What is Metal Film Resistor : Working & Its Applications

Metal film resistor is the most common type of resistor used in electronics. They have been widely used since their development in the 1960s, and are very popular for their consistency and accuracy. This is a type of resistor that has a metal oxide film deposited on the surface of a ceramic core. This is different from carbon film resistors, which have a layer of carbon instead of metal.

Metal film resistors have better performance than carbon or wire wound resistors. They are also more dependable and reliable than other types of electronics parts. For these reasons, metal film resistor is often used as replacements for other types of electronic components.

Metal Film Resistor Definition:

A metal film resistor is a passive two-terminal electrical component. It is designed to provide a specific resistance. The metal film resistor is constructed by depositing an insulating layer of oxide or nitride, which is then covered with a resistive metal film and terminated with a second insulating layer, usually oxide. These components are used in active filters, precision circuits, and timing applications.

Meta Film Resistor
Meta Film Resistor

The metal film resistor symbol is similar to a normal resistor which includes two connecting terminals like anode and cathode shown below.

Meta Film Resistor Symbol
Meta Film Resistor Symbol

Metal Film Resistor Construction

Metal Film Resistors are made of a metal film on an insulating substrate, similar to a thin-film resistor. The metal is deposited by evaporation onto the substrate. The metal film is typically 0.1 µm or thicker and has a much higher resistance than the substrate.

Metal Film Resistor Construction
Metal Film Resistor Construction

The resistors are formed by screen printing carbon paste onto the substrate in a shape that cross-sections the metal film, creating two parallel conductors with a uniform gap between them. The resistor body is coated with an epoxy coating for environmental protection and insulation.

Metal film resistors are manufactured using a photolithography process that creates high-accuracy and low-tolerance resistors. Photolithography is a photographic process that transfers images to semiconductor wafers during integrated circuit development. Metal film resistors have resistance values ranging from 0.1 ohms to 100 MΩ, with tolerances of ±0.1% or less at operating temperature ranges from -55°C to 155°C.

Working

The working of a metal film resistor is similar to the normal resistor to restrict the flow of electric current through it. So, it functions as the resistive element for the flow of electric current. The ceramic substrate of this resistor does not let heat throughout it.

Specifications

The specifications of the metal film resistor include the following.

  • The availability of typical tolerance is ±0.1%, ±0.25%, ±0.5%, ±1% & ±2%,
  • Resistance value ranges from >1Ω < 10MΩ
  • Max resistor temperature is175 °C
  • Load life (% change over 1000h) is 1
  • Voltage coefficient is 0.0 %/V
  • Max noise is 0.2 µV/V
  • Temperature coefficient ±50 to ±100 ppm/°C

Metal Film Resistor Tolerance

Metal Film Resistor tolerance is the highest level of deviation from the nominal value of a resistor. This tolerance represents an accuracy standard for resistors, and it determines how much the measured resistance will vary from the ideal resistance. The tighter the tolerance, the more accurate your measurement will be.

The tolerance of a metal film resistor is the amount the resistance of the resistor can vary from its nominal value. This is usually expressed as a percentage, and can be calculated using the formula:

(Measured Value – Nominal Value)/Nominal Value)×100

If you measure 10kΩ and it has a 5% tolerance, then you know that tolerance is 500Ω. The highest possible value would be 10.5kΩ and the lowest possible value would be 9.5kΩ.

Metal Film Resistor Power Rating

The maximum power a resistor can handle is known as the power rating for that resistor.

The power rating is determined by how much heat the resistor can take before it fails. Heat causes the resistance in the resistor to rise, which causes it to dissipate more power, which causes it to get hotter, and so on. If this process runs away, the resistor’s temperature rises quickly and it fails catastrophically.
The maximum power (Pm ) a Metal Film Resistor can handle is calculated by dividing the maximum working voltage (Um ) by the resistance value (R):

Pm = Um/R

Metal Film Resistor Size Calculation

The size of a metal film resistor can be calculated from its power rating.

After the power rating (power = voltage x current) and the maximum possible temperature rise are known, the resistor size can be calculated by adding the thermal resistance RTH of the resistor to the thermal resistance RθJA of the mounting surface.

Rth = (TJ – TA)/Pd

Here, Rth is thermal resistance

‘Tj’ is Junction temperature

‘TA’ is ambient temperature

‘Pd’ is power dissipation

Size limitation is given by:

RTHmax = (TJmax – TA)/Pd

Resistor size = Rthmax + RθJA

Metal Film Resistor Color Code

The color code for metal film resistors consists of three bands and a fourth band indicating tolerance.
The first two bands indicate the first two digits of the value.

The third band indicates the number of zeros following those first digits. For example, if the third band is red, there are two zeros following the first two digits. If it is gold, there are five zeros following those first two digits.

The fourth band indicates tolerance in percent. A brown or red band represents a 1% tolerance, while a gold or silver band represents a 10% tolerance.

If additional bands are present, they typically indicate temperature coefficient, but this varies based on the manufacturer and part number.

What is the difference between metal film and metal oxide resistors?

The difference between metal film and metal oxide resistor include the following.

Metal Film Resistor

Metal Oxide Resistor
Metal films are the most common type of resistor. Metal oxide resistors are also popular and often used for high-power applications.
Metal films typically have a smaller tolerance. Metal films typically have a higher tolerance but they handle higher power ratings.
These are cylindrical axial resistors. These are axial & fixed value resistors.
These resistors are applicable where a more precise value and high tolerance value are necessary within a circuit. These resistors are used for high endurance applications.
The frequency response of these resistors is about 100 MHz. The frequency response of metal oxide resistors is about 100 MHz.
These resistors have good characteristics for stability, TCR & tolerance. In addition, these resistors feature high linearity & low noise properties due to a low voltage coefficient. These resistors will exceed both the resistor’s performance like carbon film & metal film for different properties like voltage rating, power rating, surge capacity, high-temperature operation & overload capabilities.
The maximum temperature of the metal film resistor operates up to 150°C & with platinum sensors, they work up to 600°C. The temperature coefficient of the metal oxide resistor is approximately 300 ppm/°C.

Advantages

Metal Film Resistors have the following advantages:

High Precision: Film thickness is easy to control, the manufacturing process is mature, with stable performance.

Low Noise: The temperature coefficient of resistance is small, it is not easy to produce a microphonic effect.

High Stability: High purity ceramic core, film-coated on the surface, no oxygen in the air will not oxidize the surface of the resistor, it has good anti-vibration and anti-shock performance.

High Power Rating: Metal film resistors also have a high power-rating, which means they can be used in high-power applications. They tend to have a lower temperature coefficient than carbon film resistors, which makes them ideal for use in precision circuits where a low-temperature coefficient is essential.

Disadvantages

The disadvantages of metal film resistors include the following.

  • Metal film resistors are more expensive than carbon composition and carbon film resistors because there’s a higher cost to precision.
  • Metal film resistors have a limited power rating of 3 watts or less. If you require more power, you’ll need to use a wire wound resistor instead.
  • They are sensitive to temperature changes, both within their environment and in the current they conduct.
  • They need to be regularly maintained, cleaned, and checked to make sure they work correctly.
  • The resistor will fail before it reaches its maximum power rating if there is a sudden surge in current or voltage.
  • The metal film will erode over time due to high temperatures caused by prolonged use at higher power levels than those specified on the resistor itself
  • Metal film resistors can only operate up until temperatures around 200°C (392°F).
  • They are not suitable for use in high-frequency circuits because their parasitic capacitance limits them from being able to handle frequencies above about 100 MHz (Megahertz).

Where are Metal Film Resistors Used/ Applications?

The applications of metal film resistors include the following.

  • Metal-film resistors are used in almost every circuit where resistance is required, including power supplies and other circuits.
  • They are also used to protect sensitive electronic components from dangerous voltages.
  • Metal-film resistors have been around for a long time, but only in recent years has their popularity begun to increase. The reason for this is because they are less expensive than other types of resistors and they are very reliable. This means that they can be used in a wide variety of applications including automotive electronics and computer circuitry.
  • Its resistance to currents makes it useful in a variety of situations, from industrial equipment to consumer electronics.
  • These resistors are used in different types of equipment in different fields like Industrial, military Automotive, Professional audio & video.
  • Industrial Applications: Metal film resistors are commonly used in industrial equipment. They help regulate the flow of current and prevent damage by overheating. Metal film resistors are also frequently employed in appliances, as they help to protect motors and other components of the machines.
  • Consumer Electronics Applications: Metal film resistors are used in a wide range of consumer electronics, including cell phones, laptops, and televisions. They help regulate the current flow in these devices and prevent them from overheating when they are used for extended periods at high power levels.

What is the Difference Between Metal Film Resistor and Other Types of Resistors?

Metal film resistors are more precise than carbon composition or wire-wound resistors, but they are not as precise as thin-film resistors. There is a wider tolerance range for metal film resistors than most other types of resistors, but the tolerance can be adjusted by adjusting the process used to make them.

What should I know before selecting a metal film resistor?

There are many factors to consider when choosing a metal film resistor. Here are some considerations:

  • How much power will the resistor dissipate?
  • Is there an issue with temperature rise?
  • How accurately does the value need to be?
  • Does it need to work at high frequencies?
  • Is moisture resistance required?

How Much Power Can I Use With a Metal Film Resistor?

The amount of power that can be used with a metal film resistor depends on the size of the resistor and its temperature rating. A smaller resistor will have a lower power rating than a larger resistor because it will have less surface area over which to dissipate heat. As temperatures rise, power ratings drop because it becomes harder for heat to dissipate through the air or other means.

Thus, if you’re looking for a way to add resistance to your circuit without spending a lot of money, metal-film resistors may be just what you’re looking for. They are very durable and dependable, which means that you can use them for years without having to replace them or fix them often. Even though metal film resistors aren’t cheap, they do make up for their cost by being able to stand up to harsh conditions and still perform well. Here is a question for you, what is a metal oxide resistor?

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