Electronic – Need help with 2 Transistor Current Limiter Circuit in LTSpice

bjtcurrentmosfetspice

I am new to LTSpice. I have attached a picture of my circuit below:

The goal of the circuit is to provide current to the R1 until It is too high. Then the BJT will activate, shutting the FET off.

For some reason, the simulation is not able to finish the entire sweep. Can anyone help me with why?

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Best Answer

First off, don't do so many steps on the first pass, 100 steps is overkill. After 61mA this circuit no longer works, and the simulator can't easily find the DC operating point because it has to steadily increase the voltage in I1 becasue the mosfet doesn't want to turn on (like TimWescott said).

If you want to see this for yourself, make it easier for spice to simulate and put a 1e6 resistor across M1 and it will become apparent what is happening. The node between I1 and R3 will reach kV or MegV. Spice's solver has a hard time when simulating very large values and very small circuit values at the same time. The results aren't physical anyway, the components will burn out at after M1 breaksdown, and if you were testing this circuit in real life, I don't think you'd test with a current source that works the the kV range.

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