What is Carbon Film Resistor : Working & Its Applications

The most frequently used type of resistor in electronics applications is the carbon resistor. These components are made with solid cylindrical resistive elements including metal end caps or wire leads. Generally, resistors are made with different materials like alloys & metals which include platinum, nichrome, tungsten alloys & brass. But, most of these metals have less electrical resistivity, not like carbon resistors, so generating high resistance is difficult. Carbon resistors are available in different sizes and types like a carbon film resistor and a carbon composition resistor. These resistors generate high accurate resistance values, so used to compare & calibrate resistances. So this article discusses one of the types of carbon resistor namely carbon film resistor.


What is Carbon Film Resistor/Define Carbon Film Resistor?

A type of carbon resistor that uses carbon film to limit the flow of electric current to a particular level is known as a carbon film resistor. These are fixed value type resistors that are designed out of a ceramic carrier through a thin clean carbon film around it. Here carbon film works like the resistive material. The carbon film resistor symbol looks like a normal resistor which is shown below.

Symbol
Symbol

Construction

The construction of a carbon film resistor can be done with a deposition method like a carbon film can be deposited on a substrate of ceramic. This carbon film is very helpful in restricting the flow of current and it is an essential part of this resistor. The ceramic substrate functions as an insulator for electricity or heat. So that it protects from high temperature to protect the resistor.

Carbon Film Resistor Construction
Carbon Film Resistor Construction

The shape of this resistor is the helical shape, so it helps in receiving precise resistance from this resistor. And also it helps in enhancing the resistor length while reducing the resistor’s width. The resistance value can be simply adjusted by enhancing or reducing the helical path length. Here, this resistor can be protected by providing an epoxy coat.

In the above construction, the two end caps are metallic & the two connecting terminals are made with copper material which is connected at the two metallic end caps.

The electron flow can be restricted by a carbon film layer’s width in the resistor. A thin layer is used for maximum resistance value. When the carbon film layer width is less then electrons cannot be moved freely due to less space. In the same way, a thick layer is used for less resistance value. When the layer width is high then electrons can move very freely and so they collide more with the atoms available thereby losing energy and so less resistance towards the current which it could carry.

These resistors have a high negative temperature coefficient (NTC) and it lies in between 2.5×10^-4 Ω/°C & -8×10^-4 Ω/°C. Here the negative temperature coefficient of resistance can be defined as the rate at which point the material’s resistance value reduces when temperature increases.

Carbon Film Resistor Working Principle

The carbon film resistor works by using the carbon film on a ceramic substrate. Here carbon film works as the resistive material to restrict the flow of current whereas the ceramic substrate works as the insulating material to the flow of electric current or temperature.

In carbon film resistors, the typical tolerance values are; 2%, 5%, 10% & 20%. These resistors have a higher negative temperature coefficient because it uses pure carbon material. In this resistor, the resistance value can be determined by the thickness of the carbon film material.

These resistors are useful as they have a higher negative temperature coefficient. The temperature coefficient of resistance mainly ranges in between 2.5e^-4 ohms/1 degree C & -8e^-4/ 1 degree C. These resistors were very responsive to temperature changes. When temperature increases then the resistance will be decreased.

Specifications

As compared to the carbon composition type, the carbon film resistor has some specifications like the following.

  • The power rating range at 70 °C is from 0.125 W to 5 W.
  • The resistance ranges from 1 ohm to 10 megohms.
  • The operating temperature ranges from −55 °C to 155 °C.
  • The maximum working voltage ranges from 200 to 600 volts.
  • Typical tolerance accessibility is ±2%, ±5%, ±10%, ±20%.
  • Max noise is 20 µV/V.
  • The temperature coefficient ranges from ±200 to ±1500 ppm/°C.
  • The voltage coefficient is 0.0005 %/V.
  • Max resistor temperature is 150 °C.

Difference b/w Metal Film Resistor Vs Carbon Film Resistor

The difference between metal film resistor and carbon film resistor are discussed below.

Metal Film Resistor

Carbon Film Resistor

 Metal film resistor uses a thin metal film to restrict the flow of current. A carbon film resistor uses a thin carbon film to restrict the current flow.
As compared to carbon film resistors, these resistors have high accuracy. These resistors have less accuracy as compared to metal film resistors.
It has high stability. It has low stability.
These are small in size. These resistors are not small in size.
These resistors are not expensive. These resistors are expensive as compared to metal film-type resistors.
The tolerance of the metal film resistor is 0.1%. The tolerance of this resistor is 2%.
These resistors have a low noise design. These resistors have a high noise design.
Metal film resistors have Temperature coefficient values ranges from ±50 – >±100 (ppm/°C) & Voltage coefficient is 0.0 %/V.& Voltage coefficient is 0.0 %/V. The carbon film resistor has temperature coefficient ranges from ±200 – >±1500 ppm/°C & Voltage coefficient of 0.0005 %/V.

There are some common similarities between metal film and carbon film resistors;

  • Both the resistors are available in the same shape and size.
  • The non-conducting material used by these two resistors is ceramic.
  • The controlling of resistance for both resistors can be done by changing the width of the thin film.

Carbon Film Vs Carbon Composition Resistor

The difference between carbon film and carbon composition resistors includes the following.

Carbon Film Resistor Carbon Composition Resistor
A carbon film resistor uses a thin carbon layer on an insulating rod that is slashed to form a thin, extensive resistive path. Carbon composition resistor uses a material that is made from an insulating ceramic, carbon powder & binder material.
The resistance of this resistor can be simply controlled through the width & length of the path with 1% tolerances. The resistance of these resistors can be controlled by changing the carbon ratio to the materials of filler.
The power rating of this resistor is upto 5watts. The power rating of the carbon composition resistor is up to 1 watt.
The frequency response of these resistors is not good because the inductance & capacitance caused through the resistive path slash into the film. The frequency response of these resistors is  good so used in high-frequency based applications,

Carbon Film Resistor Color Code

Generally, resistors are available with different color bands which are used to determine the resistance, tolerance rate. The number of bands in a resistor ranges from 3 color bands to 6 color bands.
The resistance value can be indicated through the first 2 color bands, the 3rd band acts as a multiplier and the fourth band is the tolerance. On resistor, the fourth band is located away from the first three bands.

The color code of the carbon film resistor is the same as the normal resistor color code. So, please refer to this link to know about Resistor Color Code Calculation.

Characteristics

The typical characteristics of a carbon film resistor include the following.

  • The accuracy of this resistor is high that ranging from 2 to 5%.
  • The resistance range is wide that generally ranges from 2.1Ω – 10MΩ.
  • Nominal resistance is E-48.
  • High limit voltage.
  • It has good high-frequency characteristics.
  • The rated power includes 1/8Watt, 1/4Watt, 1/2Watt, 1Watt, 2Watt, 5Watt & 10Watt.
  • It has a negative temperature coefficient (NTC). So changes within voltage will have a small effect on the value of resistance.
Negative Temperature Coefficient or NTC
Negative Temperature Coefficient or NTC

Advantages

The advantages of a carbon film resistor include the following.

  • This resistor generates low noise.
  • These resistors have the capacity to endure high temperatures & voltages.
  • Tolerance is low.
  • Not expensive.
  • The operating range is wide.
  • Its size is small.
  • It is used widely in different applications.

Disadvantages

The disadvantages of a carbon film resistor include the following.

  • It has a High NTC (negative temperature coefficient) of resistance. So its resistance will be changed very easily when the temperature changes. Once temperature increases then the resistance will be decreased.
  • It has less capacity as compared to the same range of resistance.
  • Based on the resistance value, they have a temperature coefficient resistance range.

Applications

The applications of a carbon film resistor include the following.

  • They are applicable where high voltage & high temperature is required.
  • These resistors are used in shunting, current limiting, voltage division, step-down, resistance matching, load & capacitor matching filters.
  • The operating voltage is upto 15kV at 350oC temperatures.
  • These are used in X-rays, high power supplies, laser & RADAR systems.
  • Carbon film resistors are used in electrical & electronic circuits to restrict the flow of current.

Thus, this is all about an overview of the carbon film resistors and it’s working. These resistors are fixed resistors that use a carbon film for resistive operation. Here is a question for you, where can we buy carbon film resistors?

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