07-12-2012, 04:57 PM
CAPACITOR COLOUR CODES
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GENERALLY, THE ACTUAL VALUES OF CAPACITANCE, VOLTAGE OR TOLERANCE ARE MARKED ONTO THE BODY OF THE CAPACITORS IN THE FORM OF ALPHANUMERIC CHARACTERS. HOWEVER, WHEN THE VALUE OF THE CAPACITANCE IS OF A DECIMAL VALUE PROBLEMS ARISE WITH THE MARKING OF A "DECIMAL POINT" AS IT COULD EASILY NOT BE NOTICED RESULTING IN A MISREADING OF THE ACTUAL VALUE. INSTEAD LETTERS SUCH AS P (PICO) OR N (NANO) ARE USED IN PLACE OF THE DECIMAL POINT TO IDENTIFY ITS POSITION AND THE WEIGHT OF THE NUMBER.
FOR EXAMPLE, A CAPACITOR CAN BE LABELLED AS, N47 = 0.47NF, 4N7 = 4.7NF OR 47N = 47NF AND SO ON. ALSO, SOMETIMES CAPACITORS ARE MARKED WITH THE CAPITAL LETTER K TO SIGNIFY A VALUE OF ONE THOUSAND PICO-FARADS, SO FOR EXAMPLE, A CAPACITOR WITH THE MARKINGS OF 100K WOULD BE 100 X 1000PF OR 100NF.
TO REDUCE THE CONFUSION REGARDING LETTERS, NUMBERS AND DECIMAL POINTS, AN INTERNATIONAL COLOUR CODING SCHEME WAS DEVELOPED MANY YEARS AGO AS A SIMPLE WAY OF IDENTIFYING CAPACITOR VALUES AND TOLERANCES. IT CONSISTS OF COLOURED BANDS (IN SPECTRAL ORDER) KNOWN COMMONLY AS THE CAPACITOR COLOUR CODE SYSTEM AND WHOSE MEANINGS ARE ILLUSTRATED BELOW
THE CAPACITOR COLOUR CODE SYSTEM WAS USED FOR MANY YEARS ON UNPOLARISED POLYESTER AND MICA MOULDED CAPACITORS. THIS SYSTEM OF COLOUR CODING IS NOW OBSOLETE BUT THERE ARE STILL MANY "OLD" CAPACITORS AROUND. NOWADAYS, SMALL CAPACITORS SUCH AS FILM OR DISK TYPES CONFORM TO THE BS1852 STANDARD AND ITS NEW REPLACEMENT, BS EN 60062, WERE THE COLOURS HAVE BEEN REPLACED BY A LETTER OR NUMBER CODED SYSTEM.
GENERALLY THE CODE CONSISTS OF 2 OR 3 NUMBERS AND AN OPTIONAL TOLERANCE LETTER CODE TO IDENTIFY THE TOLERANCE. WHERE A TWO NUMBER CODE IS USED THE VALUE OF THE CAPACITOR ONLY IS GIVEN IN PICOFARADS, FOR EXAMPLE, 47 = 47 PF AND 100 = 100PF ETC. A THREE LETTER CODE CONSISTS OF THE TWO VALUE DIGITS AND A MULTIPLIER MUCH LIKE THE RESISTOR COLOUR CODES IN THE RESISTORS SECTION.
Calculating Resistor Values
The Resistor Colour Code system is all well and good but we need to understand how to apply it in order to get the correct value of the resistor. The "left-hand" or the most significant coloured band is the band which is nearest to a connecting lead with the colour coded bands being read from left-to-right as follows;
Digit, Digit, Multiplier = Colour, Colour x 10 colour in Ohm's (Ω's)
For example, a resistor has the following coloured markings;
Yellow Violet Red = 4 7 2 = 4 7 x 102 = 4700Ω or 4k7.
The fourth and fifth bands are used to determine the percentage tolerance of the resistor. Resistor tolerance is a measure of the resistors variation from the specified resistive value and is a consequence of the manufacturing process and is expressed as a percentage of its "nominal" or preferred value.
Typical resistor tolerances for film resistors range from 1% to 10% while carbon resistors have tolerances up to 20%. Resistors with tolerances lower than 2% are called precision resistors with the or lower tolerance resistors being more expensive. Most five band resistors are precision resistors with tolerances of either 1% or 2% while most of the four band resistors have tolerances of 5%, 10% and 20%.
The British Standard (BS 1852) Code.
Generally on larger power resistors, the resistance value, tolerance, and even the power (wattage) rating are printed onto the actual body of the resistor instead of using the resistor colour code system. Because it is very easy to "misread" the position of a decimal point or comma especially when the component is mis-coloured or dirty. An easier system for writing and printing the resistance values of the individual resistance was developed.
This system conforms to the British Standard BS 1852 Standard and its replacement, BS EN 60062, coding method were the decimal point position is replaced by the suffix letters "K" for thousands or kilohms, the letter "M" for millions or megaohms both of which denotes the multiplier value with the letter "R" used where the multiplier is equal to, or less than one, with any number coming after these letters meaning it's equivalent to a decimal point.
Tolerances, E-series & Preferred Values.
We now know that resistors come in a variety of sizes and resistance values but to have a resistor available of every possible resistance value, literally hundreds of thousands, if not millions of individual resistors would need to exist. Instead, resistors are manufactured in what are commonly known as Preferred values.
Instead of sequential values of resistance from 1Ω and upwards, certain values of resistors exist within certain tolerance limits. The tolerance of a resistor is the maximum difference between its actual value and the required value and is generally expressed as a plus or minus percentage value. For example, a 1kΩ ±20% tolerance resistor may have a maximum and minimum value of.
Surface Mount Resistors
kΩ SMD Resistor
Surface Mount Resistors or SMD Resistors, are very small rectangular shaped metal oxide film resistor. They have a ceramic substrate body onto which is deposited a thick layer of metal oxide resistance. The resistive value of the resistor is controlled by increasing the desired thickness, length or type of deposited film being used and highly accurate low tolerance resistors, down to 0.1% can be produced. They also have metal terminals or caps at either end of the body which allows them to be soldered directly onto printed circuit boards.
Surface Mount Resistors are printed with either a 3 or 4-digit numerical code which is similar to that used on the more common axial type resistors to denote their resistive value. Standard SMD resistors are marked with a three-digit code, in which the first two digits represent the first two numbers of the resistance value with the third digit being the multiplier.