# Help for absolute LT Spice beginner

#### Vilius_Zalenas

Joined Jul 24, 2022
140
Hi,

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.

#### Papabravo

Joined Feb 24, 2006
20,392
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.

#### Papabravo

Joined Feb 24, 2006
20,392
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

#### Vilius_Zalenas

Joined Jul 24, 2022
140
Alright, I only decreased the value of one capacitor by 10 pF and now I am getting the oscillations... Thank you.

#### Audioguru again

Joined Oct 21, 2019
6,161
Vilius, I think your base resistors should have 10 times the resistance of the collector resistors. Yours are wrongly the same.

#### Audioguru again

Joined Oct 21, 2019
6,161
I looked in Google for Transistors Multivibrator and found many wrong ones posted on You Tube by school kids, and found this wrong one by Engineers. The base resistors have such high resistance that the transistors do not saturate.

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