Metal film resistor is the most common type of resistor used in electronics. They have been widely used since their development in the s, and are very popular for their consistency and accuracy. This is a type of resistor that has a metal oxide film deposited on the surface of a ceramic core. This is different from carbon film resistors, which have a layer of carbon instead of metal.

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Metal film resistors have better performance than carbon or wire wound resistors. They are also more dependable and reliable than other types of electronics parts. For these reasons, metal film resistor is often used as replacements for other types of electronic components.

Metal Film Resistor Definition:

A metal film resistor is a passive two-terminal electrical component. It is designed to provide a specific resistance. The metal film resistor is constructed by depositing an insulating layer of oxide or nitride, which is then covered with a resistive metal film and terminated with a second insulating layer, usually oxide. These components are used in active filters, precision circuits, and timing applications.

The metal film resistor symbol is similar to a normal resistor which includes two connecting terminals like anode and cathode shown below.

Metal Film Resistor Construction

Metal Film Resistors are made of a metal film on an insulating substrate, similar to a thin-film resistor. The metal is deposited by evaporation onto the substrate. The metal film is typically 0.1 µm or thicker and has a much higher resistance than the substrate.

The resistors are formed by screen printing carbon paste onto the substrate in a shape that cross-sections the metal film, creating two parallel conductors with a uniform gap between them. The resistor body is coated with an epoxy coating for environmental protection and insulation.

Metal film resistors are manufactured using a photolithography process that creates high-accuracy and low-tolerance resistors. Photolithography is a photographic process that transfers images to semiconductor wafers during integrated circuit development. Metal film resistors have resistance values ranging from 0.1 ohms to 100 MΩ, with tolerances of ±0.1% or less at operating temperature ranges from -55°C to 155°C.

Working

The working of a metal film resistor is similar to the normal resistor to restrict the flow of electric current through it. So, it functions as the resistive element for the flow of electric current. The ceramic substrate of this resistor does not let heat throughout it.

Specifications

The specifications of the metal film resistor include the following.

• The availability of typical tolerance is ±0.1%, ±0.25%, ±0.5%, ±1% & ±2%,
• Resistance value ranges from >1Ω < 10MΩ
• Max resistor temperature is175 °C
• Load life (% change over h) is 1
• Voltage coefficient is 0.0 %/V
• Max noise is 0.2 µV/V
• Temperature coefficient ±50 to ±100 ppm/°C

Metal Film Resistor Tolerance

Metal Film Resistor tolerance is the highest level of deviation from the nominal value of a resistor. This tolerance represents an accuracy standard for resistors, and it determines how much the measured resistance will vary from the ideal resistance. The tighter the tolerance, the more accurate your measurement will be.

The tolerance of a metal film resistor is the amount the resistance of the resistor can vary from its nominal value. This is usually expressed as a percentage, and can be calculated using the formula:

(Measured Value &#; Nominal Value)/Nominal Value)×100

If you measure 10kΩ and it has a 5% tolerance, then you know that tolerance is 500Ω. The highest possible value would be 10.5kΩ and the lowest possible value would be 9.5kΩ.

Metal Film Resistor Power Rating

The maximum power a resistor can handle is known as the power rating for that resistor.

The power rating is determined by how much heat the resistor can take before it fails. Heat causes the resistance in the resistor to rise, which causes it to dissipate more power, which causes it to get hotter, and so on. If this process runs away, the resistor&#;s temperature rises quickly and it fails catastrophically.
The maximum power (Pm ) a Metal Film Resistor can handle is calculated by dividing the maximum working voltage (Um ) by the resistance value (R):

Pm = Um/R

Metal Film Resistor Size Calculation

The size of a metal film resistor can be calculated from its power rating.

After the power rating (power = voltage x current) and the maximum possible temperature rise are known, the resistor size can be calculated by adding the thermal resistance RTH of the resistor to the thermal resistance RθJA of the mounting surface.

Rth = (TJ &#; TA)/Pd

Here, Rth is thermal resistance

&#;Tj&#; is Junction temperature

&#;TA&#; is ambient temperature

&#;Pd&#; is power dissipation

Size limitation is given by:

RTHmax = (TJmax &#; TA)/Pd

Resistor size = Rthmax + RθJA

Metal Film Resistor Color Code

The color code for metal film resistors consists of three bands and a fourth band indicating tolerance.
The first two bands indicate the first two digits of the value.

The third band indicates the number of zeros following those first digits. For example, if the third band is red, there are two zeros following the first two digits. If it is gold, there are five zeros following those first two digits.

The fourth band indicates tolerance in percent. A brown or red band represents a 1% tolerance, while a gold or silver band represents a 10% tolerance.

If additional bands are present, they typically indicate temperature coefficient, but this varies based on the manufacturer and part number.

What is the difference between metal film and metal oxide resistors?

The difference between metal film and metal oxide resistor include the following.

Metal Film Resistor

Metal Oxide Resistor Metal films are the most common type of resistor. Metal oxide resistors are also popular and often used for high-power applications. Metal films typically have a smaller tolerance. Metal films typically have a higher tolerance but they handle higher power ratings. These are cylindrical axial resistors. These are axial & fixed value resistors. These resistors are applicable where a more precise value and high tolerance value are necessary within a circuit. These resistors are used for high endurance applications. The frequency response of these resistors is about 100 MHz. The frequency response of metal oxide resistors is about 100 MHz. These resistors have good characteristics for stability, TCR & tolerance. In addition, these resistors feature high linearity & low noise properties due to a low voltage coefficient. These resistors will exceed both the resistor&#;s performance like carbon film & metal film for different properties like voltage rating, power rating, surge capacity, high-temperature operation & overload capabilities. The maximum temperature of the metal film resistor operates up to 150°C & with platinum sensors, they work up to 600°C. The temperature coefficient of the metal oxide resistor is approximately 300 ppm/°C.

Advantages

Metal Film Resistors have the following advantages:

High Precision: Film thickness is easy to control, the manufacturing process is mature, with stable performance.

Low Noise: The temperature coefficient of resistance is small, it is not easy to produce a microphonic effect.

High Stability: High purity ceramic core, film-coated on the surface, no oxygen in the air will not oxidize the surface of the resistor, it has good anti-vibration and anti-shock performance.

High Power Rating: Metal film resistors also have a high power-rating, which means they can be used in high-power applications. They tend to have a lower temperature coefficient than carbon film resistors, which makes them ideal for use in precision circuits where a low-temperature coefficient is essential.

Disadvantages

The disadvantages of metal film resistors include the following.

• Metal film resistors are more expensive than carbon composition and carbon film resistors because there&#;s a higher cost to precision.
• Metal film resistors have a limited power rating of 3 watts or less. If you require more power, you&#;ll need to use a wire wound resistor instead.
• They are sensitive to temperature changes, both within their environment and in the current they conduct.
• They need to be regularly maintained, cleaned, and checked to make sure they work correctly.
• The resistor will fail before it reaches its maximum power rating if there is a sudden surge in current or voltage.
• The metal film will erode over time due to high temperatures caused by prolonged use at higher power levels than those specified on the resistor itself
• Metal film resistors can only operate up until temperatures around 200°C (392°F).
• They are not suitable for use in high-frequency circuits because their parasitic capacitance limits them from being able to handle frequencies above about 100 MHz (Megahertz).

Where are Metal Film Resistors Used/ Applications?

The applications of metal film resistors include the following.

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• Metal-film resistors are used in almost every circuit where resistance is required, including power supplies and other circuits.
• They are also used to protect sensitive electronic components from dangerous voltages.
• Metal-film resistors have been around for a long time, but only in recent years has their popularity begun to increase. The reason for this is because they are less expensive than other types of resistors and they are very reliable. This means that they can be used in a wide variety of applications including automotive electronics and computer circuitry.
• Its resistance to currents makes it useful in a variety of situations, from industrial equipment to consumer electronics.
• These resistors are used in different types of equipment in different fields like Industrial, military Automotive, Professional audio & video.
• Industrial Applications: Metal film resistors are commonly used in industrial equipment. They help regulate the flow of current and prevent damage by overheating. Metal film resistors are also frequently employed in appliances, as they help to protect motors and other components of the machines.
• Consumer Electronics Applications: Metal film resistors are used in a wide range of consumer electronics, including cell phones, laptops, and televisions. They help regulate the current flow in these devices and prevent them from overheating when they are used for extended periods at high power levels.

What is the Difference Between Metal Film Resistor and Other Types of Resistors?

Metal film resistors are more precise than carbon composition or wire-wound resistors, but they are not as precise as thin-film resistors. There is a wider tolerance range for metal film resistors than most other types of resistors, but the tolerance can be adjusted by adjusting the process used to make them.

What should I know before selecting a metal film resistor?

There are many factors to consider when choosing a metal film resistor. Here are some considerations:

• How much power will the resistor dissipate?
• Is there an issue with temperature rise?
• How accurately does the value need to be?
• Does it need to work at high frequencies?
• Is moisture resistance required?

How Much Power Can I Use With a Metal Film Resistor?

The amount of power that can be used with a metal film resistor depends on the size of the resistor and its temperature rating. A smaller resistor will have a lower power rating than a larger resistor because it will have less surface area over which to dissipate heat. As temperatures rise, power ratings drop because it becomes harder for heat to dissipate through the air or other means.

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Comparing Carbon Film Resistors and Metal Film Resistors - Advantages and Disadvantages, Performance, Structure

In electronic circuit design, resistors are indispensable basic components, among which metal film resistors and carbon film resistors are two common resistor types. They differ in structure, manufacturing process, performance, and application scenarios. Metal film resistors, usually represented by the symbol RJ, use special metals or alloys as resistance materials, and form a resistance film layer on a ceramic or glass substrate through vacuum evaporation or sputtering. This manufacturing process results in metal film resistors with superior electrical properties, including better heat resistance, low noise potential, and stable temperature and voltage coefficients. In contrast, carbon film resistors are formed by coating a suspension containing organic binders, carbon ink, graphite, and fillers on an insulating substrate, and then forming a carbon film through a pyrolysis reaction in a high-temperature and vacuum environment. Although carbon film resistors are cheaper to manufacture and easier to mass produces, their electrical performance and stability are not as good as metal film resistors. This article will compare in detail the advantages and disadvantages, manufacturing processes, and applicability of these two resistors in different applications, providing readers with comprehensive technical analysis and practical guidance.

Catalog

1. What is a metal film resistor?

Metal film resistors, usually represented by the symbol RJ, are resistors that use special metals or alloys as resistance materials and form a resistance film layer on ceramics or glass through vacuum evaporation or sputtering. During the manufacturing process, the alloy is heated in a vacuum, causing it to evaporate and form a conductive metal film on the surface of the porcelain rod. By grooving and changing the thickness of the metal film, the resistance value can be precisely controlled. The electrical performance of this resistor is better than that of carbon film resistors, with better heat resistance, low noise potential, stable temperature coefficient, and voltage coefficient. The manufacturing process of metal film resistors is very flexible. The resistance value can be adjusted by adjusting the material composition, film thickness, and grooves, thereby producing resistors with excellent performance and a wide resistance range.

The main advantages of metal film resistors include:

The cost is relatively low: the production process is mature and material costs are controllable.

Less noise: Low electrical noise, suitable for high-precision applications.

High stability: Excellent performance in various environments, suitable for long-term use.

Small size: easy to integrate into small electronic devices.

High precision: The resistance value is accurate and can be used in circuit design that requires high precision.

2. What is a carbon film resistor?

Carbon film resistors are made by coating a suspension containing organic binders, carbon ink, graphite, and fillers on an insulating substrate and then polymerizing it through heating. During its manufacturing process, gaseous hydrocarbons are decomposed under high temperatures and vacuum, and carbon is deposited on the porcelain rod or tube to form a crystalline carbon film. By changing the thickness of the carbon film and the length of the groove, different resistance values can be obtained. Carbon film resistors have lower cost, but their electrical properties and stability are poor, so they are generally not used as general-purpose resistors. However, due to the ease of making high-resistance films, carbon film resistors are mainly used for high-resistance, high-voltage resistors, and their applications are similar to high-voltage resistors.

Advantages and Disadvantages of Carbon Film Resistors

advantage

Low cost: The manufacturing process is simple and the material cost is low.

High resistance: Easy to make high resistance films, suitable for high resistance and high voltage applications.

Easy to mass produce: suitable for large-scale, low-cost production needs.

shortcoming

Poor electrical performance: Not as stable and accurate as metal film resistors.

High noise: The electrical noise generated when the current passes through is large and is not suitable for high-precision applications.

High-temperature coefficient: The resistance value changes greatly with temperature, which affects the stability in high-temperature environments.

3. Carbon Film Resistors vs. Metal Film Resistors: Side-by-Side Comparison

Understanding the differences between metal film resistors and carbon film resistors helps in choosing the right component for specific applications. This comparison focuses on key performance metrics such as the temperature coefficient of resistance (TCR), voltage coefficient of resistance (VCR), and overall stability.

Temperature Coefficient of Resistance (TCR)

The TCR indicates how the resistance value changes with temperature fluctuations. Typically, the resistance increases when the temperature rises and decreases when it drops. Metal film resistors exhibit a TCR ranging from ±20 to ±200 parts per million per Kelvin (ppm/K), suggesting minor changes in resistance with temperature variations. In contrast, carbon film resistors have a TCR of -200 to - ppm/K, indicating a more significant resistance fluctuation under temperature changes. This makes metal film resistors more dependable for applications involving extreme temperatures. Moreover, they generate less thermal noise, enhancing their suitability for sensitive electronic devices like radios and radar systems.

Voltage Coefficient of Resistance (VCR)

The VCR measures how resistance changes with voltage application. When voltage is applied to a resistor, typically, the resistance slightly decreases. Metal film resistors have a low VCR, nearly 1 ppm/V, showing minimal change in resistance when subjected to voltage. Carbon film resistors, however, have a higher VCR of -10 ppm/V, indicating a more noticeable resistance change with voltage application. This property makes metal film resistors more stable and reliable under high-voltage conditions.

Stability

Stability is crucial for resistors operating in diverse environments. Metal film resistors excel in stability, performing robustly against external factors like radiation, heat, and cold. This makes them particularly well-suited for high-demand environments. Although carbon film resistors also offer decent stability, they do not perform as well as metal film resistors under similar conditions.

4. Carbon film resistors and metal film resistors: structural comparison

There are significant differences in the construction and manufacturing processes of carbon film and metal film resistors, with each resistor being customized for specific applications based on its structural properties and performance characteristics.

Manufacturing Carbon Film Resistors

The process begins with a ceramic substrate that is coated with a mixture of carbon ink, graphite, and a binder suspension. The component is then exposed to high temperatures, triggering a cleavage reaction. As the temperature increases, the carbon compound decomposes, leaving behind a layer of pure graphite that crystallizes and forms a durable carbon film on the ceramic base. This graphite layer is responsible for the resistor's ability to conduct electricity. To protect the fragile carbon film from thermal damage and environmental factors, the resistors are encased in a protective plastic housing. Copper leads are attached to both ends of the ceramic base to provide electrical connection points.

Manufacturing Metal Film Resistors

Metal film resistors are produced through a more complex process. These resistors typically use metals such as nickel-chromium (NiCr), but other materials such as tantalum nitride, gold, platinum or tin and antimony can be used depending on the required specifications. The metal of choice is vapor deposited onto a substrate to form a thin layer. This metal layer is then artificially aged at low temperatures for an extended period of time to enhance its electrical properties. Fine-tuning the resistor value by etching spiral grooves into the metal film allows for precise control of the electrical path and therefore the total resistance provided by the resistor.

Performance comparison

Carbon film resistors are generally cheaper to produce and simpler to design but have lower electrical performance and stability. They are most effective in high resistance and high voltage settings, but because they are susceptible to temperature changes and aging, they are not ideal for applications requiring high accuracy.

In contrast, metal film resistors, although more expensive and complex to manufacture, provide superior electrical performance and stability. Careful control of film thickness and etching processes enables these resistors to operate reliably in applications requiring high precision and stability, such as sensitive measurement instruments and critical electronic circuits.

5. Carbon Film Resistors vs. Metal Film Resistors: Availability

Both carbon film and metal film resistors are essential components in various electronic applications, each with unique specifications that suit different needs.

Carbon Film Resistors

Carbon film resistors are available in resistance values ranging from 1 ohm to 25 megohms, with power ratings from one-quarter watts up to five watts. Their widespread use can be attributed to the low cost of production and their adaptability across a broad spectrum of applications. The resistance value of carbon film resistors is determined by several factors. The resistance decreases as the cross-sectional area of the carbon-coated rod increases. Conversely, the length of the rod enhances the resistance&#;the longer the rod, the higher the resistance. Additionally, the resistance lowers as the concentration of carbon in the coating increases, allowing for a versatile range of resistances by adjusting these dimensions during manufacturing.

Metal Film Resistors

Metal film resistors, known for their precision and stability, are offered in a range of tolerances including 2%, 1%, 0.5%, 0.25%, and 0.1%. They have temperature coefficients (TCR) that vary from 10 to 100 ppm/ K, making them ideal for use in sophisticated electronic devices where accuracy is crucial. The factors influencing the resistance of metal film resistors are closely tied to their manufacturing process. The resistance decreases as the width of the cut spiral grooves on the metal film increases&#; wider cuts result in lower resistance. Furthermore, increasing the number of layers of the metal film also results in decreased resistance. This allows manufacturers to achieve precise control over the resistor's properties by adjusting the depth and the number of spiral cuts.

6. Carbon film resistors VS metal film resistors: Disadvantages

Although metal film resistors perform best in terms of frequency response, carbon film resistors still have their unique advantages due to their low cost and high resistance. However, both also have their drawbacks.

Disadvantages of Carbon Film Resistors

Temperature Sensitivity: Carbon film resistors have a significant temperature coefficient of resistance (TCR), typically ranging from -200 to - ppm/K. This high TCR means that their resistance changes significantly with temperature, which can be problematic in temperature-sensitive applications.

Stability Issues: The resistance value of carbon film resistors can change significantly over time or under extreme environmental conditions. This instability can lead to unpredictable performance, especially in precision electronic circuits where consistent resistance values are critical.

Physical Size: Generally, carbon film resistors are larger compared to other types of resistors. For compact electronic devices where space is limited and miniaturization is a priority, the larger size can be a disadvantage.

Disadvantages of Metal Film Resistors

Cost Factor: Compared to carbon film resistors, the production of metal film resistors involves more complex processes and higher quality materials, resulting in higher manufacturing costs. As a result, they are more expensive, which can be a significant disadvantage for projects on a tight budget.

Power Dissipation: Metal film resistors may dissipate more power in applications where high current flows through the resistor. This increased power consumption may cause efficiency issues in power-sensitive circuits, which may result in higher operating costs and heat generation.

7. Summary

Through a detailed comparison of the structure, manufacturing process, electrical performance, and application of metal film resistors and carbon film resistors, their respective advantages and disadvantages can be seen. Metal film resistors have become the first choice for demanding electronic equipment due to their high precision, high stability, and low noise. Although their manufacturing costs are higher, they are undoubtedly the best choice for applications that require long-term reliability and precise control. Carbon film resistors are widely used in high-resistance and high-voltage circuits that do not require high electrical performance due to their low cost, high resistance, and ease of mass production. In actual operation, engineers should carefully weigh the characteristics of these two resistors and select the most appropriate resistor type based on specific application requirements to ensure the reliability and performance stability of the circuit design. Both metal film resistors and carbon film resistors play an important role in modern electronic technology, and their continuous development and improvement will continue to promote the progress and innovation of electronic technology.

Frequently Asked Questions [FAQ]

1. What is the difference between metal film resistors and carbon film resistors?

Metal oxide film resistors operate in a wide range of resistors and can withstand a higher temperature than the resistors of the carbon film. Noise Design In contrast to carbon film resistors, metal oxide film resistors have a low noise design. They keep the minimum current. Therefore, it ensures less noise.

2. Can metal film resistors be used instead of carbon film resistors?

Replacing them is your choice. Both carbon film and metal film will work the same. Some like the super accuracy of metal films (1%, 2%, etc.). Some like carbon films because they're similar to the originals elsewhere in the receiver.

3. Why are carbon resistors no longer the most common?

Carbon resistors usually are not designed to carry large currents. If too much current passes through this type of resistor, it will heat to the point that permanent damage results. Even currents that are slightly too large may cause changes in the resistivity of the carbon material.

4. How to tell whether a resistor is a metal film resistor?

Carbon film resistors have a layer of black protective paint on the surface. We can distinguish by looking at the color of the protective paint. Metal film resistors usually use a bright white protective film.