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What is Carbon Resistor use for? What do you Mean by Color Code of Carbon Resistors?
In electrical experiments resistances starting from about 0.1 Ω to 1000 Ω are widely used. To manufacture these types of resistances, generally copper or any other conducting metal or alloys are used. On the other hand in the experiments of electronics, resistances less than 1000 Ω are seldom used. For these high resistances graphite and gas carbon are widely used nowadays. These are called carbon resistors.
If we observe the table of specific resistance of the conductors it will be found that the specific resistances of the allotropes of carbon are over a thousand times greater than those of metals. So carbon is very effective as an element of high resistance. Another advantage is that carbon resistors are cheaper than metal resistors.
To manufacture a carbon resistor, a cylindrical shell made of a bad conductor is taken. Its length is more or less 1 cm and diameter is within 2 mm to S mm [Fig.]. Conducting carbon powder is introduced in a controlled manner into the shell. The value of the resistance depends on the length and the diameter of the shell and also on the amount of carbon powder. Two conducting metal wires are taken out from either side of the shell along its axis through which the resistor is connected to the external circuit. The shell has four different coloured rings or bands A, B, C and D on its surface. The fourth band D is painted further off with respect to the bands A, B, C. The resistance of the resistor is obtained from the colour of these four bands according to an acknowledged code.
Colour Code of Carbon Resistors
According to this code ten single digits from 0 (zero) to 9 corre-spond to ten colours. The code is as follows:
The first two bands (A and B) represent the first two digits to ascertain their values in ohms. The third band (C) represents multiplier and the fourth band (D) tolerance.
If D is absent, it will indicate that tolerance of the resistor is 20%.
In some resistors a 5 -band or 6 -band coding is used. In these cases the third band represents the third digit. In that case, the fourth and fifth bands represent the multiplier and tolerance respectively. For a six-banded resistor, the sixth band denotes temperature coefficient.
The determining digits are shown against colour codes in the following table:
Mnemonics: The sequence of the colour code given above can be remembered by the following sentence.
B B ROY of Great Britain has a Very Good Wife wearing Gold Silver necklace.
Example:
Suppose the colour bands on a carbon resistor are in the sequence yellow, violet, red and silver.
A : Colour yellow ; so from the table the digit is 4.
B : Colour violet ; so from the table the digit is 7.
C : Colour red ; so from the table the digit is 2
i.e., the multiplier is 102.
So, the value of the resistance is 47 × 102 or 4700 Ω.
D : Colour silver ; so tolerance is 10%
Hence, the value of the resistance is 4700Ω± 10% i.e., lies between 4230 Ω and 5170 Ω.
However in electronics no special importance is given on the band D.