Carbon Composition Resistor
is a device which is used to oppose or minimizing the current
flow in a circuit. The resistor
offers a voltage drop across it so that the voltage
across the load is reduced so that the voltage across the load is reduced and hence the current flow is reduced. The resistor can be made in many ways, we can make a resistor by using a wire which has high resistivity
. In order to make the resistance almost constant, we select the material which has the low temperature coefficient. But the main disadvantage of doing this is that the power loss is more and also the cost increases. Its size also increases and the resistor will not be economic. So in order to remove that disadvantage, we use carbon resistor
. These resistors have very small size as well as less power loss. Also, the resistor can have very high value up to a few mega Ohm.These are so small that we cannot even indicate the value. For that we use color codes.
Construction of Carbon Resistors
They are made of carbon clay and leads are constructed of tinned copper. It is enclosed in plastic case so that there will be no entry of moisture and other harmful particles in the resistor. They have diameters less than 10 mm. For higher power dissipation the size of the resistor should be increased. The shape of the resistor is of cylindrical type.
Color Code of Carbon Resistors
The color coding is provided since its size is too small and hence we cannot indicate the value of the resistor in the plate. The color code is as follows -
There are two types of color codes that we use -
- General Color Coding.
- Precision Color Coding.
General Color Coding
They are used if it has four bands and also they are used in the case of general purpose resistor. They have tolerance of ±5% or more.
To calculate resistance value, see the color of first digit and number it according to color code. Next, take the second color code and indicate the digit according to the color code. Third band is the multiplier band so put that the power of ten and multiply it with the previous number we got from the two digits, that will indicate the value and for tolerance see the fourth band and indicate the tolerance accordingly.
For example, if a resistor has a color code of Brown, Grey, Brown and Gold then the first digit we get is 1 as the brown indicates the digit 1.
Secondly the second band indicates the code to be 8. So the number we got from those two digits is 18.
Third band indicates the multiplier the multiplier so the multiplier is 10 as brown indicates 1.
Fourth band indicates the tolerance value, Gold has the tolerance of ± 5 %. So the resistance value we got is 180 ± 5 %.
Precision Color Coding
In this case we have five bands to indicate the resistor value. The first band is not black as in case of general purpose resistor. The only difference from the general resistors is that it has five bands and the first three will indicate the value, fourth one will indicate the multiplier and the last color indicates the tolerance value. They have the tolerance less than ± 2 %. So the color code is used accordingly.
Advantages of Carbon Resistors
- They are cheap.
- They have small size.
Disadvantages of Carbon Resistors
- They are noisy.
- The power dissipation capacity is small.
The carbon resistor is very widely used in electronic circuits and integrated circuits due to their small size. They can be used to make electronic circuits because the circuits will require a very small current so the carbon resistor can handle safely. They are very sensitive to temperature changes. Also the small power handling capacity is the main drawback of these types of resistors. Also, these types of resistor tend to produce noise. Due to these they reasons they cannot be used in all the electric circuits but due to their low cost and small size, they are very popular.