How Does metal oxide film resistor Work?

Release: -03-28, Update: -08-24, M.P.

Carbon film resistors

Carbon film resistors, commonly called "carbon resistors," use a carbon-based baked film. They have the lowest cost and are available in a wide range of resistance values. Since their temperature coefficient is not very good, they are not used in applications requiring high accuracy. Carbon film resistors have become the mainstream type of general resistors due to their low cost and performance aspect.

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Solid resistors

Solid resistors use a baked kneaded mixture of a carbon-based resistive material and ceramic, etc., as the resistive element. These resistors have the feature of being robust and capable of being used in harsh conditions. Ultra-small resistors such as 1/16 W types can be produced, and in addition, a wide range of resistance values can be covered. However, from the cost aspect and overall characteristics aspect, carbon film resistors are better.

Metal film resistors

There are two types of metal film resistors: Thick film types in which the resistive element is a heated and fired metal-based paste, and thin-film types that use a metal vapor deposition film as the resistive element. They have relatively high accuracy (around ±1%) and better characteristics than carbon film resistors. However, they are somewhat expensive and cost 2 to 3 times as much as carbon film resistors.

Metal oxide film resistors

Metal oxide film resistors use a film in which metal oxides are mixed into the resistive element. They are used in relatively intermediate power applications of around several watts, and are inexpensive. The temperature characteristics are inferior to metal film resistors at about ±350 ppm/°C, which is not very good.

Wire-wound resistors, cement resistors

Wire-wound resistors are resistors made by winding a thin metal wire around a ceramic bobbin. Since the structure of winding a wire around a bobbin is the same as a coil, these resistors have an inductance.

Resistor network

A resistor network is a type of composite component consisting of several resistors in a single package. It has the merits of reducing the number of components, saving labor, having high density, etc.

Semi-fixed resistor

A variable-resistance resistor (trimmer) which is intended to be adjusted in the factory. It can be adjusted by turning with an adjustment screwdriver.

The first alternative to a carbon composition resistor is a metal oxide film resistor. These resistors are very easy to manufacture as compared to metal film resistors. Firstly we have to know that metal oxide is one kind of metallic compound that is formed due to the chemical bonding among the oxygen atom as well as other elements. So it includes a minimum of two or above different kinds of atoms like a tin with oxygen. Thus, tin oxide is utilized for designing these resistors. These resistors are the most frequently used resistors because they have superior performance as compared to carbon film & carbon composition resistors. So this article discusses an overview of a metal oxide film resistor, it's working, and its applications.

What is Metal Oxide Film Resistor?

Metal oxide resistor definition is; the fixed value and axial resistors which are made with the ceramic rod that is covered with a metal oxide thin film like tin oxide are known as metal oxide film resistor. These resistors should not be confused with metal oxide varistors because they are made with silicon carbide or zinc oxide. These resistors are used mostly wherever power dissipation is necessary. The metal oxide resistor diagram is shown below.

Specifications

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

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• The available tolerance typically is ±1%, ±2%, ±5%, ±10%,
• Its resistance value ranges 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 resistor temperature maximum is 300°C.
• It has outstanding long-time stability.
• It has overload or high surge capability.
• The standard resistance tolerance is ±5%.

Construction of Metal Oxide Film Resistor

The metal oxide film resistor construction is the same as the metal film resistor except for the material used to construct the film. This resistor uses tin oxide to construct the film which is made with tin oxide as well as antimony oxide which works as the resistive element for the electric current. Thus, this material restricts the current flow to a certain level.

The manufacturing procedure of a metal oxide film can be done with chemical deposition techniques & almost ceramic carriers are used as substrates always. The deposition procedure is obtained by the pure metals reaction with gas at low pressure & high temperature. The metal oxide film is most frequently produced with a chemical deposition process. Here tin oxide is the common metal oxide film that is created by heating the body of this resistor within a tin chloride steam.

Usually, other metal oxide film uses a different deposition procedure. Initially, a thin metal film is applied, after that it is reacted by oxygen. So the desired composition is attained by simply measuring the resistance.

The working of this resistor is; once the film is applied to the body of the resistor then the final value of resistance can be achieved by applying a helical cut. Usually, this can be done by laser cutting but earlier it was done by sandblasting or grinding. Here, the spiral cut on the resistor will make the resistance path longer & also increases the value of resistance up to a hundred times higher. The value of resistance can be controlled precisely by cutting. The resistance of his resistor mainly depends on the antimony oxide amount of added to the tin oxide; the helical metal oxide film cut width & the metal oxide film layer thickness.

Characteristics

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

• These resistors give better performance as compared to other resistors like carbon film & metal film resistors like voltage rating, power rating, surge capacity, high-temperature operation & overload capabilities.
• So these resistors are preferred mainly for high-endurance applications.
• These resistors' stability properties are lower than metal film resistors.
• These have very poor properties for tolerance & low values.
• The temperature coefficient of metal oxide film resistors is higher as compared to metal film resistors like 300 ppm/0C.
• These resistors use tin oxide as a resistive material that is contaminated by antimony oxide to enhance resistivity.
• The antimony oxide amount added will determine the resistance level, the metal oxide film layer thickness & the helical metal oxide film cut width.
• These resistors can withstand high temperatures as compared to metal film or carbon resistors. The noise property of these resistors is the same of carbon resistors.

Metal Oxide Film Resistor Color Code

Generally, cylindrical resistors are color coded with bands/markings to identify the values of resistance & tolerance very quickly like metal film, carbon film, fuse, winding, metal film, and metal oxide film resistors. So the resistance value can be found based on the color code of the resistor. Resistors are available in four to six color bands on the resistors which specify the value of resistance.
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.

• The performance of this resistor is high compared to metal film & carbon film resistors.
• These resistors are used by many designers in high-endurance applications.
• These are not expensive as compared to carbon composition type resistors.
• They have superior high surge current capacity.
• These operate at higher temperatures.
• They have high stability & reliability.
• These are available in small sizes.
• They produce less noise.

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

• These are not appropriate for high-frequency-based applications.
• Nowadays these are not commonly used.
• Its installation is difficult.

Applications

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

• These are the main types of resistors within basic circuits.
• These resistors are used where power dissipation is required above 1 W.
• These are used where high stability is necessary.
• These are used in medical equipment & telecommunication.
• These are used in power applications around several watts