On Tue, Apr 1, 2025 at 08:51 AM, Christopher Paul wrote:
It¡¯s unclear to me which of the four separate circuits you are referring to, or why you are suggesting a reconfiguration and what goal you are pursuing. All are configured as I intended. They are independent of one another except for sharing a power supply.
I think eT was getting confused about the fact that there were two current sources, which he thought both fed current into the string of diodes.? They don't, of course.? (Even if they did, they do not conflict or fight.? Current sources can be directly added without problem.)
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The op-amp+MOSFET current source controls the 100-times-larger LM317A current source, by controlling its voltage sensing pins (the voltage between its OUT and ADJ pins).? We don't know exactly why Chris has it arranged that way, but I'm guessing it is either to toggle the LM317's current source on and off, or to adjust its current level.
Although we still don¡¯t know, the simulation completes and runs (acceptably) slowly with Andy¡¯s suggestion of the alternate solver.
You can speed up the simulations.? With the Alternate solver, or the pull-down resistor, LTspice wastes a lot of time trying (and failing) to find the initial operating point.? By examining the results in the Error Log file, you can see which of its algorithms failed, and then add appropriate .OPTIONS commands to skip those algorithms and save time.? For example:
? ? .OPTIONS NoOpIter GminSteps=0 SrcSteps=0
makes it go straight to the Pseudo-Tran algorithm, which is the only one that succeeded with just the Alternate solver.? If I remember correctly, the Alternate Source Stepping algorithm succeeded after adding the pulldown resistor, so omit "SrcSteps=0" in that case.
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See LTspice's Help > LTspice > Dot Commands > .OP -- Find the DC Operating Point.? The information on that page applies to all simulations, not only the ones that have an ".OP" simulation command.
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I don't know what causes the first two or three algorithms to fail.? Sometimes that just happens and we need to live with it.? That is why LTspice has four algorithms, in case some of them fail.
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Andy
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