What are the advantages of carbon film resistors?

Carbon film resistor - Definition, construction, advantages

Carbon film resistor definition

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A carbon film resistor is a type of fixed resistor that uses a carbon film to restrict electric current to a certain level. These resistors are widely used in electronic circuits.

Carbon film resistor construction

A carbon film resistor is constructed by placing a carbon film or layer on a ceramic substrate. The carbon film acts as the resistive material, blocking some amount of electrical current. The ceramic substrate serves as an insulating material against heat and electricity, enabling these resistors to withstand higher temperatures without damage.

Metallic end caps are fitted at both ends of the resistive material, and copper leads are joined to these end caps. The resistive element of the carbon film resistor is coated with epoxy for protection.

Carbon film resistors are preferable over carbon composition resistors because they generate less noise. They also have a lower tolerance value. The tolerance of a resistor is the difference between the desired resistance value (e.g., 200Ω) and the actual manufactured resistance value (e.g., 202.3Ω). Carbon film resistors are available in ranges from 1Ω (one ohm) to 10MΩ (ten mega ohms).

In carbon film resistors, the desired resistance value can be achieved by either trimming the thickness of the carbon layer or by cutting the carbon metal in a helical manner along its length. This is usually done using lasers, with cutting stopped once the desired resistance value is reached.

High negative temperature coefficient of resistance

The rate at which the resistance of the material decreases with increasing temperature is called the negative temperature coefficient of resistance. Carbon film resistors have a high negative temperature coefficient, meaning their resistance decreases easily with an increase in temperature.

Dependence on carbon film layer and width of helical cut

Carbon film layer

The resistance of a carbon film resistor is inversely proportional to the thickness of the carbon film layer. A thicker carbon film provides more space for free electrons to move, reducing the likelihood of collisions with atoms and allowing a larger current to flow. Conversely, a thinner carbon film restricts the space available for free electrons, increasing collisions and thereby limiting the current.

Width of helical carbon film cut

The resistance of a carbon film resistor is inversely proportional to the width of the helical carbon film cut. A wider cut provides less resistance because the electrical current travels a shorter distance through the resistive path, resulting in fewer electron collisions. A narrower cut increases resistance by making the electrons travel a longer distance and increasing collisions.

Advantages and disadvantages of carbon film resistors

Advantages

• Generates less noise than carbon composition resistors
• Low tolerance
• Low cost
• Wide operating range

Disadvantages

• High negative temperature coefficient of resistance

Applications of carbon film resistors

• RADAR
• X-rays
• Power supplies

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