Basic Circuit Capture and Simulation

Adding elements in LTspice is fairly straightforward. Pressing r, l, c, d, or g will change your cursor into a resistor, inductor, capacitor, diode, or ground icon, respectively. You can rotate the cursor by pressing CTRL+r, and place the components with your trusty left mouse button. Right clicking or pressing esc will get you back to the default mode.

F2 brings up a menu of less common components, and I'll bring them up as they become useful. The first one you'll probably use is a voltage source. Bring up the component menu, and type "voltage" into the text box to select the component. Pressing ENTER will let you place as many voltage sources as you like.

Once the components are placed, right-click on the component icon to change its parameters. You can use SI prefixes like k, m, u, etc. as usual, except that you use MEG for mega.

To delete a component, first press the 'delete' key and your cursor will change into a pair of scissors. Clicking on a component then removes it. There are two different ways to move a component, called "drag" and "move." The difference is that "drag" pulls any wires connected to the component along with it, while "move" breaks the connections. Press 'F8' to drag components, and 'F7' to move them.

F3 lets you start running wires between components, which will come in handy. Holding down 'ctrl' lets you put diagonal wires while still snapping the endpoints to the grid. As in Pspice, you'll need a ground connection to be able to run a simulation.

Speaking of simulation, click on the little running man icon on top to bring up the simulation menu. For DC circuits you'll want to click on the "DC op pnt" tab and click OK. This will find out the steady state of your circuit. The first thing you'll see is a text list of the voltages at all the nodes and the currents through each component. It's not immediately obvious which node is which, so if you close the text window you can just let your mouse hover over a node or component and see the pertinent info on the status bar at the bottom.

A note about currents: Watch your signs! When you hover your mouse over a component, the current is taken to be positive if it is running down or right, and negative if it is running up or left.

Next time I'll go into parametric sweeps. Fun.