Metal Oxide Film Resistor : Construction, Working and Its ...

What Distinguishes Metal Film Resistors from Metal Oxide Resistors?

The distinctions between metal film and metal oxide resistors include several key aspects.

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Metal Film Resistors

Metal Oxide Resistors represent one of the most common resistor types. They are frequently utilized in high-power applications. While metal film resistors exhibit smaller tolerances, metal oxide resistors tend to manage higher power ratings. Both categories consist of cylindrical axial resistors designed to meet specific circuit requirements where precise values and elevated tolerance are crucial. These resistors are ideal for applications demanding high endurance. Typically, their frequency response hovers around 100 MHz, while metal oxide resistors also maintain a comparable frequency response. Noteworthy features include stability, TCR (temperature coefficient of resistance), and tolerance. Furthermore, these resistors demonstrate high linearity and low noise characteristics due to minimal voltage coefficients. They outperform carbon film and metal film resistors in terms of voltage rating, power rating, surge capacity, high-temperature operation, and overload capabilities. The maximum operational temperature for metal film resistors stands at 150°C, while with platinum sensors, they can function up to 600°C. The temperature coefficient for metal oxide resistors measures approximately 300 ppm/°C.

Advantages

Metal Film Resistors come with numerous benefits:

High Precision: The control over film thickness along with a mature manufacturing process, lends these resistors stability in performance.

Low Noise: Their small temperature coefficient minimizes the microphonic effect.

High Stability: The core is made from high-purity ceramic, and the film is coated, ensuring it resists oxidation and provides excellent anti-vibration and shock characteristics.

High Power Rating: Metal film resistors' higher power rating qualifies them for high-power applications. Their lower temperature coefficient compared to carbon film resistors makes them suitable for precision circuits.

Disadvantages

Drawbacks of metal film resistors include the following:

Metal film resistors tend to be more costly compared to carbon composition or carbon film resistors due to higher precision costs.

They typically have a maximum power rating limited to 3 watts. For applications requiring higher power, wire wound resistors are recommended.

Temperature fluctuations can influence their performance, both from environmental changes and current flow.

Regular maintenance and checks are necessary to ensure their functionality.

A sudden surge in current or voltage can lead to premature failure, even before reaching maximum power rating.

Operating at high temperatures can cause the metal film to degrade over time.

Metal film resistors can perform optimally only up to temperatures around 200°C (392°F).

These resistors are less suited for high-frequency circuits as their parasitic capacitance restricts performance beyond approximately 100 MHz.

Applications of Metal Film Resistors

The applications of metal film resistors encompass various domains:

Metal film resistors are essential in virtually all circuits requiring resistance, such as power supplies.

They also safeguard sensitive electronic components against potentially harmful voltages.

Despite being around for many years, their usage has surged recently due to their cost-effectiveness and reliability, making them versatile for applications in automotive electronics and computer circuitry.

Their robustness against currents has rendered them valuable across diverse applications, from industrial machinery to consumer electronics.

These resistors feature across industries such as Industrial, Military, Automotive, and Professional Audio & Video.

Industrial Applications: Metal film resistors are prevalent in industrial machines, aiding in current regulation and protecting against overheating. They are used extensively in appliances to safeguard motors and other components.

Consumer Electronics Applications: You can find metal film resistors in a variety of consumer electronics, including cell phones, laptops, and televisions. They regulate current flow to prevent overheating during prolonged use at elevated power levels.

Differences between Metal Film Resistors and Other Types: Metal film resistors offer greater precision compared to carbon composition or wire-wound resistors, although they don't match the precision of thin-film resistors. Their tolerance range is wider than most other resistors, though the manufacturing process can adjust this.

Selecting a Metal Film Resistor: Several factors should influence your choice of a metal film resistor:

How much power will the resistor need to dissipate?

Is there a potential for increased temperature?

How precise does the resistance value need to be?

Will it be used in high-frequency applications?

Is moisture resistance a consideration?

Power Utilization with Metal Film Resistors: Power capacity for metal film resistors is contingent on size and temperature rating. Typically, a smaller resistor will handle lower power due to reduced heat dissipation surface area. As ambient temperatures increase, power ratings drop due to impaired heat dissipation.

For resistance needs without excessive spending, metal film resistors offer durability and dependability, lasting years without frequent replacements or repairs. Though their costs are higher, their ability to endure harsh conditions makes them worth the investment.

Now, here’s a question for you: What exactly is a metal oxide resistor?

Understanding Metal Film Resistor: Structure, Function & Its Uses

Metal film resistors are widely regarded as the most common resistors in electronics, with a strong history of development since the 1970s. They stand out for their consistency and precision. Unlike carbon film resistors, these have a metal oxide film deposited onto a ceramic core.

Metal film resistors significantly outperform carbon or wire wound alternatives regarding reliability and dependability. For these reasons, metal film resistors are often favored as substitutes for various electronic components.

Definition of Metal Film Resistor:

A metal film resistor is a passive two-terminal electrical element designed to present specific resistance. It features an insulating layer of oxide or nitride coated with a resistive metal film, topped with another insulating layer, usually oxide. These components are commonplace in active filters, precision circuits, and timing applications.

The symbol for a metal film resistor is akin to that of a typical resistor, featuring two connecting terminals paralleling anode and cathode as depicted below.

Construction of Metal Film Resistors

Metal film resistors consist of a thin metal film over an insulating substrate, akin to thin-film resistors. The metal layer typically deposited onto the substrate is at least 0.1 µm thick and exhibits significantly higher resistance relative to the substrate.

Resistor formation involves screen printing carbon paste onto the substrate in a configuration that intersects the metal film, resulting in two parallel conductors with a consistent gap between them. An epoxy coating protects the resistor body against environmental factors.

The production of metal film resistors employs a photolithography process that yields high-accuracy, low-tolerance resistors. This method transfers images to semiconductor wafers during integrated circuit development. Resistance values range from 0.1 ohms to 100 MΩ, with tolerances of ±0.1% or lower at operating temperatures spanning -55°C to 155°C.

Operational Mechanics

The mechanism of metal film resistors parallels that of standard resistors, restricting electric current flow. They serve as resistive elements impeding current flow. The ceramic substrate minimizes heat transport throughout the device.

Specifications

The specifications for metal film resistors encompass the following:

• Typical tolerance options include ±0.1%, ±0.25%, ±0.5%, ±1% & ±2%
• Resistance values range from >1Ω to <10MΩ
• Maximum resistor temperature reaches 175°C
• Load life (% change over hours) estimated at 1%
• Voltage coefficient reaches 0.0%/V
• Peak noise tops out at 0.2 µV/V
• Temperature coefficient varies from ±50 to ±100 ppm/°C

Tolerance of Metal Film Resistors: Tolerance represents positional deviation from a resistor's nominal value. This assessment signifies an accuracy standard for resistors, indicating the extent of resistance variation from its targeted value. The higher the tolerance level, the more precise your measurements will be.

The tolerance of a metal film resistor indicates the extent to which its resistance can deviate from its nominal value, commonly expressed as a percentage. This can be calculated as follows:

(Measured Value - Nominal Value) / Nominal Value) × 100

For instance, if the measured resistance is 10kΩ with a 5% tolerance, this implies a tolerance of 500Ω, resulting in the highest permissible value at 10.5kΩ and the lowest at 9.5kΩ.

Power Rating for Metal Film Resistors: The maximum power a resistor can withstand is quantified as its power rating. This assessment depends on the heat the resistor can endure before failure. Increased heat elevates resistance, causing higher power dissipation and quick temperature escalation leading to catastrophic failure.

The maximum power (Pm) that a Metal Film Resistor can handle is determined through the formula:

Pm = Um/R

Calculating Metal Film Resistor Size: The size of a metal film resistor is ascertainable from its power rating. It’s critical to calculate power (power = voltage × current) and the potential temperature rise to deduce resistor size by incorporating the thermal resistance RTH of the device along with the mounting surface thermal resistance RθJA.

Rth = (TJ - TA) / Pd

Where:

• Rth signifies thermal resistance
• TJ denotes junction temperature
• TA identifies ambient temperature
• Pd indicates power dissipation

The size limitation is represented by:

RTHmax = (TJmax - TA) / Pd

Resistor size = Rthmax + RθJA

Metal Film Resistor Color Code: The color code of metal film resistors typically features three bands along with a fourth band for tolerance annotation. The first two bands identify the initial digits of the resistance value.

The third band signifies the total number of zeros following the digits. If the third band is red, two zeros follow, while a gold band indicates five zeros.

The fourth band specifies tolerance percentage—brown or red indicating 1% precision, with gold or silver representing 10% variance.

Additional bands may delineate temperature coefficients, varying based on manufacturer or part number.

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Comparing Carbon Film Resistors and Metal Film Resistors

Introduced in the year by Boykin, resistors have become staples in almost every electronic circuit. Defined as devices that impede current flow, early resistors were relatively large, featuring tolerance values as high as 10%. Originally made from compressed carbon, resistor technology has evolved, with contemporary resistors predominantly made using metal films, now available in small SMD packages with tolerances as low as 2%, and even tighter for precision types. India accounts for roughly 34% of the demand for passive components like resistors, with several domestic manufacturers including Carmet, KWK, and Epcos India Pvt Ltd.

If you’re looking to deepen your understanding of resistors, consider reading this article where we explain the differences between Carbon Film Resistors and Metal Film Resistors.

A Concise Introduction to Resistors

The term 'resistor' stems from the concept of 'resistance,' or withstanding electric flow. By harnessing conductive materials, resistors effectively regulate or direct the flow of electrons, functioning via the established principles of electrical engineering. Resistors can connect in series or parallel, in accordance with current and voltage demands.

These devices fulfill various roles, including voltage regulation, attenuation, or current reduction, without serving as power sources. Current can pass through resistors, leading to energy loss as heat. The operational principle adheres to Ohm's Law—a fundamental tenet every electronics student becomes acquainted with.

There exists a diverse spectrum of resistor types—composition, film, and wire-wound resistors—differentiated by various characteristics such as physical size, reliability, temperature ratings, noise, temperature coefficients, and voltage coefficients.

Today, we shall focus on contrasting two pivotal resistor types that significantly influence electronic circuits.

Defining Carbon Film Resistors

Before delving further, it’s essential first to clarify what film resistors are. These components are created by depositing an oxide or pure metal film onto a substrate, typically an insulating ceramic. The thin layer of carbon film, in turn, restricts electric current flow. The insulating ceramic base prevents heat transfer, hence carbon film resistors can sustain extreme temperature conditions without damage. Available resistances span from 1 ohm to 1 Mega ohm with decent tolerance values.

Moreover, carbon film resistors exhibit a pronounced negative temperature coefficient of resistance, indicating susceptibility to resistance decreases with rising temperatures. Cost-effectiveness and broad operating range are additional attributes. They find extensive applications in X-rays, power supplies, and RADAR.

Understanding Metal Oxide Film Resistors

Unlike their carbon film counterparts, metal oxide film resistors employ thin metal oxide films over an insulating ceramic rod. The combination of oxygen and various atoms creates an effective resistive coating. Typically utilizing tin oxide, these resistors may also incorporate antimony oxide to enhance resistance.

These resistors withstand elevated temperatures effectively due to the heat-blocking nature of the ceramic base. The resistance level primarily correlates with antimony content; higher antimony levels lead to increased resistance. The nuances hinge on the metal oxide film's thickness and the width of the helical cuts, where thicker widths inversely affect resistance.

What sets them apart is their cost-effectiveness alongside their capacity for high-temperature endurance and minimal noise generation. Additionally, their compact size contributes to high reliability and stability, marking them as a favorable choice.

Comparing Carbon Film Resistor with Metal Oxide Film Resistor

Engineers often face a dilemma: whether to utilize carbon film resistors or opt for metal oxide film variants. Let’s dissect this comparison methodically to guide your decision-making.

In terms of performance, Metal Oxide Film Resistors have clear advantages over Carbon Film Resistors, attributable to distinct properties outlined below.

Voltage and Temperature Coefficients

Metal Oxide Film Resistors present superior voltage and temperature coefficients compared to carbon film variants. The voltage coefficient measures resistance variation concerning voltage changes—the greater the stability, the wider the operational resistance range, facilitating resilience against higher temperatures compared to carbon film resistors.

Noise Design The construction of Metal Oxide Film Resistors allows for reduced noise production as they minimize current flow, yielding less audible interference.

These resistors also emerge as optimal for high-frequency applications compared to their carbon counterparts.

Tolerance Metal Oxide Film Resistors offer a minimum tolerance as low as 0.1%, significantly outperforming carbon film resistors, which start at 2% tolerance.

Size

Finally, metal oxide film resistors typically exhibit a smaller footprint compared to carbon film resistors, making them a more suitable option for compact designs.

In sum, having discussed the attributes of these two resistor types, you’ll be well-equipped to choose the right one for your needs.

Resistors: Varieties and Their Applications

By H. Ward Silver

Though it may seem that an ohm is just an ohm, there exists an array of resistor types. Selecting the proper resistor is crucial for ensuring circuit functionality and durability. In the following sections, you’ll explore the common resistor types alongside their unique characteristics.

Essential Principles of Resistors

George Ohm.

Each conductor offers some level of resistance against electrical charge (with superconductors being exceptions). George Ohm formulated the precise interrelation among voltage (V), current (I), and resistance (R), encapsulated in the law that bears his name and is commonly taught to electronics students:

V = I x R or I = V / R or R = V / I

As electrons navigate through a material influenced by an electric field, they encounter atomic collisions. These collisions impart energy to the atoms, resulting in vibrational energy that subsequently elevates the material's temperature. The energy manifested as heat represents the power losses, calculable as:

Power (P) = I² x R or P = V² / R

A perfect resistor, unbothered by whether AC or DC current flows through it, facilitates electron movement regardless of directional flow.

Practically, construction choices inherently instigate parasitic effects, prompting actual resistor behavior to deviate from the ideal circuit model. The circuit model illustrated in this section highlights how a resistor's characteristics influence its electrical behavior contingent on frequency and applied current and voltage.

Types of Resistors

To convert a conductive material into a practical resistor, electrodes and leads must be connected for current flow. The body of the resistor is insulated to guard against environmental effects and vice versa. There exists a variation of resistor construction methods along with various packages tailored for voltage, power dissipation, and other parameters.

Carbon Composition
Composition means the resistive material consists of a blend of carbon and stabilizing agents. The carbon quantity dictates the material's resistance level. A small cylinder, akin to a pencil lead, held between two electrodes is coated with resin or phenolic, resulting in a non-inductive resistor with low inductance used in RF circuits.

These resistors come in power ratings from 1/4 to 2 watts and showcase stability against temporary overloads, in contrast to film resistors, due to heat evenly distributing throughout the resistive material. However, their susceptibility to temperature and humidity limits their precision, rendering them unfit for circuits necessitating stable resistance values.

Film Resistors
In film resistors, a thin layer of carbon or metal coats an insulating substrate like glass or ceramic. Resistance values arise from the film's thickness along with the carbon/metal composition. These resistors exhibit distinguished accuracy and stability.

A drawback arises from their incapacity to manage substantial power, given their uniform thinness. Overloads can inflict damage on the film, leading to 'hot spots' and permanent value shifts. Adjustments to film values are sometimes performed pre-sealing through laser trimming techniques.

When the resistive film envelops the interior of a tube, trimming generates a helical path for current, elevating the series inductance of the resistor. In high-frequency applications, ensure selected resistors showcase minimal inductance values.

Surface-mount resistors are generally film resistors, incorporating no leads, thus achieving remarkably low inductance. As a result of their reduced size, these resistors typically possess low power ratings, often ranging from 1/10 to 1/4 watt.

Wire Wound Resistors
Commonly implemented in power supplies and other high-dissipation applications, wire wound resistors consist of high-resistance wire wrapped around an insulating ceramic tube, attached to electrodes at each end. These resistors are engineered to dissipate significant power, available in sizes from one watt to hundreds.

Designed explicitly for air cooling, some versions feature a metal housing to facilitate heat dissipation. However, the winding results in high series inductance; therefore, wire wound resistors are unsuitable for audio and RF circuits. Exercise caution when repurposing resistors from discarded electronic components.

Compact wire wound resistors may visually resemble film or carbon composition types. Typically, these feature wide color bands, although not universally. To confirm selection, test resonant properties at the expected frequencies.

Ceramic and Metal Oxide Resistors:
Should you require a high-power non-inductive solution, cermet (ceramic-metal blend) or metal oxide resistors can be appropriate. These consist of the same structure as carbon comp resistors but replace carbon materials with cermet or metal oxides.

Adjustable Resistors
Different adjustable resistors exist, from simple wire wound designs, exposing portions of wire for movable electrode connection, to those requiring rotary shafts for adjustments. These components can act as voltage dividers or as adjustable resistance in circuits, available in various configurations.

Power Dissipation and Voltage Ratings

After resistance value, the power dissipation characteristic takes precedence for resistors. An overloaded resistor does not only change its value but may also reach temperatures hazardous enough for damage or failure. Most circuit designers have come to recognize the distinct scent of burnt resistors.

A practical guideline involves estimating power dissipation requirements and opting for resistors with higher ratings—preferably a factor of two above initial calculations. This applies particularly for resistors dissipating power above a watt, with adequate airflow to facilitate cooling while mounting resistors horizontally when feasible.

Another vital rating entails maximum voltage rating, where voltages exceeding designated thresholds can induce arcing between terminals! For high-voltage applications, cleanliness is critical; oily residues or dust can lead to undesired current paths, heightening leakage and leading to arcing potential. Thus, high-voltage resistors are typically elongated and slender to minimize leakage paths while enhancing their ability to withstand elevated voltages.

HOW TO INTERPRET A RESISTOR

Understanding the resistor color code is a rite of passage for electronics technicians universally. Many online guides are available for reference or simply search "resistor color code" on any search engine. Surface mount and power resistors may also display values through three or four-digit codes, with the final digit serving as an exponent.

TOLERANCE AND TEMPERATURE COEFFICIENT

Resistors incorporate nominal values along with tolerances, denoting acceptable variation that can lead to fluctuation from nominal values. Tolerances of 1%, 5%, or 10% are common, with tighter tolerances obtainable based on the resistor's series standard. For example, in the 5% series, values are regularly spaced to ensure a tolerance ratio averaging 10% across other resistor values.

Furthermore, resistors alter their values in relation to temperature, governed by the temperature coefficient or tempco, marked as parts per million (ppm) or percentage change per degree Celsius. A positive tempco indicates that the resistance escalates with temperature. For circuits employing precision resistors (1% or tighter tolerances), maintaining stable temperature conditions proves paramount.

GUIDE TO CHOOSING RESISTORS

Special applications requiring unique resistor types require thoughtful selection. While guidelines are not definitive, they assist in steering initial considerations. For the majority of circuits, carbon film or carbon composition resistors suffice.

ESD and Transient Protection: Carbon composition, metal oxide (adequate for short pulse overloads and low inductance)

Audio and Instrumentation Circuits: Metal film (low noise)

High Voltage: Wire wound and metal oxide with high-voltage body styles

RF Applications: Carbon composition and metal oxide provide low inductance.

Precision Circuits: Carbon or metal film (fixed value) and cermet (trimming controls)

Ultimately, as you prioritize value, power, voltage, stability, and cost, identify resistor types that align with your project needs.

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