An ever-present challenge in electronic circuit design is selecting suitable components that not only perform their intended task but also will survive under foreseeable operating conditions. A big part of that process is making sure that your components will stay within their safe operating limits in terms of current, voltage, and power. Of those three, the "power" portion is often the most difficult (for both newcomers and experts) because the safe operating area can depend so strongly on the particulars of the situation.
In what follows, we'll introduce some of the basic concepts of power dissipation in electronic components, with an eye towards understanding how to select components for simple circuits with power limitations in mind.
— STARTING SIMPLE —
Let's begin with one of the simplest circuits imaginable: A battery hooked up to a single resistor:
Here, we have a single 9 V battery, and a single 100 Ω (100 Ohm) resistor, hooked up with wires to form a complete circuit.
Easy enough, right? But now a question: If you want to actually build this circuit, how "big" of a 100 Ω resistor do you need to use to make sure that it doesn't overheat? That is to say, can we just use a "regular" ¼ W resistor, like the one shown below, or do we need to go bigger?
To find out, we need to be able to calculate the amount of power that the resistor will dissipate. Here's the general rule for calculating power dissipation:
Power Rule: P = I × V If a current I flows through through a given element in your circuit, losing voltage V in the process, then the power dissipated by that circuit element is the product of that current and voltage: P = I × V.