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:
Rs. With no load (open-circuited terminals), all of V_S falls across the output; the output voltage is V_S. However, the circuit will behave differently if a load is added. Therefore, 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. For example, 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 charge to flow. This current places a voltage drop across R_S, so the voltage at the output terminal is no longer V_S. The output voltage can be determined by the
voltage division rule: :V_{OUT} = V_S \cdot \frac{R_{L}}{R_{L} + R_S} If the source resistance is not negligibly small compared to the load impedance, the output voltage will fall. This illustration uses simple
resistances, but a similar discussion can be applied in
alternating current circuits using resistive, capacitive, and inductive elements. ==See also==