Georg Simon Ohm has found the nature of electrically conductive materials to oppose the flow of electrical current through them and it is named as resistor. To indicate the resistor values or ratings an electronic colour code is used, and this is called Resistor Colour Code.
There are many different types of resistors available that can be used in electrical and electronic circuits to regulate current flow or in many different ways to produce a voltage drop. But the actual resistor must have some sort of “resistive” or “resistance” value in order to do this. Resistors are available from fractions of an Ohm to millions of Ohms in a range of different resistance values.
Clearly, it would be impossible to have available resistors of any possible value, for example, 1Ω, 2Ω, 3Ω, 4Ω etc, because literally tens of hundreds of thousands, if not tens of millions of different resistors would have to exist in order to cover all possible values. Alternatively, resistors are produced with their resistance value written in colour ed ink on their body in what are called “preferred values.”
The resistance value, tolerance, and wattage rating are usually printed as numbers or letters on the resistor body when the body of the resistors is large enough to read the print, such as large power resistors. But when the resistor is small, such as a 1/4 watt carbon or film form, it is necessary to show these requirements in some other way as the print would be too small to read.
Small resistors use coloured painted bands to indicate both their resistive value and tolerance with the resistor’s physical size indicating its wattage rating. Such coloured painted bands create an identification system generally known as a Resistors colour Code.
An global and universally accepted colour code scheme for resistors was created several years ago as a quick and easy way to identify an ohmic value for resistors regardless of their size or condition. This consists of a series of spectrally ordered individual coloured rings or bands representing each digit of the value of the resistors.
The markings of the resistor colour code are always read one band at a time from left to right, with the wider band of sensitivity oriented to the right side showing its tolerance. By comparing the colour of the first band with the corresponding number in the colour chart column below the first digit, the first digit of the resistance value is defined.
Again, we get the second digit of the resistance value and so on by matching the colour of the second band with its associated number in the colour chart’s digit column. Then the colour code of the resistor will be read from left to right as shown below:
The Standard Resistor Colour Code Chart-
The Resistor Colour Code Table-
How calculate Resistor Colour Code –
The Resistor Colour code system is all well and good, but to get the correct resistor value, we need to understand how to apply it. The band that is closest to a connecting lead with the colour coded bands being read from left to right as follows is the “left-hand” or the most significant coloured band:
Digit – Digit – Multiplier = Colour- Colour x 10 colour in Ohm’s (Ω)
Assume, a resistor has the following colour mark;
Yellow Violet Red = 4 7 2 = 4 7 x 102 = 4700Ω.
The 4th and 5th bands are used to determine the resistor’s percentage tolerance Resistor tolerance is a measure of the variance of the resistors from the specified resistive value and is a result of the manufacturing process and is expressed as a percentage of its “nominal” or desired value.
Typical film resistor tolerances range from 1% to 10%, whereas carbon resistors have tolerances of up to 20%. Resistors with tolerances below 2 percent are referred to as precision resistors with more costly and lower tolerance resistors.
Most of the five band resistors are precision resistors with either 1% or 2% tolerances, while most of the four band resistors have 5%, 10% and 20% tolerances. The colour code used to denote a resistor’s tolerance is as follows:
Brown = 1%
Red = 2%
Gold = 5%
Silver = 10 %
If the resistor does not have a fourth tolerance band, the default tolerance is 20%.
The British Standard (BS 1852) Code-
In general, the resistor colour code systems are not needed on larger power resistors because the resistor value tolerance and even the power (wattage) rating are printed on the actual body of the resistor instead of using the resistor colour code system. Because the position of a decimal point or comma is very easy to “misread,” particularly when the component is discoloured or dirty. An simpler system was developed to write and print the resistance values of the individual resistance.
This scheme conforms to the British Standard BS 1852 Standard and its update, BS EN 60062, the coding method in which the decimal point position is substituted by the suffix letters “K” for kilohms, the letter “M” for megaohms, both of which denote the multiplier value with the letter “R” used where the multiplier is equivalent to, or less than, any number which follows.
Resistor Tolerance, E-series & Preferred Values-
Hopefully we now understand that resistors come in a variety of sizes and values of resistance but have a resistor available with every possible value of resistance, literally hundreds of thousands, if not millions of individual resistors. Instead, in what are commonly known as Preferred values, resistors are made.
Instead of sequential resistance values from 1 Ω and upwards, there are certain resistor values within certain ranges of tolerance. A resistor’s tolerance is the total difference between its actual value and the necessary value and is typically expressed as a percentage value plus or minus. For example, a tolerance resistor of 1k within ±20 percent may have a maximum and minimum resistive value of:
Maximum Resistance Value
1kΩ or 1000Ω + 20% = 1,200Ω
Minimum Resistance Value
1kΩ or 1000Ω – 20% = 800Ω
Therefore, using our example above, a 1kΩ resistor tolerance of ±20 percent may have a maximum value of 1200 Ω and a minimum value of 800 Ω resulting in a difference of some 400Ω For the resistor of the same value
This broad 20% tolerance of the same resistor is usually not a concern in most electrical or electronic circuits, but when near tolerance resistors are specified for high-precision circuits such as filters, oscillators or amplifiers, etc., then the appropriate tolerance resistor should be used as a 20% tolerance resistor, which can typically be used to replace 2% or even 1%.
The resistor colour code of five and six bands resistor is more commonly associated with the high precision of 1 percent and 2 percent film types, while the common garden variety prefers to use the colour code of four bands resistor of 5 percent and 10 percent general purpose kinds. Resistors are present in a range of tolerances, but the E12 and E24 series are the two most common.
Resistor Colour Code exceptions-
Resistors produced in accordance with military specifications sometimes include an additional band indicating reliability. This is stated in the rate of failure (percentage) per 1000 service hours. In commercial electronics, this is rarely used. It is mostly possible to find the reliability band on four band resistors. Further reliability information can be found in the MIL-HDBK-199 US military handbook.
Single black band or zero-ohm resistor
A resistor is called a zero-ohm resistor with a single black band. It’s essentially a wire connection that only connects traces on a PCB. Using the resistor package has the advantage of being able to place components on a circuit board using the same automated machines.
5 bands of resistor with a 4th band of gold or silver
An exception is the use of five band resistors with a fourth band of gold or silver on modified and older resistors. The first two bands reflect the significant numbers, the third the multiply component, the fourth the resistance and the fifth the coefficient of temperature (ppm / K).
The gold and silver colours are often replaced with yellow and gray for high voltage resistors. This is to prevent the coating containing metal particles.
Also Read – Measurement of resistance