What are the disadvantages of metal oxide resistors?

The metal oxide film resistor is the first alternative to a carbon composition resistor. These resistors are easier to manufacture compared to metal film resistors. To understand their construction, it's important to know that a metal oxide is a metallic compound formed through the chemical bonding of oxygen atoms with other elements. This means it must include at least two different atoms, such as tin and oxygen. Consequently, tin oxide is commonly used to design these resistors. Due to superior performance over both carbon film and carbon composition resistors, metal oxide film resistors are widely utilized. This article provides an overview of metal oxide film resistors, their working principle, and applications.

What is Metal Oxide Film Resistor?

A metal oxide film resistor is defined as a fixed value, axial resistor made with a ceramic rod coated in a thin film of a metal oxide, such as tin oxide. These should not be mistaken for metal oxide varistors, which are made from silicon carbide or zinc oxide. Metal oxide resistors are primarily used in applications where power dissipation is necessary. A diagram representing a metal oxide resistor is shown below.

Specifications

Below are specifications for the performance guide of the metal oxide film resistor:

• The available tolerances typically range from ±1%, ±2%, ±5%, to ±10%.
• Resistance values span from 1Ω to 10MΩ.
• Maximum noise is 0.2 μV/V.
• The temperature coefficient is ±300 ppm/°C.
• The voltage coefficient is 0. %/V.
• The maximum resistor temperature is 300°C.
• It boasts outstanding long-term stability.
• It has overload and high surge capabilities.
• The standard resistance tolerance is ±5%.

Construction of Metal Oxide Film Resistor

The construction of metal oxide film resistors is similar to that of metal film resistors, differing primarily in the materials used to create the film. These resistors utilize tin oxide combined with antimony oxide as the resistive element for electric current. This compound restricts current flow to a predetermined level.

The manufacturing process employs chemical deposition techniques, typically utilizing ceramic carriers as substrates. The metal oxide film is predominantly produced through a chemical deposition process, involving the heating of the resistor body in tin chloride vapor.

Generally, other metal oxide films utilize different deposition processes. A thin metal film is first applied and subsequently reacted with oxygen to achieve the desired composition through resistance measurement.

The resistor’s working principle is that once the film is applied, its final resistance value can be determined by applying a helical cut. This is often done using laser cutting, although it was formerly executed through sandblasting or grinding. The helical cut on the resistor extends the resistance path, significantly increasing the resistance value, sometimes up to a hundred times. The total resistance depends mainly on the amount of antimony oxide added, the helical cut width, and the film layer thickness.

Characteristics

The characteristics of metal oxide film resistors include:

• Better performance compared to carbon film and metal film resistors in aspects such as voltage rating, power rating, surge capacity, high-temperature operation, and overload capabilities.
• These resistors are preferentially used in high-endurance applications.
• The stability properties are lower than those of metal film resistors.
• Poor tolerance and low resistance values.
• The temperature coefficient is higher compared to metal film resistors, measuring around 300 ppm/°C.
• Utilizes tin oxide as a resistive material, enhanced by antimony oxide.
• Resistance levels depend on the added amount of antimony oxide, the thickness of the metal oxide film layer, and the helical film cut width.
• Can withstand higher temperatures than carbon or metal film resistors, with noise properties similar to those of carbon resistors.

Metal Oxide Film Resistor Color Code

Cylindrical resistors, including metal film, carbon film, and metal oxide film resistors, are color-coded with bands or markings for quick identification of resistance and tolerance values. Resistors typically employ four to six color bands to indicate their resistance values. Refer to the link for more information on the resistor color code.

Advantages & Disadvantages

Advantages of metal oxide film resistors include:

• Superior performance compared to metal film and carbon film resistors.
• Widely used by designers in high-endurance applications.
• More economical compared to carbon composition resistors.
• High surge current capacity.
• Ability to operate at elevated temperatures.
• High stability and reliability.
• Compact size.
• Generate less noise.

However, disadvantages include:

• Not suitable for high-frequency applications.
• Less common and increasingly obsolete in recent times.
• Installation can be challenging.

Applications

Metal oxide film resistors find applications in various areas, including:

• Primary types of resistors in basic circuits.
• Used where power dissipation exceeds 1W.
• Applications demanding high stability.
• Utilized in medical devices and telecommunication technology.
• Effective in power applications handling several watts.

This overview summarizes the characteristics, working, advantages, disadvantages, and applications of metal oxide film resistors. They serve as fixed-type resistors that use a resistive element like metal oxide film to limit electric current flow.