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I think it does look like a paradox. It one were doing a Laplace analysis with pencil and paper, I(L1)? = 10 A would definitely be an 'initial condition'. But I did conclude that it isn't in LTspice.

On Mon, Feb 24, 2025 at 09:11 AM, John Woodgate wrote:

On the face of it, setting IC of L1 = 10, but also specifying UIC is paradoxical.

That is actually normal for SPICE syntax.? Specifying UIC tells SPICE to definitely use that IC setting.? No paradox.

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There may be some difference in the details, between how different SPICE programs handle it.? But I think it is basically like this:? Specifying initial conditions (either IC= or .IC) works differently depending on whether UIC is also used.? Without UIC, SPICE applies the initial conditions, lets the circuit converge, then it might remove the enforcing conditions and lets it converge again.? Therefore, whether or not it does the second step, it starts with a self-consistent set of voltages and currents.? Whereas, when you add UIC, SPICE omits trying to converge.? It just accepts the initial conditions you specify (including 0 for any not specified), and accepts them as is.? ?No attempt to solve the network equations with those conditions.? Therefore it is almost guaranteed to "burp" when the transient simulation begins.? You got what you asked for, even if it is garbage.

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I can't guarantee that what I described above happens in all SPICE programs, but this was my understanding, from decades ago.

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I suppose IC = 10 doesn't count as an 'initial condition' despite the name.

It does count as an "initial condition".? But it needs to be taken with care.

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Andy

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