When I read about Thevenin equivalent, it is mentioned it only applies to "linear circuits".
But I also see in texts and tutorials showing how to measure the output impedance of an actual device which is complex such as a power supply or a transducer. They measure the output impedance and equate the device to a single power/signal source with a single output resistance/impedance.
But these devices are composed of non-linear circuits. And the whole idea of Thevenin applies to linear circuits.
How come we can employ Thevenin in these devices conceptually? Is a power supply or a transducer a linear circuit?