Metal oxide film resistor - Definition, construction and ...

Metal oxide film resistor definition

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The metal oxide film resistor is a type of fixed resistor, which uses metal oxide film as the resistive element to limit the flow of electric current to certain level.

What is metal oxide?

Metal oxide is a metallic compound, which is formed because of the chemical bonding between the oxygen atom and other elements. Metallic compound consists of two or more different types of atoms such as (tin with oxygen). In metal oxide film resistors, the film is constructed by using the tin oxide.

What is tin oxide?

Tin oxide is a type of metallic compound, which is formed because of the chemical bonding between the oxygen atom and tin.

Metal oxide film resistor construction

The construction of metal oxide film resistor is almost similar to the metal film resistors. The main difference between the metal oxide film resistor and the metal film resistor is material used for constructing the film.

In metal film resistor, the film is constructed by using the metals such as nickel chromium whereas in metal oxide film resistors, the film is constructed by using the metal oxide such as tin oxide.

The metal oxide film resistor is made by coating the ceramic core with metal oxide such as tin oxide. The antimony oxide is added to the tin oxide to increase its resistivity. The resistivity of the metal oxide film is mainly depends on the amount of antimony oxide added to the tin oxide.

The film made of tin oxide and antimony oxide acts as the resistive element to the electric current. Hence, metal oxide film restricts the electric current to certain level. The ceramic core acts the insulating material to the electricity. Hence, the ceramic core does not allow heat through it. Thus, these resistors can withstand at high temperatures.

The metal end caps are fitted at both ends of the resistive element. The leads made of copper are joined at two ends of these metallic end caps. The metal oxide film resistors are able to withstand at higher temperatures than metal film and carbon film resistors.

In metal oxide film resistor, the desired resistance is achieved by cutting the metal oxide film in a helical manner along its length. Once the desired resistance value is achieved, the cutting of metal oxide film is stopped. Lasers are generally used to cut the metal oxide film in a helical manner.

Resistance of metal oxide film resistor is depends on the amount of antimony oxide added, the thickness of metal oxide film layer, and the width of helical metal oxide film cut

The resistance of the metal oxide film resistor is depends on the amount of antimony oxide added to the tin oxide, the thickness of the metal oxide film layer, and the width of helical metal oxide film cut.

Amount of antimony oxide added

The antimony oxide is added to the tin oxide to increase the resistance of the metal oxide film. The resistivity of the metal oxide film is directly proportional to the amount of antimony oxide added.

If large amount of antimony oxide is added to the tin oxide, the resistance of the metal oxide film to the electric current increases highly. Therefore, a large amount of electric current is blocked.

If small amount of antimony oxide is added to the tin oxide, the resistance of the metal oxide film to the electric current increases slightly. Therefore, only a small amount of electric current is blocked.

Width of helical metal oxide film cut

Cutting the metal oxide film in a helical manner with laser will increase the resistive path of the electric current. Hence, the resistance to the electric current increases. The resistance provided by the metal oxide film resistor is depends on the width of metal oxide film cut.

The helical film cut with more width provide less resistance to the electric current because the free electrons have to travel only a small distance through the resistive path. Hence, the possibility of free electrons collision with the atoms is less. Thus, only a small number of free electrons collide with the atoms and the remaining large number of free electrons moves freely from one place to another place by carrying the electric current.

Therefore, the metal oxide film resistors with wide helical metal film cut allows large amount of electric current.

The helical film cut with less width provides more resistance to the electric current because the free electrons have to travel a large distance through the resistive path. Hence, the possibility of free electrons collision with the atoms is high. Therefore, a large number of free electrons collide with the atoms and the remaining small number of free electrons moves freely by carrying the electric current.

Metal oxide film layer

The helical film cut with less width provides more resistance to the electric current because the free electrons have to travel a large distance through the resistive path. Hence, the possibility of free electrons collision with the atoms is high. Therefore, a large number of free electrons collide with the atoms and the remaining small number of free electrons moves freely by carrying the electric current.Therefore, the metal oxide film resistors with narrow helical film cut allows only a small amount of electric current.

The resistance of the metal oxide film resistor is inversely proportional to the thickness of the metal oxide film layer.

Metal oxide film resistors with very thick film provide large space for the free electrons to move freely from one place to another place. Hence, the possibility of free electrons collision with the atoms is very less. Therefore, only a small number of free electrons collide with the atoms. The small number of free electrons, which collides with the atoms, loses their energy in the form of heat and the remaining large number of free electrons moves freely from one place to another place by carrying the electric current. Therefore, the metal oxide film resistors with very thick film layer allows large amount of electric current.

Metal oxide film resistors with very thin film provide less space for the free electrons. Hence, the possibility of free electrons collision with the atoms is high. Therefore, a large number of free electrons collide with the atoms. The large number of free electrons, which collides with the atoms, loses their energy in the form of heat and the remaining small number of free electrons moves freely by carrying the electric current. Therefore, the metal oxide film resistors with very thin film layer allow only a small amount of electric current.

Advantages of metal oxide film resistors

• Low cost compared to the carbon composition resistor.
• Metal oxide film resistors operate at higher temperatures.
• Produce low noise.
• Small size
• High stability
• High reliability

        

                              

Carbon Film Resistor VS Metal Film: Definition

A carbon film resistor is a type of resistor that slows the flow of electricity by using a thin layer of carbon. Unfortunately, carbon film resistors are more expensive to make than other types of thin film resistors. They also generate a lot of thermal noise and can portray low accuracy. 

(carbon film resistor)

Despite its challenges, it can withstand high-energy pulses and thus are suitable for applications like surge protection electronic circuits, computers, test equipment, DC power supplies, and high-frequency applications. 

A metal film resistor limits the flow of electricity by using a thin metal film. It is also the most popular type of thin film resistor. Moreover, metal resistors are cost-effective, compact, have low tolerance, generate less noise, and operate widely compared to carbon resistors. 

(metal film resistors)

Therefore, you&#;ll find them in bridge circuits, active filters, and high-frequency applications. 

Carbon Film Resistor vs Metal Film Resistor: Side-by-Side Comparison

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Now that we&#;ve defined these resistor types individually, let&#;s look at a head-to-head comparison of them both. This will make it easier to know which use cases would suit each resistor type best. The properties this section will focus on are the voltage coefficient of resistance, stability, and temperature coefficient of resistance.

Temperature Coefficient of Resistance (TCR)

Similar to how the amount of voltage applied to a resistor can affect its resistance, changes in temperature have an effect, too.

Of course, this phenomenon isn&#;t exclusive to resistors. All conductors have resistance of some sort which increases in high temperatures and decreases in low temperatures.

We express this value as parts per million per degree Centigrade (ppm/°C) or Kelvin (ppm/K). As with the previous property the resistance value of metal film resistors is less susceptible to change in temperature than carbon film resistors.

Metal film resistors have a TCR between  ± 20 and ± 200 ppm/k. On the other hand, carbon film resistors have a TCR between -200 to - ppm/k.

As such, the carbon film resistor TCR is substantially greater than the metal film resistors. This makes metal film resistors more reliable under extreme temperatures (both hot and cold).   

Moreover, metal film resistors produce less thermal noise. This makes them ideal for devices such as radios, intercoms and radars. 

Carbon Film Resistor Green Color

Voltage Coefficient of Resistance (VCR)

Applying voltage to a resistor may cause changes in its resistance value. Usually, applying voltage to a resistor causes the resistance to drop in near proportionality.

Regardless of decrease or increase, we represent the change in resistance against each applied volt (per volt).

This measurement is known as the Voltage Coefficient of Resistance (VCR) and is represented by the metric: parts per million per volt (ppm/V).

The VCR is important because it helps us predict and determine how well a resistor performs in high-voltage situations. While carbon film resistors perform well in high-voltage applications, their metal film counterparts perform even better. Metal film resistors have a VCR of just under 1 ppm/V, whereas carbon film resistors have a -10 ppm/V. This means metal film resistors are less susceptible to changes in resistance caused by applied voltage.

Stability

Since resistors are a part of machinery and electronics operating in various environments, we must know how they are fair against external stressors. These stressors can (potentially) affect the resistance value of the resistor. We refer to a resistor&#;s tolerance to environmental stressors over time as stability.

Some of the most common stressors are exposure to radiation, heat (high temperatures), and cold (low temperatures). Sudden or prolonged power spikes can also be categorized as stressors that can challenge the stability of the resistor.

Nevertheless, stability is another area in which metal-film resistors outperform carbon-film resistors. This is not to say that carbon-film resistors are unstable. They have moderate stability. On the other hand, metal film resistors exhibit good to exceptional stability.

Carbon Film Resistor VS Metal Film: Construction

The two resistor types have different processes.

Carbon film resistor construction

Manufacturers make carbon composition resistors from a solid, cylinder-shaped resistive element with wire leads and metal caps on the ends. Further on, the materials in the resistive element are a mixture of clay-based ceramic and graphite or carbon powder. Generally, the carbon powder lets electricity flow through it well.

(Internal part of a carbon film resistor) 

So, during the deposition process, manufacturers start by heating the ceramic substrate in a hydrocarbon like methane. Next, at high temperatures of approximately °C, cracking reactions will occur in the gas. The pure graphite&#;s crystalline carbon then sticks to the ceramic. 

Afterward, the resistive material gets a plastic covering to keep it safe from the heat generated from outside. Manufacturers finally apply copper to make the wire leads at both ends of the resistive element. 

Metal film resistor construction

Contrarily, manufacturers use a special method for metal film resistors, where they artificially age the deposited metal in low temperatures for an extended time. 

Here, the material employed as the resistance material is Nickel chromium (NiCr). However, in other applications, you&#;ll find other materials like tantalum nitride, gold with platinum, and tin plus antimony.

Then, the resistor&#;s stability and resistance often depend on the metal film thickness. Thus, a larger thickness will mean a lower resistance value and greater stability on both films ends. 

Manufacturers use connection leads to press a metal cover for the final touches. They can now achieve the desired resistance via a spiral-shaped slot cut in the thin metal thickness.

Carbon Film Resistor VS Metal Film: Availability

Carbon Film Resistor

You&#;ll find carbon resistors in 1 ohm to 25megaohms with a power rating ranging between one-quarter watt and five watts. 

The three factors affecting the resistance of carbon film resistors are;

• The solid cylindrical rod&#;s cross-sectional area is inversely proportional to the resistance of the resistor.
• Then, we have the rod&#;s length directly proportional to the resistance.
• Lastly, the amount of carbon you add to the resistor is inversely proportional to the resistor&#;s resistance.

Metal Film

Usually, the available metal film resistors in the market have tolerances of 2%, 1%, 0.5%, 0.25%, and 0.1%. Additionally, the resistors are likely to have temperature coefficients ranging from 10 to 100 ppm/K and some resistance. 

The two main factors affecting the resistance of metal film resistors include:

• The width of the spiral metal film cut is inversely proportional to the metal film resistor resistance.
• Secondly, the layer of the metal film is also inversely proportional to the resistance. 

Carbon Film Resistor VS Metal Film: Similarities and Differences

• Similarities

The film resistors are similar in the following ways;

• First, you can regulate both by controlling their thin film thickness.
• Then, their non-conducting material is ceramic.
• They also have the same size.
• Lastly, their shape and appearance are similar. 

(Film resistors in a circuit board)

• Differences

We have summarized the differences between the metal film and carbon film resistors in the table below.

Carbon Film Resistor VS Metal Film: Drawbacks

Both these resistor types have advantages. While metal film resistors have the best frequency response of all resistors, carbon film resistors are more affordable and have high resistance. However, both these resistor types have drawbacks and limitations.

For instance, as we previously discussed, one of the biggest disadvantages of using carbon film resistors is their high-temperature coefficient of resistance. In addition to this drawback, they also have comparatively subpar stability, especially when compared against wire-wound and metal-film resistors.

Another somewhat cumbersome feature of carbon-film resistors is their bulkiness.

As a consequence of their larger size, they can be a little difficult to maneuver and place, especially by hand. Contrastingly, metal-film resistors are compact, which makes them ideal for smaller electronics.    

While, in many ways, metal-film resistors may be superior to carbon-film and carbon-composite resistors, they aren&#;t without their own drawbacks.

For one, they&#;re generally more pricier than carbon film resistors. Moreover, they have higher power dissipation.

Some may see this as an advantage, but it&#;s an attribute that makes them unsuitable for use cases that require low-power dissipation resistors.

Metal Film Resistor On Green Blur

Conclusion

Both carbon film and metal film resistors limit current flow through circuits. However, carbon film resistors use carbon, while metal film resistors use metal to reduce current flow.

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