Transient Analysis

The transient analysis computes the transient output variables as a function of time over a user-specified time interval. The initial conditions are automatically determined by a dc analysis. All sources which are not time dependent (for example, power supplies) are set to their dc value. The transient time interval is specified on a .TRAN control line.

.TRAN <step_time> <stop_time> [start_time [maximum_step]] [UIC]



step_time is the nominal time increment used in the analysis, however the actual timestep is varied automatically by the simulator in order to achieve convergence.


Is the final time.


Is the initial time. A Transient analysis always begins at time zero. In the time interval between zero and start_time, the simulator analyzes the circuit but does not store the results.


The maximum_step limits the varying size of the timestep that the simulator can use when calculating the transient data; by default it chooses either step_time or (stop_time - start_time)/50, whichever is smaller. Typically step_time and maximum_step are set to the same value.


UIC (use initial conditions) is an optional keyword which indicates that the user does not want SPICE to solve for the quiescent operating point before beginning the transient analysis. If this keyword is specified, SPICE uses the values specified using IC=... on the various elements as the initial transient condition and proceeds with the analysis. If the .IC control line has been specified, then the node voltages on the .IC line are used to compute the initial conditions for the devices. An initial value of zero is assumed for a component which does not have the Initial Condition defined. Look at the description on the .IC control line for its interpretation when UIC is not specified.


An Operating Point analysis is automatically performed prior to a Transient analysis to determine the DC bias of the circuit, unless the UIC option is enabled.


.TRAN 1n 100n
.TRAN 100u 1m 0 10u
.TRAN 10n 1u UIC

See also