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Understanding metal film resistor

Metal film resistors are one of the most widely used types as they are superior to carbon film resistors in performance. They have a thin metal layer as a resistive element on a non-conducting body and are one of the most common types of axial resistors. Carbon film and thick and thin-film resistors are also the other categories that use film.

The metal film resistors are often said to be cylindrical axial resistors, but, thin chip resistors work in the same principle for the metal layer. Also, the carbon film resistors are much similar in appearance to the metal film types. However, their properties for stability, accuracy, and reliability are considerably better.

Read more: Understanding resistors

In this article, you’ll get to know the definition, applications, diagram, construction, specifications, and working of metal film resistors. You’ll also get to know the advantages and disadvantages of these resistors.

metal film resistors

What are metal film resistors?

As the name indicates, metal film resistors are axial leaded resistors made from a thin film of metal that is deposited into a ceramic rod. The thin metal film serves as the resistive element and acts the same way as a resistance wire. Metal film resistors can be produced to a very high tolerance because their thickness, width, and length can be accurately controlled. Long term stability of these types of resistors is good because even as they are pre-aged, the metal protects and ensures it does not degrade over time. For leaded axial resistors, it is the popular choice and they are used in huge quantities.

Read more: Understanding carbon composition resistor

Applications of metal film resistors

Due to the good advantages of metal film resistors, they are used in almost all electronic circuits. They give superior performance in many respects to metal oxide film resistors and they stand the position of standard resistors commonly used. They are used when a high tolerance or more accurate value is needed in a circuit. They are also used for bridge circuits, filter circuits, and low-noise analog signal circuits. Metal film resistors are not suitable for applications that expose them to surge transients. This is due to their size and construction, unlike, the carbon composition resistors that are able to tolerate transients way better.

Because of the helical cut to trim the resistance, the resistors are slightly inductive. Although they are still used for several applications, although their performance may be affected by the inductance caused by the helical cut when used in the microwave region. Surface-mount resistors may be suitable for these applications due to their high-frequency tolerance. The metal film resistors are normally applied to the axial leaded components, although thin film surface mount chip resistors use the same technology to create the resistors.

Construction of metal film resistors

The construction of metal film resistors is done by vacuum depositing a metal layer onto a high-purity ceramic cylindrical rod. The thicker the metal film deposited, the more stable the resistor value. Typically, the film thicknesses between 50 and 250nm are used as material thicknesses in most regions because it provides a greater long-term stability level. Different techniques can be used for the construction of metal film resistors but the most common method is Sputtering. Metal normally deposited is nickel Chromium, NiCr, other metals including gold with platinum, or tantalum nitride may be used for specialized applications.

As soon as the film is deposited a metal end cap is pressed over the deposited metal. This makes contact with the resistive film and has the leads incorporated. The next stage in the process is trimming the value of the resistance to the required figure. This can be achieved by using lasers to cut a helix into the metal film. With this, the length of the metal element is extended and also reduces the width of the current-carrying track whilst keeping the deposited material thickness with the optimum range for long stability.

Read more: Understanding carbon firm resistor

Diagram of metal film resistors:

diagram of a metal film resistors

The trimming process means the metal film resistor can be produced to very close tolerance. Tolerances of 0.1%, 0.25%, 0.5%, as well as 1% and 2%. The 1% and 2% are widely available and their tight tolerance enables circuits to maintain good repeatability from one unit to the next. These types of resistors also maintain a good temperature coefficient of resistance which can normally fall between 50 and 100 ppm / 0K.

Finally, with the construction of metal film resistors is to add protective coating and markings. The protective coating consists of a resin that is added in several layers which is individually baked on. The marking rings are added to indicate the value of the metal film resistor and the other relevant characteristics.

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Specifications

The table below shows the specifications of metal film resistors:

METAL FILM RESISTOR PERFORMANCE GUIDE
METAL FILM RESISTOR PARAMETER METAL FILM RESISTOR PERFORMANCE
Typical tolerance availability ±0.1%, ±0.25%, ±0.5%, ±1%, ±2%,
Value range <1Ω – ~10MΩ
Load life (% change over 1000h) 1
Max noise (µV/V) 0.2
Temperature coefficient (ppm/°C) ±50 – >±100
Voltage coefficient (%/V) 0.0
Max resistor temperature (°C) 175

Read more: Understanding capacitor

watch the video below to learn the working of metal film resistors:

Advantages and disadvantages of metal film resistors

Advantages:

One advantage of metal film resistors over carbon composition resistors is that they don’t change their value with age and their tolerance is better than the carbon resistor.  Other benefits include:

  • Better performance
  • Lower cost
  • Universally adopted
  • High tolerance
  • Good temperature coefficient of resistance
  • Use less space in the circuit

Disadvantages:

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

  • They are slightly inductive
  • Not suitable for high-frequency applications
  • Installation may be difficult
  • Professional is required for their working.

That is all for this article where the definition, applications, diagram, construction, specifications, and working of metal film resistors are being discussed. You also learned the advantages and disadvantages of these resistors. I hope you get a lot from the reading, if so, kindly share with other students. Thanks for reading, see you around!

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