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

The first alternative to a carbon composition resistor is a metal oxide film resistor. These resistors are very easy to manufacture compared to metal film resistors. Firstly, we should understand that metal oxide is a type of metallic compound formed from the chemical bonding of oxygen atoms with other elements. It consists of at least two different types of atoms, like tin and oxygen. Consequently, tin oxide is utilized for creating these resistors. They are commonly favored due to their superior performance compared to carbon film and carbon composition resistors. This article provides an overview of a metal oxide film resistor, including its working principles and applications.

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What is Metal Oxide Film Resistor?

A metal oxide resistor is defined as fixed value axial resistors made with a ceramic rod that is coated with a thin layer of metal oxide, such as tin oxide. These resistors should not be confused with metal oxide varistors, which are made with silicon carbide or zinc oxide. They are primarily used in applications where power dissipation is necessary. The following diagram illustrates a metal oxide resistor.

Specifications

The performance guide of the metal oxide film resistor is outlined below.

• The available tolerance typically is ±1%, ±2%, ±5%, ±10%,
• Resistance values range from 1Ω to 10MΩ.
• Maximum noise is 0.2 μV/V.
• Temperature coefficient is ±300 ppm/°C.
• Voltage coefficient is 0. %/V.
• Maximum resistor temperature is 300°C.
• It features outstanding long-term stability.
• It has overload or high surge capability.
• The standard resistance tolerance is ±5%.

Construction of Metal Oxide Film Resistor

The construction of a metal oxide film resistor is similar to that of a metal film resistor, except for the material used for the film. This resistor utilizes tin oxide as the film material, along with antimony oxide, which serves as the resistive element for electric current. Thus, this material limits the current flow to a designated level.

The manufacturing process of a metal oxide film involves chemical deposition techniques, usually using ceramic carriers as substrates. The deposition process results from the reaction of pure metals with gas under low pressure and high temperature. The metal oxide film is most frequently produced through a chemical deposition process. In this process, tin oxide is synthesized by heating the resistor body within a tin chloride steam.

Other metal oxide films may use different deposition methods. Initially, a thin metal film is applied, which is then reacted with oxygen to achieve the desired composition by measuring the resistance.

When the film is applied to the body of the resistor, the final resistance value can be achieved through a helical cut. This is commonly done by laser cutting, whereas previously, it was performed through sandblasting or grinding. The spiral cut on the resistor elongates the resistance path, effectively increasing the resistance value up to a hundred times higher. The resistance value can be precisely controlled by adjusting the cut. The resistance of this resistor primarily depends on the amount of antimony oxide mixed with tin oxide, the width of the helical cut, and the thickness of the metal oxide film layer.

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Characteristics

The characteristics of a metal oxide film resistor are detailed below.

• These resistors outperform others, including carbon film and metal film resistors, in voltage rating, power rating, surge capacity, high-temperature operation, and overload capabilities.
• They are typically chosen for high-endurance applications.
• The stability of these resistors is lower than that of metal film resistors.
• They have poor tolerance and low values.
• The temperature coefficient of metal oxide film resistors is higher than that of metal film resistors, measuring around 300 ppm/°C.
• These resistors use tin oxide as the resistive material, enhanced by adding antimony oxide.
• The resistance level is determined by the amount of antimony oxide added, the cut width, and the film layer thickness.
• These resistors can withstand higher temperatures than metal film or carbon resistors. The noise characteristics are comparable to those of carbon resistors.

Metal Oxide Film Resistor Color Code

Generally, cylindrical resistors are color-coded with bands to quickly identify resistance values and tolerances, similar to metal film, carbon film, fuse, winding, and metal oxide film resistors. The resistance value can be determined based on the resistor's color code, with four to six color bands indicating the resistance value.
Please refer to this link for: Resistor Color Code.

Please refer to this article for the Difference between Metal Oxide Resistor & Metal Film Resistor.

Advantages & Disadvantages

The advantages of a metal oxide film resistor are discussed below.

• Its performance is superior compared to metal film and carbon film resistors.
• They are commonly used by designers in high-endurance applications.
• They are less expensive than carbon composition resistors.
• They possess high surge current capacity.
• These resistors operate well at higher temperatures.
• They offer high stability and reliability.
• They are available in compact sizes.
• They produce minimal noise.

The disadvantages of a metal oxide film resistor are discussed below.

• These are not suitable for high-frequency applications.
• Currently, they are not as commonly used.
• Installation can be challenging.

Applications

The applications of a metal oxide film resistor are as follows.

• These resistors are essential components in basic circuits.
• They are used in applications requiring power dissipation above 1 W.
• They provide high stability in various uses.
• They are employed in medical equipment and telecommunications.
• They are used in power applications concerning several watts.

In summary, this article has provided an overview of the construction, working principles, characteristics, advantages, disadvantages, and applications of the metal oxide film resistor. This type of fixed resistor utilizes a resistive element made from metal oxide film to control electric current flow. Here’s a question for you: what is a carbon film resistor?