transient analysis

If the operating point is a freeze-frame and AC analysis is a frequency chart, transient analysis is the movie: it computes how every voltage and current actually changes over time, step by step, exactly what you would see on an oscilloscope. Switch on the power, feed in a square wave, and watch a capacitor charge up or an oscillator burst into life.

The command sets a stop time and an interval, like '.tran 0 10m', run from 0 to 10 milliseconds. SPICE marches forward in tiny time steps, at each one solving the full nonlinear circuit while honouring the real behaviour of capacitors and inductors (a capacitor's voltage cannot jump; an inductor's current cannot jump). This is the only analysis that captures large signals truthfully: clipping against the rails, slew-rate limiting, oscillator start-up, the ringing on a fast edge, switching-supply ripple. A capacitor charging through a resistor rises with time constant tau = R times C, reaching about 63 percent of the way in one tau.

Transient is the most general and trustworthy analysis, but also the slowest and the one most prone to convergence and accuracy problems. Choose the stop time long enough to see what you care about and let SPICE pick small enough steps to resolve the fastest edges; too coarse a step and a sharp spike is simply missed. It is the closest thing in software to plugging in a scope, with all the same need to set your timebase sensibly.