Electrical load

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 ${\displaystyle V_{S}}$  falls across the output; the output voltage is ${\displaystyle V_{S}}$ . 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 ${\displaystyle R_{S}}$ , so the voltage at the output terminal is no longer ${\displaystyle V_{S}}$ . The output voltage can be determined by the voltage division rule:

${\displaystyle V_{OUT}=V_{S}\cdot {\frac {R_{L}}{R_{L}+R_{S}}}}$ 

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.

• Dummy load