Help for absolute LT Spice beginner

Thread Starter


Joined Jul 24, 2022

This is my first time ever doing a simulation on LT Spice. After watching a couple of tutorials I decided to simulate a simple ZVS circuit or the astable RC multivibrator... However, I am not getting any signal probing the nodes, where the oscillations should appear... I guess this is some kind of very simple mistake to fix, but I can not find it on my own... Any advice? Thank you.1.JPG


Joined Feb 24, 2006
Real circuits oscillate because of small difference in the components. In a simulation you are (probably) using ideal components. The lack of small differences is why your circuit is not oscillating. A noise source or an initial condition might prove helpful. Alternatively look for examples of oscillators in LTspice for additional ideas.

I used slightly different capacitor values and I slowly ramped up the supply voltage.

Here is a zoomed in view of what happens after the supply reaches its target value of +12V.


Joined Feb 24, 2006
Initial conditions are implemented with the .ic spice directive. Here is the help file paragraph on this:

.IC -- Set Initial Conditions
The .ic directive allows initial conditions for transient analysis to be specified. Node voltages and inductor currents may be specified. A DC solution is performed using the initial conditions as constraints. Note that although inductors are normally treated as short circuits in the DC solution in other SPICE programs, if an initial current is specified, they are treated as infinite-impedance current sources in LTspice.
Syntax: .ic [V(<n1>)=<voltage>] [I(<inductor>)=<current>]
Example: .ic V(in)=2 V(out)=5 V(vc)=1.8 I(L1)=300m