Resistors are simple electrical components found in virtually every electronic circuit. They limit current and divide voltage according to a simple relationship known as Ohm's Law. A resistor has two important values: resistance, measured in ohms, and power, rated in watts. Selecting a resistor is a two-stage process: first, you calculate the component's resistance and power values using Ohm's Law. You then choose a resistor based on the values you calculated.
Calculate the resistor's ohm resistance value by dividing the circuit's voltage by its current. Units of voltage and current are volts and amps, respectively. For example, if the voltage across the resistor is 9 volts and the current through the resistor is 72 milliamps, divide 9 by .072 to get a resistance of 125 ohms.
Calculate the resistor's power in watts by squaring the current and multiplying by the resistance. Continuing the example, .072 squared times 125 is 0.005184 times 125, or .648 watts.
Multiply the calculated power rating by 133 percent, or 1.33, to get a final power value. For example, multiply .648 watts times 1.33 to get .862 watts. The additional 33 percent follows good engineering practice by adding a safety factor to the original calculated value.
Find the resistance closest to your calculated ohm rating in a table of standard resistor value. The resistance you computed in the above example, 125 ohms, does not exist as a standard 5 percent resistor, but there are two equally close values, 120 ohms and 130 ohms. In this case, it makes no difference which one you choose, so to continue the example, choose the 120 ohm resistor.
Convert the ohm value into the standard resistor color code. The color code has three value bands followed by a tolerance band. The first two bands are the significant digits in the ohm value; the third band is a power-of-10 multiplier. For this example, 120 ohms, the color code is brown-red-brown. The first two bands, brown-red, signify the digits 1 and 2. The third band, brown, is 10 to the first power, or 10. Multiplying 12 by 10 gives you the resistor's value, 120 ohms. The fourth band, gold, signifies 5 percent tolerance. The resistor's color code is brown-red-brown-gold.
Select a standard power rating that is just higher than your calculated rating. Standard resistors come in ratings of one-eighth, one-quarter, one-half, one and two watts. In the example, you calculated a power of .862 watts. The next biggest standard resistor power rating is 1 watt. This will comfortably handle the expected .648 watts with a generous margin of safety.
- For high-power applications such as power supplies and amplifiers, you may need a resistor rated at 5, 10 or 20 watts. Such resistors are standard, although they do not have a color code. Instead, they have the ohm value and power rating printed on the body of the part.
- Resistor power values are maximum values. A 2-watt, 100-ohm resistor will work just as well electrically as a 1-watt, 100-ohm resistor, although the 2-watt part will cost more.
- Resistor value chart
- Comstock/Comstock/Getty Images