Carbon Resistors

Introduction

Otis Boykin invented an improved version of the electrical resistor and got its patent in 1959. The resistor allowed a precise electric current to flow through a component. He made this invention in 1959. Two years later, he invented an even better resistor. This new resistor could be manufactured easily at a considerably low cost. It could also withstand extreme temperature changes and shock. This was the carbon resistor.

A resistor is a two-terminal passive fundamental device used in all electronic devices. A resistor is a device that opposes the flow of current flowing through it. It also has other important uses, like adjusting signal levels and dividing voltages, among other services. Carbon resistor is the most common type of resistor used. Carbon resistors are made of some resistive material. It has metal caps at the two ends of it. It has a power dissipation limiting range from 1 watt to ⅛ watt. Its physical sizes vary according to the power dissipation limits.

Resistor

A resistor is found in almost all electrical circuits. It has two terminals and is a passive component (which can not produce energy). It is used to reduce the passage of electric current in a circuit to which it is connected in series.

Definition of Carbon Resistors

We can define carbon resistors as a particular type of resistors. It is made up of tiny carbon fragments. It is generally cylindrical. It is baked with lead terminals on both its ends. There are a variety of materials that can provide resistance. These are metals or alloys like platinum, brass, nichrome, and tungsten. But there is a disadvantage with these materials. These materials can not provide a very high resistance value without being very bulky, making them difficult to use. This drawback is overcome by carbon resistors which can provide high resistance without having a bigger size. This is possible because it has a high value of resistivity. As we can see in the following relation

Resistance ∝ {Length ×Resistivity}

Another great advantage of carbon resistors is that they provide a highly accurate value of resistance. This advantage makes it applicable to perform some important functions like comparing resistances. It is also used to compare different resistances. 

In fact, for most practical purposes, carbon resistors are preferred. 

Symbol of Carbon Resistor

The most common symbol used for carbon resistors in electronic and electrical circuits is a simple zig-zag line. Its value is indicated in ohms. Fixed resistors have only one value shown, whereas variable resistors have a range of values mentioned from zero to a maximum value.

Parts of a Carbon Resistor

The parts of carbon resistors are described below. Even though carbon resistors can have various constituent materials, it generally has below-listed constituents.

1. A core made of ceramic
2. Cap made of nickel 
3. The film is made of carbon
4. Lead terminals
5. A protective epoxy layer.

Manufacture of Carbon Resistors

The method used to manufacture carbon resistors is the deposition method. A carbon layer is placed on the resistor’s ceramic body. This is important because the carbon does not allow current to pass through it. In the procedure, hydrocarbons such as benzene and methane are shattered at a very high temperature of about 1000 degrees Celsius. A ceramic mould is added, which helps to insulate the resistor against high temperature or electricity. Pure graphite(carbon) is evenly distributed over the ceramic body.

The resistance of carbon resistors is determined by knowing the ratio of material filled to carbon. If the carbon composition is high, it is considered a good conductor, and its resistance value is less—the value of resistance changes in the presence of high voltages. The internal moisture is also necessary as the soldering heat of the resistor can create a non-reversible transform within the value of resistance. 

The resistors are relatively expensive but are still accessible. So, it is used in welding controls and power supplies. The range of resistance varies from one ohm to ten megaohms.

Working of Carbon Resistors

Carbon resistors are connected to the circuit to limit the electric current by a specific amount. The exact value of resistance must be induced in the circuit board. Pure carbon or graphite is deposited on the ceramic core. The carbon is deposited like a wire coiled on the ceramic core—the diameter. The Pitch and length of the carbon spiral vary as per resistance required. Both ends of the core are sealed with Nickel Caps so that the carbon can make excellent contact with the lead terminals. These metal caps also function as heat sinks. These caps carry away the resistor’s heat and prevent the carbon resistor from burning up.

Advantages of Carbon Resistors

This is because of the following reasons:

1. They are cheap to produce
2. They are compact
3. They can be printed onto circuit boards like computer processors in phones and tablets.
4. They can produce resistance reasonably well within a practical range.
5. Carbon, which is its primary ingredient, is abundantly available.
6. They can withstand very high-energy pulses. This means they do not get burned out when a high-energy electric current passes through them.
7. It can also tolerate temperatures of very high value up to 350 degrees celsius.

Disadvantages of Carbon Resistors

With every technology comes a few disadvantages along with it. The drawbacks of carbon resistors are as follows:

1. They have a significant value of the negative coefficient of resistance. Because of this, the resistance varies rapidly with the temperature change. In other words, the carbon resistor is very sensitive to temperature changes.
2. The value of resistance varies with ageing.

Uses of Carbon Resistors

The uses of carbon resistors are as given.

1. The carbon resistor can withstand very high energy impulses even with its relatively small size.
2. It can also tolerate temperatures of very high value up to 350 degrees celsius and voltage up to 15 kV. Therefore, it can be used for power supplies of X-ray, lasers, and radar systems. It is also used in the protection of circuits. It protects the circuit against surge or discharge. 
3. It is also used in sensitive measuring instruments such as medical defibrillators attached to the patient to protect them against high energy pulse. 
4. It is also used for welding.

Conclusion 

Carbon resistors have numerous advantages over their very few drawbacks. So, it is widely used in many critical conditions where it becomes tough to apply other kinds of resistors. Besides its simple and robust construction, it is also easy to manufacture.