Let's try using LTspice - Dot Command Edition (7): Initial Condition Command (.ic)

Narrow down by specifying conditions

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This article is part of a series about the dot commands available in SPICE. This seventh installment explains the command (.ic) used to set initial conditions within LTspice.

By completing the final survey, you can download supplementary materials for the Initial Conditions (.ic) command found in LTspice Help. While the Help documentation is netlist-based, if you would like to obtain explanatory materials and sample circuits that perform schematic-based simulations, as in this article, please complete the survey.

Initial conditions (.ic command)

The `.ic` command is used to define initial conditions that can be used within LTspice. It allows you to set initial conditions for node voltages to which capacitors are connected and inductor currents. In particular, in transient analysis, as illustrated in the next chapter's example, if initial conditions are not specified, the analysis starts with capacitors in an open state and inductors in a short state. This can lead to unintended results, so it may be necessary to use the `.ic` command to set these conditions.

Syntax of initial conditions (.ic)

This section explains the netlist notation for initial conditions (.ic). The LTspice syntax is as follows: < > are required fields during configuration. [ ] are optional.

.ic [ V (<n1>) = <voltage> ] [ I (<inductor>) = <current> ]

Item	content
n1	Specify the node name
voltage	Node voltage settings
inductor	Specify the inductor.
current	Setting the inductor current

Example of using the initial conditions command

Here, we will create the following circuit to verify how to use the initial conditions (.ic) command.

We will create a circuit that constructs an RC circuit and checks the charging characteristics of the capacitors connected to nodes A and B when a voltage is applied to the capacitors. The power supply voltage is 20V supplied from V1.

Figure 1: Initial condition (.ic) command verification circuit

Setting initial conditions

Initial conditions are set using the .ic command.

Click on SPICE Directive and enter the following to set the initial conditions using the .ic command.

.ic V(a)=0

Set the simulation to start with the voltage at node A set to 0V (zero).

simulation result

The simulation results are as follows: After execution, the voltage V(a) at node A (green waveform) rises according to the RC time constant because the initial condition is set to 0V (zero). The voltage at node B is treated as open immediately after the simulation is run because no initial conditions are met, so the voltage at node B becomes 20V, which is equal to the set voltage of V1. Therefore, the simulation result shows that the voltage at node B is constant at 20V.

Figure 2: Simulation results

The Help documentation provided by Macnica explains the initial conditions using a circuit diagram as an example. If you would like to obtain the documentation and sample circuits, please cooperate by answering the survey.

Examples of setting initial conditions

1. When the voltage setting of the node to which the capacitor is connected is set.

2. When the initial current value of the inductor is set.

3. When the discharge characteristics of the capacitor are checked.

Download supplementary material for user-defined functions (.func) commands

You can download documentation and sample circuits explaining the initial conditions (.ic) command from the LTspice Help. After completing the survey, we will send you an email with a URL to download the supplementary materials.

Supplementary information for the initial conditions (.ic) command.

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