The RJ Journal - Electronics

How do we know what value a resistor has? Of course, it can be measured with a multimeter. If the resistor is mounted on a circuit board a measurement can give the wrong reading depending on how it is connected to other resistors and components on the circuit board and in this case (and many other) it is more reliable to use the marking on the component. Resistors are marked, either directly with numbers, with an alpha numeric code or with a color code. If the resistor is big enough, the resistor value and sometimes also the tolerance is clearly written on the component body. It may be that the decimal point is replaced with a letter that indicates the multiplier of the value. Letters used are:

Letter	Multiplier
E or R	1
K	1000 (kilo)
M	1000000 (mega)

0.22Ω is marked as E22 or R22
4.7Ω is marked as 4E7 or 4R7
46kΩ is marked as 46K
3.3MΩ is marked as 3M3

It may also be that a 3 or 4 digit number without a letter is written on the component. In this case the first digits (2 or 3) holds the value and the last digit is the multiplier (number of zeroes to follow).

To complicate things further I have also seen combinations where both a letter and a multiplier has been used. In this case only the letter R which is a decimal point or a filler where no more digits are needed instead of putting a zero in the beginning.

When the component is very small and doesn't have room for a proper marking the manufacturer have sometimes chosen to put a 3 digit/letter code on the component instead of the resistance value. In this case you have to consult the manufacturer data sheet to decode the actual resistor value.

The axial leaded rod shaped resistors, such as metal film, metal oxide, carbon film and carbon composition shown above, almost always have the resistance value printed on them with color coded bands. Besides the resistor value, the tolerance and sometimes also the temperature coefficient is included in the code. There can be 4, 5 or 6 bands which are arranged as in the picture below.

The colors with their equivalent values are: Black=0, Brown=1, Red=2, Orange=3, Yellow=4, Green=5, Blue=6, Purple=7, Grey=8 and White=9. For tolerance and multiplier there are also Silver and Gold. The color band for the first digit of the resistor value is the band that are closest to the edge.

Available resistance values for resistors are divided into series of standardized values, so called E-series. The E-series specify the values for various tolerances. Today the most usual E-series are the E12, E24, E48, E96 and E192 series. The number after the "E" specifies the number of logarithmic steps per decade. For the E24 series the values in the decade between 100Ω and 1000Ω are 100, 110, 120, 130, 150, 160, 180, 200, 220, 240, 270, 300, 330, 360, 390, 430, 470, 510, 560, 620, 680, 750, 820, 910.

E12 specifies values for 10% tolerance, E24 for 5%, E48 for 2%, E96 for 1% and E192 for 0.5% and higher.

Resistors are specified with their resistor value, of course, type, maximum power dissipation, maximum voltage and operating temperature. Next chapter will explain power and power dissipation in more detail but first a word of caution about the specified maximum power dissipation for a resistor; The specified maximum rating is valid up to a certain temperature and at higher temperatures the power rating deteriorates linearly down to 0W when the resistors maximum operating temperature is reached. The maximum temperature at which maximum power dissipation is specified is normally around 70°C-80°C. Above this temperature the maximum power dissipation is deteriorated by a certain amount of power per degree of increased temperature. This has to be taken under consideration especially when the resistor is mounted in a closed, unventilated compartment and when the equipment in which the resistor is going to be used has to work in a high maximum ambient temperature (55°C for example). This means that the actual maximum power dissipation for the resistor is much lower than what it would appear at a first glance.