Resistors and the Colour Code 

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Resistors  

These are components made to provide resistance in circuits. They are made in various resistance values, physical sizes, and can be of fixed or variable value.  

The circuit symbols used for different types of resistors are shown in the diagram The symbols at A are those used for fixed value resistors, at B are those for variable resistors, at C those for preset resistors or trimpots, and at D a variable resistor used as a potentiometer, with provision for tapping off part of the voltage across the resistor, and being able to vary the tap. 

 

 

 

Resistor power ratings: 

Fixed resistors are available in a wide range of power ratings. The power rating of a resistor depends on its ability to dissipate heat and to keep its operating temperature below a value at which its resistance value is seriously affected. To do this, it must have a certain surface area. The larger the power rating, the larger the resistor. 

Usual ratings are 0.125, 0.25, 0.5, 1, and 2 watt in carbon resistors; above these levels, wire-wound or other types are used.   

 

Resistor markings: 

Sometimes the value of a resistor is printed on the component itself.  The letters "R" and "k" have a purpose here. Note in this table how these letters act as the decimal indicator: 

Circuit
Marking 

Resistance value

R05

0.05 ohms

1R5

1.5 ohms

15R

15 ohms

1k5

1500 ohms or 1.5 kilohms

15k

15 000 ohms or 15 kilohms

1M5

1.5 megohms

 

The “Resistor Colour Code”:  

Resistor values are usually marked on the resistor itself by using colour rings or bands. Because the range of resistor values used in electronics goes from fractions of an ohm to tens of megohms, it is impossible to manufacture resistors in every possible value, so resistors are produced in “preferred values”. Resistors are marked with their preferred value, and a tolerance value, which indicates the spread of the resistance value you can expect. 

 

  1st ring = 2nd ring = 3rd ring = 4th ring =
Colour  1st digit 2nd digit multiplier tolerance
  of value of value   %
None       ±20%
Silver      10-2 Ω = 0.01 Ω ±10%
Gold      10-1 Ω = 0.1 Ω ±5%
Black 0 0  100 Ω = 1.0 Ω  
Brown 1 1  101 Ω = 10 Ω ±1%
Red 2 2  102 Ω = 100 Ω ±2%
Orange 3 3  103 Ω = 1 kΩ  
Yellow 4 4  104 Ω = 10 kΩ  
Green 5 5  105 Ω = 100 kΩ ±0.5%
Blue 6 6  106 Ω = 1 MΩ  
Violet 7 7  107 Ω = 10 MΩ  
Grey 8 8  108 Ω = 100 MΩ  
White 9 9  109 Ω = 1000 MΩ  

 

The Table shows the colour code and the tolerance colours for the four ring colour code. Note that the third band, the "multiplier" value, gives the number of "zeros" that follow the first two values. 

Some equipment requires increased precision. To meet this need resistors with a precision of ±0.5% are required. For these types of resistor a five ring or band colour code is used to give a third digit value for more precise resistor values and tolerances. 

 

Remembering the Colour Code: 

The colouring starts with Black (which is Zero). The sequence is: 

0                1            2           3            4             5           6         7           8                9

Black, Brown, Red, Orange, Yellow, Green. Blue, Violet, Grey, and White. 

 Just remember the ditty:  

"Better  Be  Right  Or  Your  Great  Big  Venture  Goes  West"

Remember that it starts at Zero which is Black! 

An example: 

This figure below shows a fixed resistor with its colour rings. To read the colour code, put the tolerance ring to your right and read off the colours as number values. The first two rings give a number between 10 and 100 and the third ring is the power of 10 or multiplier needed for the final value. The fourth ring indicates the “tolerance”. 

For example, a resistor coded brown, red, yellow, silver is a 12 x 104 or 120 kilohm resistor, and the tolerance of 10% indicates that the value could lie between 108 kilohms and 132 kilohms. 

Gold and silver are also used as multipliers. Gold = multiply the value by l0–1 (multiply the value by 0.1) and silver = multiply the value by 10–2 (multiply the value by 0.01). 

 

Preferred ranges for resistors: 

The preferred ranges depend on the tolerance. 

For 10% resistors the preferred values are 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, and multiples of each value. For example, you can have a 10 ohm, a 10 kilohm, and a 10 megohm resistor value.  

For 5% resistors the preferred values are 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91, and multiples of each value. 

For 1% resistors the “Preferred values of Resistor” are 100, 102, 105, 107, 110, 113, 115, 118, 121, 124, 127, 130, 133, 137, 140, 143, 147, 150, 154, 158, 162, 165, 169, 174, 178, 182, 187, 191, 196, 200, 205, 210, 215, 221, 226, 232, 237, 243, 249, 255, 261, 267, 274, 280, 287, 294, 301, 309, 316, 324, 332, 340, 348, 357, 365, 374, 383, 392, 402, 412, 422, 432, 442, 453, 464, 475, 487, 499, 511, 523, 536, 549, 562, 576, 590, 604, 619, 634, 649, 665, 681, 698, 715, 732, 750, 768, 787, 806, 825, 845, 866, 887, 909, 931, 953, 976, and multiples of each value. For example, you can have a 16.5 ohm, a 165 kilohm, and a 16.5 megohm resistor value.

 

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