Calculating the resistor's power dissipation

General-purpose resistors like the ones we're using in our breadboards are commonly rated to be 1/8 Watt, 1/4 Watt, or 1/2 Watt. If you supply too much power to a resistor, it will burn out with a puff of smoke and give off a horrible smell.

Here is how we calculate the power dissipation of our 200Ω resistor when we have a 3.3-volt power source:

1. The power dissipated by a resistor can be calculated with the following formula. Note that the voltage V is the voltage drop across the resistor in volts, while R is the resistance in Ohms:

2. Therefore, when we substitute our resistor's voltage drop and resistance value in the formula, we get the following:

= 0.0072 Watts, or 7.2 milliwatts (or mW)

3. Our power value of 7.2 mW is below even a 0.25 Watt-rated resistor, so a 1/8 Watt or above resistor is safe in our circuits and will not burn out in a puff of smoke.

If you think the power equation looks different...