Electronics Components: Resistor Power Ratings
Resistors are like brakes for current flowing through an electronic circuit. Like the brakes in your car, resistors work by applying the electrical equivalent of friction to flowing current. This friction inhibits the flow of current by absorbing some of the current's energy and dissipating it in the form of heat. Whenever you use a resistor in a circuit, make sure that the resistor is capable of handling the heat.
The power rating of a resistor indicates how much power a resistor can handle before it becomes too hot and burns up. Power is measured in units called watts. The more watts a resistor can handle, the larger and more expensive the resistor is.
Most resistors are designed to handle 1/8 W or 1/4 W. You can also find resistors rated for 1/2 W or 1 W, but they're rarely needed in the hobbiest types of electronic projects.
Unfortunately, you can't tell a resistor's power rating just by looking at it. Unlike resistance and tolerance, there are no color codes for wattage. However, the size of the resistor is a good indicator of its power rating.
Power ratings are written on the packaging when you buy new resistors. After you work with them for a while, you'll come to quickly recognize the size difference between resistors of different power ratings.
If you want to be safe, you can calculate the power demands required of a particular resistor in your circuits. First, use Ohm's law to calculate the voltage across the resistor and the current that will pass through the resistor.
For example, if a 100 Ω resistor will have 3 V across it, you can calculate that 30 mA of current will flow through the resistor by dividing the voltage by the resistance (3 V ÷ 100 Ω = 0.03 A, which is 30 mA).
Once you know the voltage and the current, you can calculate the power that will be dissipated by the resistor by using the power formula:
P = I V
Thus, the power dissipated by the resistor will be just 0.09 W, well under the maximum that can be handled by a 1/4 W (0.25 W) resistor. (A 1/8 W resistor should be able to handle this amount of power too, but it's always better to err on the large side when it comes to power ratings.)