When prototyping or testing a device, engineers often want to measure the amount of power the device consumes. Especially considering trends in consumer electronics, understanding power consumption enables engineers to optimize circuit designs for size, heat transfer, and critical functionality.
A power measurement application requires one measurement device to capture the voltage across the terminals of a load, and the second device to capture the current going through the load. However, the actual power calculation depends on the resistive and reactive components (capacitors and/or inductors) in the circuit. The power dissipation in a purely resistive circuit is always a function of the voltage drop and current draw through the circuit.
Reactive circuits appear to function like resistive circuits because they produce voltage drops and draw current. However, reactive circuits actually store or return power. The reactive components (e.g. capacitors, inductors) cause a phase shift up to 90 degrees between the voltage and current waveforms which reduces the overlap between the two curves and effectively decreases the power delivered to the loads. This phenomenon is represented by three different power measurements: reactive power, apparent power, and real power. These three power measurements have a phase relationship that can be visualized in the power triangle, shown below.
Figure 1: Power Triangle