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Best Free Circuit Simulator January 21, 2006

I have been using LTspice (a.k.a. SwitcherCAD III) for about 3 years as my main circuit simulation tool and have been very happy with it. Linear Technology has generously made it available free on their web site, and it is full-featured, fast, and seems to converge very well. I’ll be posting some of my simulation tips and tricks, so I highly recommend that you download it and get familiar with it. They also have a great user’s group to help get you going.


1. Amit Sawant - February 22, 2006

very good

2. Nick Maris - May 9, 2006

I have also been using LTSpice (aka SwitcherCAD III) for some time and have been very pleased with it. The users group on Yahoo is especially helpful. If you need a good and FREE SPICE -based simulator it fits the bill!

3. tamilmohan - October 8, 2006

sir, i am tamilmohan from india. i saw your website greate. but connot downlaod the free simulation software. how do download in your software.

4. Brad - October 9, 2006


You should be able to get it from the Linear Technology web site. Follow the link in the original post. If that does not work, search for it on Google. Good luck!


5. C. Zarowski - November 9, 2006

I recently acquired SwitcherCAD III. My experiments with it have
mainly been successful except for the following problem.

The thing that is giving me trouble is that I am not able to create
a symbol for a schematic that can be used in other schematics.
The scad3.pdf manual has not helped me in solving this problem.
I believe I am following the instructions properly, and yet I am
not successful.

I create a schematic of the entity I wish to represent as a symbol.
I create a symbol. I create another schematic that uses the symbol.
But I get error messages such as

unknown subcircuit called in:
xu1 n001 0 seriesrlc.asc seriesrlc series rlc

when I attempt a simple simulation (e.g., operating point).

In this ‘example’ I tried to create a block that is a series RLC
circuit (just as a simple experiment in symbol creation).

Thanks for any help you can provide me in getting past this

6. Brad - November 9, 2006

Hi C. Zarowski,

I have been able to get the hierarchical symbols to work in the past, but I will have to revisit that and see how I did it. I will let you know. Meanwhile, you might want to try the official Yahoo Group for LTSPICE here:


The group has been very helpful to me in the past.



I was able to get it to work. I created a symbol called “test.asy” and a schematic for it called “test.asc”. I placed the symbol in another schematic and it was able to simulate. All files were in the same directory.

Since you got the error message

unknown subcircuit called in:
xu1 n001 0 seriesrlc.asc seriesrlc series rlc

I looked at my netlist (View – SPICE Netlist) and saw

XX1 N001 0 test

For some reason, it looks like your sub-circuit call had extra information.

7. CJZ - November 10, 2006

Does the order of construction matter ? That is, do you
create a symbol first and then create a schematic for it ?
(My experiments all began with a schematic first and then
creating a symbol.)

Maybe my problem is setting up attributes ?

I did have all of my files in the same directory so I guess that’s
not the problem.

8. C. Zarowski - November 10, 2006

Hello again.

I just tried changing the order of operations. That is, create
the symbol (without attributes) first. Then create the schematic
for it.

This seems to work.

I wonder if I should be surprised that the order matters.

In any case, it was your update that put me on to this

I therefore thank you.


9. Brad - November 10, 2006

Hi C,

I don’t think the order matters, but I will have to try it later. One thing I thought of is that I did not edit the symbol attributes at all - just left them all defaults (blank). Your symbol appears maybe to have attributes.


10. CJZ - November 11, 2006

Yes, I did originally put in attributes. But then I found a statement
at the end of the scad3.pdf manual section relevant to symbol
creation that said not to do this.

However, the revised ‘experiment’ was not successful until
I created the symbol first, and then the schematic for the
entity. I do not know if there is a ‘bug’ in the SwCADII
release I have, or if I did something else wrong that is more


11. Brad - November 11, 2006


I tried making the schematic first then the symbol and it worked. I wonder if someting about the name of the files matters like having a space in them??? Once again, I’ve got to run for now. Catch you later.


12. CJZ - November 12, 2006

Yesterday afternoon I tried schematic first and then symbol.
It worked.

I do not know why my first attempt at this ordering did not
work. Very strange. Maybe just a random ‘hiccup.’

Thanks again for your assistance.


13. Kevin - April 26, 2007

LTSpice is a very good analog simulator. If you are looking for a great mixed-mode simulator that is also free check out CircuitLogix at www.CircuitLogix.com


14. Charles Bert - October 28, 2007

Another good free circuit simulator is TopSpice Demo. This is a fully featured Berkeley Spice 3F analog simulator with a great PostProcessor name TopView. Node limit is 50 nodes.
TopView allows for mathematical operations in the plotted data, such as log, exp, SQRT, etc etc. Further the plot data expression may be complex using brackets and standard algebraic rules.
Up to four plot variable may be plotted on the same plot or in four separate windows, all using separate colors.
Subcircuits are allowed.
A subcircuit is described as

.subckt opamp IN+ IN- OUT

To insert into a circuit description,

X1 1 2 3 opamp

This is all standard Berkeley stuff.

Analyses include AC small signal, DC, DC stepped source, Transient, FFT.

Charles Bert