Electrical Load - A More Technical Approach

A More Technical Approach

(Two sidenotes on generality, for advanced readers: This discussion assumes circuits are linear. It uses simple resistances, but can be readily generalized to impedances for AC analysis.)

When discussing the effect of load on a circuit, it is helpful to disregard the circuit's actual design and consider only the Thévenin equivalent. (The Norton equivalent could be used instead, with the same results.) The Thévenin equivalent of a circuit looks like this:

With no load (open-circuited terminals), all of falls across the output; the output voltage is . However, the circuit will behave differently if a load is added. We would like to ignore the details of the load circuit, as we did for the power supply, and represent it as simply as possible. If we use an input resistance to represent the load, the complete circuit looks like this:

Whereas the voltage source by itself was an open circuit, adding the load makes a closed circuit and allows current to flow. This current places a voltage drop across, so the voltage at the output terminal is no longer . The output voltage can be determined by the voltage division rule:

If the source resistance is not negligibly small compared to the load impedance, the output voltage will fall.