When the current through the load (blue) is greater than the set level, current through the 12Vrelay coil (green) stops. The simulation does not show the relay opening or closing the power supply switch (red).
The maximum current level is set by the trimmer across the base of the right hand side transistor.
In practice, the circuit needs a kicker momentary switch to bypass the relay and send power to the load to switch on the coil. The circuit will then run until the current reaches the threshold and switches it's own power off.
Can be used on DC motors that do not provide their own overload protection.
