A: (originally posted by Skylined)
Reading a Resistor:
The color bands on a resistor give its value and tolerance (accuracy). The vast majority of resistors you'll encounter will be 5% tolerance, which makes reading them easier. 5% tolerance resistors have a gold band on one end, usually separated from the other three bands. With the resistor oriented so that the gold band is on the right, read off the color bands from left to right according to the following chart.
The first two bands will indicate the value of the resistor to two significant figures. Multiply this value by the correct factor given in the multiplier band to find the total resistance. The actual resistance will be within 5% of this value. For example:
Yellow = 4
Violet = 7
Orange = 1,000
Gold = 5%
The value of the resistor is 47x1,000 or 47,000 ohms with a tolerance of 5%, so the actual value could be anywhere from 44650 to 49350 ohms.
E24 Standard Values:
Because a resistor's value is not exact (it can vary within the tolerance range), only certain discrete values of resistors are manufactured. For example, 102 ohm 5% tolerance resistors are not made because it is possible for a 100 ohm 5% tolerance resistor (an E24 standard value) to have the same resistance. Therefore, 5% resistors can be found only in the following values:
And any multiple of 10 thereof (such as 62 or 240000 ohms). If a more accurate resistance value is necessary, a variable resistor, resistors connected in series (resistances add in series), or a fixed resistor with a tighter tolerance (and therefore come in more standard values) may be used. Tighter tolerance resistors necessarily have more value bands. These are read normally. For example:
Red = 2
Red = 2
Blue = 6
Yellow = 10,000
Red = 2%
The value of the resistor is 226x10,000 or 2,260,000 ohms with a tolerance of 2%. Notice that the tolerance band is red, making proper orientation of the resistor for reading difficult. However, the tolerance band is usually set apart from the value and multiplier bands.
*Note for modders: when selecting a current limiting resistor for an LED, if the calculated value for the resistor is not a standard value, the next greater standard value should be used.
Different ways of writing resistances:
When reading schematics, you may encounter different systems for indicating resistances. In some places, the United States, for example, 470 ohms is usually written as 470, 2,200 ohms is written as 2.2K, 1,200,000 ohms is 1.2M. In other places, such as the UK, 470 ohms is written as 470R, 2,200 ohms is 2K2, and 1,200,000 ohms is 1M2, where the metric prefix replaces the decimal point.
The most commonly encountered resistors come in 1/8 (0.125) watt, 1/4 (.25) watt, and 1/2 (0.5) watt flavors. Which one is right for you? The power dissapated by a resistor can be calculated by the following equations:
P = (I^2)xR
P = (V^2)/R
Where P is the power dissipated by the resistor, measured in watts; I is the current through the resistor, measured in amps; V is the voltage across the resistor, measured in volts; and R is the resistance of the resistor, measured in ohms. Example:
A LED draws 20 milliamps at 3.7 volts. You want to run it off of the 12 volt rail with a 415 ohm resistor (the calculated current limiting resistor, value is not standard, but we will use it for the purposes of this example). What power resistor should you purchase?
Using the current, the power dissipated is 0.02 amps (current through the resistor) squared multiplied by 415 ohms, which is 0.166 watts.
Using the voltage, the power dissipated is 8.3 volts (voltage across the resistor, 12 volts - 3.7 volts) squared divided by 415 ohms, which is 0.166 watts.
Since 0.166 watts is more than 1/8 watt, a 1/4 watt resistor should be used.
x24 has created a downloadable program which will translate resistor values and calculate current limiting resistors for LEDs. It is available at:http://mywebpages.comcast.net/x24/prog_LEDcalc.htm