More Than You Ever Wanted to Know About Electrical Engineering: Circuit Loading
Here’s another neat trick you can do with Thévenin equivalents. Suppose we have a generic circuit, represented by its Thévenin equivalent, and we’re attaching a load to it.
What voltage does our load see here? This circuit is just a basic voltage divider, so we can find the answer pretty easily.
Take a look at that for a minute, and think about what happens if the circuit’s Thévenin resistance is large compared to the resistance of the load. You’ll get a much smaller voltage across the load. In other words, a circuit with a large output impedance makes a lousy power supply, since the voltage it can provide drops when you put a load on it.
When this happens, it’s called circuit loading, and it’s a phenomenon with powerful and far-reaching consequences. Real power supplies (and other devices required to power others) often have output resistances measured in milliOhms. Conversely, devices which are designed for measurement (like a scope or a mulitmeter) or designed to accept power from another device are made with very high input resistances to reduce the chance of their loading down the source.