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Understanding metal oxide film resistors

Metal oxide film resistors are one of the most widely used resistors because they are superior in performance to the carbon composition and carbon film resistors. As the name indicated, metal oxide film resistors axially loaded resistors are made from a thin film of metal oxide that has been deposited into a ceramic rod. They are mostly used where power dissipation is required. Today you’ll get to know the definition, history, applications, properties, manufacturing and construction, and working of metal oxide film resistors. You’ll also get to know the advantages and disadvantages of these metal oxide film resistors.

metal oxide film resistor

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What is a metal oxide film resistor?

Firstly, you should know metal oxide is a metallic compound formed because of the chemical bonding between the oxygen atom and other elements. It consists of two or more different types of atoms such as (tin with oxygen). So, tin oxide is used in the construction of metal oxide film resistors. Therefore, the definition of metal oxide film resistors is said to be resistors with a fixed value, axial resistors made of the ceramic rod that is coated with a thin film of metal oxides. Metal oxide film resistors should not be confused with metal oxide varistors, made of zinc oxide, or silicon carbide.

The metal oxide film resistor is the first alternatives to carbon composition resistors. They can be easily manufactured than that metal film resistors, but they are not commonly used nowadays.

Applications

Even though metal oxide film resistors are similar to metal film resistors in properties. They are currently the predominant types of resistors in basic circuit use and they are cheap in price compared to a carbon film. Metal oxide film resistors are common in applications where the power dissipation requirements are above 1 W and high stability is required. Carbon film resistors are more cost-efficient in this situation.

Diagram of metal oxide film resistors:

diagram of metal oxide film resistor

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Properties

Metal oxide film resistors offer better performance than metal film and carbon film resistors in the following properties: power rating, voltage rating, overload capabilities, surge capacity, and high-temperature operation. This is why the types of resistors are chosen for high endurance applications. Also, their stability properties are inferior to metal film resistors and they have poor properties for low values and tolerance. The temperature coefficient is about 300 ppm/0C, which is higher than that of metal film resistors.

These resistors have their resistance material as a tin oxide that is contaminated with antimony oxide to increase resistivity. The amount of antimony oxide added determines the level of resistance, the thickness of the metal oxide film layer, and the width of the helical metal oxide film cut. Also, they can withstand higher temperatures than carbon or metal film resistors. The noise properties are similar to carbon resistors.

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The table below shows the performance of metal oxide film resistors:

METAL OXIDE FILM RESISTOR PERFORMANCE GUIDE
PARAMETER  PERFORMANCE
Typical tolerance availability ±1%, ±2%, ±5%, ±10%,
Value range <1Ω – ~10MΩ
Load life (% change over 1000h) 2
Max noise (µV/V) 0.2
Temperature coefficient (ppm/°C) ~±300
Voltage coefficient (%/V) 0.0005
Max resistor temperature (°C) 300

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Construction or Manufacturing

The construction of metal oxide film resistors is similar to metal film resistors. Their difference is the material used in constructing the film. For metal film resistors, the film is constructed by employing metals such as nickel-chromium whereas metal oxide film resistors use tin oxide in constructing the film. Just as mentioned, this film is made of tin oxide and antimony oxide which acts as the resistive element to the electric current. So, metal oxide film restricts the electric current to a certain level. The ceramic core acts as the insulating material to the electricity. Therefore, the ceramic core does not allow heat through it, allowing the resistors to withstand high temperatures.

You should know the manufacturing process of a metal oxide film is done using chemical deposition methods and ceramic carrier is almost always used as substrate. The deposition process is achieved with the reaction of pure metals with gas at high temperature and low pressure.

Working

The working principle of metal oxide film resistors is less complex and can be easily understood. The video below explains in-depth the working of metal oxide resistors:

Advantages and disadvantages of metal oxide film resistors

Advantages:

Below are the benefits of metal oxide film resistors in their various applications:

  • High stability
  • High reliability
  • They can operate at a higher temperature
  • Low cost compared to the carbon composition resistor
  • Low noise produce
  • Smaller in size.

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Disadvantages:

Despite the good advantages of these resistors, some limitations still occur. Below are the disadvantages of metal oxide film resistors in their various applications:

  • There are better options
  • Not commonly used nowadays
  • Installation may be difficult
  • Not suitable for high-frequency applications.

That is all for this article, where the definition, history, applications, properties, manufacturing and construction, working advantages and disadvantages of metal oxide film resistors are being discussed. I hope you get a lot from the reading, if so, kindly share with students. Thanks for reading, see you around!

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