External electric load
If an electric circuit has a well-defined output terminal, the circuit connected to this terminal is the load.
Load affects the performance of circuits that output
voltages or
currents, such as sensors,
voltage sources, and amplifiers. A household's
power outlets provide an easy example: they are a voltage source, outputting 120
V AC, with the household's appliances collectively making up the load. When a power-hungry appliance switches on, it dramatically reduces the load
impedance, causing the output voltage to drop.
Encyclopedia
If an electric circuit has a well-defined output terminal, the circuit connected to this terminal is the
load.
Load affects the performance of circuits that output
voltages or
currents, such as sensors,
voltage sources, and amplifiers. A household's
power outlets provide an easy example: they are a voltage source, outputting 120
V AC, with the household's appliances collectively making up the load. When a power-hungry appliance switches on, it dramatically reduces the load
impedance, causing the output voltage to drop. This drop is easily observed; for instance, turning on a
vacuum cleaner dims the lights.
A more technical approach
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 Thèvenin equivalent of a circuit looks like this:
With no load , 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:
References
See also
