LTSpice/SwitcherCAD question

[Speakerguy]

I've made a circuit that self-oscillates in LTSpice. How do I kick it off/get it going into oscillation? If I simulate it, it just sits in a stable DC state.

I have previously hooked up a sinusoidal voltage source through a 1Meg resistor to a random part of the ckt to get it going, but it isn't working this time and I can't figure out how to set up a starting voltage value on the timing cap.

Any help is appreciated!

 

[Roff]

Try running the attached file as is, and then with the .ic (initial condition) statement commented out.
You can also .ic an inductor current.

 

[Torben]

Another thing that's tripped me up a few times is that I just had the sim set to too short a time to see the oscillations start. If the sim isn't so complex that it would take too long to run, try using a longer sim time and see if the oscillations kick in at some time after what you can currently see.

That said I haven't tried what Ron said. I'll have to revisit some old circuits and see if that helps. I suspect it probably will.


Torben

 

[Roff]

I used to use dummy PWL current sources (or sinks) that were turned off after a short period of time. They usually work, but tend to make the schematic look kinda funny.

 

[Torben]

I used to use dummy PWL current sources (or sinks) that were turned off after a short period of time. They usually work, but tend to make the schematic look kinda funny.

I've often resorted to various kinds of jiggery-pokery to get things to work, then some time later stumbled upon the Right Way, usually by accident (like in a thread such as this).

Amazing how much easier and more reliable doing things the right way can be.


Torben

 

[Roff]

I've often resorted to various kinds of jiggery-pokery to get things to work, then some time later stumbled upon the Right Way, usually by accident (like in a thread such as this).

Amazing how much easier and more reliable doing things the right way can be.


Torben

Yeah, I think .ic comes close to the way the real world works. The trouble with spice is that it calculates the steady-state DC condition before it "turns things loose", unless you add one or more initial conditions. I think you are also supposed to add UIC (use initial conditions) to the end of your .Tran statement, but LTspice seems to work without it.
Once the steady-state is established, many oscillators won't start in the absence of noise, which of course is present in all semiconductors (and resistors), but not in spice.

 

[Torben]

Yeah, I think .ic comes close to the way the real world works. The trouble with spice is that it calculates the steady-state DC condition before it "turns things loose", unless you add one or more initial conditions. I think you are also supposed to add UIC (use initial conditions) to the end of your .Tran statement, but LTspice seems to work without it.
Once the steady-state is established, many oscillators won't start in the absence of noise, which of course is present in all semiconductors (and resistors), but not in spice.

OK, that makes sense. Thanks for the explanation!


Torben

 

[Speakerguy]

Muchos gracias. I knew there had to be some sort of initial condition thingamabober.

 

[Hero999]

Try running the attached file as is, and then with the .ic (initial condition) statement commented out.
You can also .ic an inductor current.

It seems you have some symbols and models that I don't have as I had the following errors:

"Couldn't find symbol(s): 2pol" when opening the file.
"Couldn't open library file TL072.SUB" when trying to simulate it.

 

[Roff]

It seems you have some symbols and models that I don't have as I had the following errors:

"Couldn't find symbol(s): 2pol" when opening the file.

It's 2pole, and it's a standard op amp macromodel that LTspice has had for ages. Unless you have moved them the symbol is in C:\Program Files\LTC\SwCADIII\lib\sym\Opamps\2pole.asy, and the subcircuit is found in C:\Program Files\LTC\SwCADIII\lib\sub\2pole.sub. I'll include them here just in case.

"Couldn't open library file TL072.SUB" when trying to simulate it.

Sorry. I added that to my library years ago. I forgot that it was not included in the LTspice download. I'll include it here.

 

[Speakerguy]

Hey Roff,

I just now got your user name. I am sooo dense.

 

[Roff]

Hey Roff,

I just now got your user name. I am sooo dense.

Yeah, I explained it once when I changed it, back several months ago. It actually was sort of an inside joke at Micron, where I worked. I spent a lot of time analyzing the output resistance of driver circuits. We called the output resistance Ron, pronounced are-on. One of the guys started calling me Roff, pronounced are-off.

 

[Speakerguy]

If I ever have two dogs, let's just say I have names picked out now If I only have one it will be Nikolay for a male and Tess (Tesla) for a female.

 

[Roff]

If I ever have two dogs, let's just say I have names picked out now If I only have one it will be Nikolay for a male and Tess (Tesla) for a female.

Oh, great. What an honor. I think.

 

[Speakerguy]

Hey, you're up there with Tesla man. Don't complain about the end use