Imagine you have to design an intermediate stage of an existing circuit because it requires a new functionality. How do you model the rest of the circuit that yours will feed into?
For instance, if you need to create a protection circuit between the power supply and the rest of the circuit which draws 10mA @ 10V, how do you model the rest of the circuit? Do you just throw in a 1k resistor?
What if your load circuit has inductance and capacitance, how do you model that in your analysis?
If you are intending to model something, what that means is that you are specifying which parts of its behaviour are important to you, and which parts you can ignore.
If you want to model a load, especially if that load is to test a protection circuit that you're designing, then that model has to be able to die as easily as the real load, if the protection is for the load.
For instance, a pure resistor will double current consumption when the supply voltage doubles. If instead the load is perhaps LEDs with an 8V forward voltage, with 2v being dropped across a 200 ohm resistor, then the current consumption will increase by a factor of 6 for a doubling of input voltage. If the protection circuit has to protect against a voltage surge from the power supply, the difference is very significant.
If the protection circuit is to protect the power supply from the load, then the load model has to be able to generate the problems that the real load will throw at the supply.
For instance, if your 10V 10mA load is a relay coil, then you at least have to include the inductance in the model, which stores energy, and throws a high voltage back at the supply when it tries to stop the current flowing. Or if the load has a big electrolytic across the supply, this will draw an inrush current when the supply goes on, and will hold the voltage up when the supply goes off, both of which could damage the supply, so needs to be included.
Quite apart from modelling the load, designing a protection circuit is not possible until you've specified what the threat is. Are you intending to protect against the user mis-using it, or the result of any single component failure, or multiple component failures, or reverse battery connection, or lightning strike? Specify the protection level, and know your load, before you start.
