In electrical or electronic circuits, one of the most commonly used “passive devices” is a resistor. Because of its wide range of applications, they exist in different varieties. Depending on the type of resistance they provide, these devices are usually classified as fixed or variable resistors. However, whether the resistance is fixed or variable, the structure is different. So we named them according to their structure type. Carbon film resistors are one type of resistance.
What is a Carbon Film Resistor?
It is clear from the name “carbon film resistor” itself that this type of resistor is made of carbon film. The carbon film is deposited on the ceramic precursor. This is actually a fixed type of resistor, which means that it only provides a resistance value. The function of the carbon film is to limit the current flow in the circuit. We can define carbon film resistors as: Carbon film resistor is a fixed resistor. It uses a carbon film deposited on a ceramic precursor to limit the flow of current.
The accuracy of the resistance value provided by this type of resistor is due to the spiral cut that is usually made into a thin film. This and other architectural features are discussed in the “Construction” section of this article.
Now this resistor has a special symbol to indicate its schematic diagram, you might think. In fact, this symbol is the same as the symbol for resistors, zigzag lines, or rectangular boxes (IEC standard).
When using carbon film resistors in the circuit, we use “CR” to indicate. For example, if we used a 120 KΩ carbon film resistor, then we would write it as CR120kΩ to represent the same situation.
Carbon film resistors are built or manufactured using a deposition process. Its ceramic substrate is a carbon film. This carbon film restricts flow and is therefore an essential part of this resistance. It is for this reason that the resistor is named “carbon film resistor”. A hydrocarbon, such as methane or benzene, cracks at a high temperature of 1,000 degrees Celsius, where the ceramic carrier is broken. Since pure graphite (carbon) is used for distribution on the ceramic substrate without being combined with it, the noise generated by the carbon film resistor is very low. The role of the ceramic substrate is the heat or electricity of the insulator. Therefore, its existence enables this resistor to withstand high temperatures without causing great damage.
The Structure of Carbon Film Resistors
The two end caps shown in the figure are actually metal, and two connecting wires made of copper are connected to the two ends of these metal caps. We observe that the carbon film is spiral. It helps to obtain an accurate resistance from this resistance. This helps increase the effective length of the resistor while reducing the width of the resistor. The resistance value is adjusted by increasing or decreasing the length of the spiral path. In order to protect the carbon film, it has an epoxy coating.
So How Does the Carbon Film Help Limit the Flow of Electrons?
This actually depends on the width of the carbon film layer. For higher resistance values, choose a thin layer. When the width becomes smaller, the space for free electrons to move becomes smaller. Likewise, for low resistance values, thick layers are chosen. Since the greater the width, the greater the space for free electrons to flow, so the current restriction is smaller than the restriction on the thin layer. Due to the use of pure carbon, the resistor has a high negative temperature coefficient, in the range of 2.5×10^-4Ω/°C and -8×10^-4Ω/°C.
Advantages and Disadvantages of Carbon Film Resistors
Compare carbon film resistors with carbon component resistors. The former has the advantage of low noise. Compared with carbon composition resistors, it has lower resistance.
It is easy to pocket because it is cheap cost.
It has a wide operating range, so it is widely used.
However, its high negative resistance coefficient makes the resistance easily change with temperature. An increase in temperature will reduce the resistance of the resistor, which is definitely a huge disadvantage.
Application
This resistor can withstand high temperature and high pressure. They have found applications where these conditions need to be met. Its operating temperature can reach 350°C and the voltage is 15 kV. Carbon film resistors are mostly used in high temperature and high pressure applications. It is widely used in high-power power supplies, X-rays, radar systems and lasers.
Summarize
The carbon film resistor is a fixed resistor in which the carbon film is used for its resistance operation. Here, the deposition process is used to build and carbon film, usually cut into a spiral, is deposited on a ceramic substrate. This substrate acts as an insulator, helping to withstand resistance at high temperatures and high voltages. The width of the carbon film determines the value of the resistance. If the width is small, that is, a thin carbon film is used, the resistor provides a higher resistance value. Similarly, if a thick carbon film is used, the resistance value of the resistor will be very low. In addition, the spiral shape of the carbon film can be adjusted to change the effective length of the carbon film according to the resistance value. The effective length is proportional to the resistance. Its negative temperature coefficient is very high. This is also the main disadvantage of this type of resistor, because its structure uses pure carbon.