coefficient of Resistance (VCR)

Applying voltage to a resistor can provoke alterations in its resistance value, generally causing a corresponding decrease in resistance. This change is represented in parts per million per volt (ppm/V), known as the Voltage Coefficient of Resistance (VCR).

The significance of VCR lies in its ability to predict performance in high-voltage scenarios. While carbon film resistors handle high-voltage applications adequately, metal film counterparts typically outperform them, with metal film resistors demonstrating a VCR of just under 1 ppm/V compared to -10 ppm/V for carbon film resistors. This indicates greater resilience in resistance changes due to voltage application for metal film resistors.

Stability

As resistors are integrated into machinery and electronics operating in diverse environments, stability against external stressors is essential. Such stressors, which can potentially influence resistance values, include heat, cold, radiation, and sudden power spikes.

Overall, metal film resistors maintain superior stability compared to carbon film resistors. That said, while carbon film resistors have moderate stability, metal film resistors exhibit good to excellent stability levels.

Carbon Film Resistor VS Metal Film: Construction

These two resistor types employ different fabrication processes.

Carbon film resistor construction

Manufacturers create carbon composition resistors from a solid, cylindrical resistive element with wire leads and metal caps at both ends. The resistive element comprises a mix of clay-based ceramic and graphite or carbon powder, allowing electricity to flow efficiently.

(Internal part of a carbon film resistor)

The manufacturing process begins by heating a ceramic substrate in a hydrocarbon like methane. This results in cracking reactions at high temperatures around 1000°C, causing crystalline carbon from the pure graphite to bond with the ceramic structure.

Afterward, the resistive material receives a plastic covering for protection against external heat generation. Finally, copper is applied to form wire leads at both ends of the resistive element.

Metal film resistor construction

Conversely, metal film resistors involve an aging process where deposited metal undergoes low-temperature treatment over an extended duration.

Nickel chromium (NiCr) serves as the common resistance material, although other materials like tantalum nitride, platinum combined with gold, and tin with antimony are also utilized.

Resistance often depends on the metal film thickness, where greater thickness correlates with lower resistance and enhanced stability across the film. Manufacturers finalize the product by applying connection leads and forming a spiral-shaped slot in the thin metal to achieve the desired resistance.

Carbon Film Resistor VS Metal Film: Availability

Carbon Film Resistor

Carbon resistors are generally available in ohmic values ranging from 1 ohm to 25 megaohms, with power ratings between one-quarter watt and five watts.

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Three main factors affecting resistance in carbon film resistors include:

• The cross-sectional area of the solid cylindrical rod, which has an inversely proportional relationship with resistance.
• The length of the rod, which directly correlates with resistance.
• The quantity of carbon included in the resistor, also inversely proportional to resistance.

Metal Film

Metal film resistors are typically offered in tolerances of 2%, 1%, 0.5%, 0.25%, and 0.1%. They can also have temperature coefficients ranging from 10 to 100 ppm/K.

Factors influencing resistance in metal film resistors consist of:

• The width of the spiral metal film cut, which has an inversely proportional relationship with resistance.
• The thickness of the metal film, which is also inversely proportional to resistance.

Carbon Film Resistor VS Metal Film: Similarities and Differences

• Similarities

Both types of film resistors share several commonalities:

• Both can be regulated by modifying their thin film thickness.
• Both utilize ceramic as a non-conducting material.
• Both are of similar size.
• Both exhibit similar shape and appearance.

(Film resistors in a circuit board)

• Differences

Below is a summary table highlighting the differences between metal film and carbon film resistors.

Carbon Film Resistor VS Metal Film: Drawbacks

Both resistor types provide certain advantages. While metal film resistors offer superior frequency response, carbon film resistors are generally more cost-effective and may have higher resistance values. Yet, each type also has its limitations.

One major drawback of carbon film resistors is their high temperature coefficient of resistance, alongside relatively low stability when compared to wire-wound and metal film resistors.

Additionally, carbon film resistors tend to be bulkier, making them somewhat challenging to utilize and position accurately, especially when handled manually. Conversely, the compact nature of metal film resistors renders them ideal for use in smaller electronic devices.

Despite their advantages, metal film resistors feature their own shortcomings.

Primarily, they are often more expensive than carbon film resistors and typically exhibit higher power dissipation characteristics.

Although some may perceive higher power dissipation as advantageous, it can render metal film resistors unsuitable for applications requiring low-power dissipation.

Metal Film Resistor On Green Blur

Conclusion

Both carbon film and metal film resistors function to curtail current flow in circuits. The primary distinction is that carbon film resistors utilize carbon, whereas metal film resistors use metal.

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