Capacitors, inductors and TR

c and l take the same four fields as a resistor, plus an optional fifth — the initial voltage on a capacitor, the initial current in an inductor.

c1,2,0,10'u        no initial charge
c1,2,0,10'u,5      starting at 5 V
l1,2,0,2'm,0.3     starting at 300 mA

The fifth field is read by TR and FD only. DC and AC ignore it, which is correct: in DC a capacitor is an open circuit and the initial condition has nothing to say.

TR

Choose TR and the answers come back as functions of t.

e,1,0,10
r,1,2,1'k
c,2,0,1'u

The familiar step response, τ = RC = 1 ms, and derived rather than recalled. Answers are valid for t ≥ 0.

Give the capacitor a starting voltage and only the coefficient changes:

e,1,0,10
r,1,2,1'k
c,2,0,1'u,4

It starts at 4 and climbs to 10, so the swing is 6 rather than 10.

No source at all

A natural response needs no source — the energy is already in the element:

r,1,0,1'k
c,1,0,1'u,10

Second-order circuits are no different: add an inductor and the answer comes back over-, critically or under-damped as the values dictate. You do not select the case; the algebra does.

Switched circuits, in two runs

There is no switch element and no t < 0. A switched problem is two runs, and this is the whole method:

1. Run the t < 0 circuit in DC. Read the capacitor voltages and inductor currents.
2. Write the t > 0 circuit, putting those numbers in the fifth field.
3. Run it in TR.

That is the same thing you would do by hand, and step 1 is where the initial conditions come from rather than being assumed.

Two things that make TR bearable

Limit the results. Every TR answer costs an inverse Laplace transform, so a circuit with a dozen elements pays for dozens of them. Ask for the one or two you want and the wait collapses. Limiting the results to save time has it.

Plot it. The Plot card takes Plot a function of time (TR), an answer name, and a time range. The s-domain, transfer functions and plots covers all four plot types.