An Open-Source, Free Circuit Simulator

The original circuit simulation software, called the Simulation Program with Integrated Circuit Emphasis, or SPICE as it is more commonly known, was originally developed at the University of Califorina Berkeley in the 1970s with an open-source license. That’s the reason for the vast versions of SPICE available now decades after the original was released, not all of which are as open or free as we might like. Qucs is a GPL circuit simulator. And if you want the GUI option, you might want to try out QucsStudio, which uses Qucs under the hood, and is free to use, but binary-only.

(Editor’s note: the author was confused between the GPL open-source Qucs and the closed-source, binary-only QucsStudio. We’ve cleaned that up.)

QucsStudio supports a wide range of circuit components and models much in the same fashion as other more popular SPICE programs, including semiconductor devices, passive components, and digital logic gates. Qucs also utilizes SPICE-based simulation, which can model various types of circuit behavior, such as DC, AC, transient, and small-signal analysis.

Unfortunately there are only Windows versions available, and although some might have some success running it under WINE. There are plenty of other options for those of us running non-Windows operating systems though. Here’s a review of 30 of them.

Thanks to [Electroagenda] for the tip!

60 thoughts on “An Open-Source, Free Circuit Simulator

      1. uses an invalid security certificate.

        The certificate is not trusted because it is self-signed.


        [Accept the risk and continue… I did and nothing bad happined… yet.]

    1. It´s closed source (but you can´t really find out before you download, it´s not that explicitly mentioned on the website, and also it´s not obvious what it brings over Qucs (except “based on but not compatible with Qucs”)

      1. In the FAQ there is a section “What will the future of QucsStudio look like?”, and there it says “it will remain free but releasing the source code is not intended”.

        1. Call me cynical but every single “it will be free forever” claim sooner or later gets monetized somehow through ads or eventual licensing. Thanks but you can keep your closed source. I’ll find something else.

      2. “and also it´s not obvious what it brings over Qucs”

        QucsStudio does definitely have a better transmission line calculator, although all of those tools are completely separate so you can just lift the binary itself out of the package. Although neither of them are really as good as just using a proper 2D field solver like ATLC.

      1. Not when they are also the original author of QUCS.
        As far as I understand, they removed and rewrote theparts of QUCS not by them, so QUCS Studio only contains code by them.
        As it is their copyright, they can change the license.
        The GPL2.0+ QUCS is still out there, but the development seems rather dead.

    1. That is my experience. I have access to PSPICE with my company’s Cadence package, but I like LTspice better. It’s pretty much the standard tool that everybody uses.

      What’s cool about LTspice is that since it’s free, you can send out a circuit to anybody and they can run it, as opposed to tools like MATLAB or PSPICE, which only people with an expensive license can run.

      1. Plus, I found an app in that lets you import the .s1p file of the NanoVNA. I just scanned my HF antenna, and now I can design custom preselector filters for the Ham bands.
        Smaller than my ATU.

    2. Spice and its variants are primarily 𝒍𝒖𝒎𝒑𝒆𝒅-element, time-domain simulators. The article fails to point out that Qucs and Qucs Studio can also simulate in the frequency domain and also handle 𝒅𝒊𝒔𝒕𝒓𝒊𝒃𝒖𝒕𝒆𝒅 elements (using 2½D electromagnetics analytical models), such as edge-coupled microwave filters and even patch antennas, I think, in the case of Qucs Studio. Also, through the use of Analog Verilog, they can handle more than just small-signal analysis. With some care, Qucs Studio can even do the layout of a distributed filter in KiCAD! LTSpice has a cumbersome notion of transmission lines, but you would not be able to use it to design, say, a hairline filter. One can do microwave circuit design with Qucs or Qucs Studio. Qucs Studio runs fine on Ubuntu 22.04LTS under Wine, and I suspect this is true also of other Linux distributions. Qucs Studio seems to be better maintained than Qucs. Last time I checked, Qucs could not be installed on my Ubuntu 22.04LTS because of its obsolete qt dependencies. The only advantage I see Qucs has (if you can install it) over Qucs Studio is that it can be controlled externally from Python.

      1. Thank you for this synopsis, that’s really useful. I found Qucs-S slightly better presentation-wise, it has antialiasing, but QucsStudio better in most other respects. Do you have any links to PCB filter design in Qucs?

    3. Qucs is much more of an RF tool, like Keysight ADS. It has a ton of RF related features that LT spice is missing – realistic transmission lines, transmission line layout, 2.5D FEM, S-parameter simulation, touchstone file handling. I use it for a lot RF work after hours, and we even use it at work for some simpler stuff since Keysight ADS licenses are very very expensive.

    1. I’ve been learning it lately. Overall I like it, but can you really not measure current natively? As far as I can tell, that is not supported and you have to do some sort of “hack” like inserting a small resistor and computing current from the voltage drop.

  1. “That’s the reason for the vast versions of SPICE available now decades after the original was released, not all of which are as open or free as we might like.”

    Ah history you fickle beast.

    “The license originally included distribution restrictions for countries not considered friendly to the US, but the source code is currently covered by the BSD license. ”

  2. LTspice runs fine in Wine – at least it used to. Unfortunately I run Linux Mint Cinnamon now and it seems Wine is no longer a simple install option. All but one of my computers are ticking time bombs that will not be allowed to run Windows 11, ever. I seem to remember long ago QUCS used to run in Linux, not Windows. Now it runs as QucsStudio in Windows only? What is the back story with that?

    1. If you want you could check out Tiny 11. I just installed it on a core 2 duo (6400) with 8 gig of ddr2. Haven’t tried pushing with any apps as I just started testing it.

    2. QucsStudio runs happily under Wine. That is, until you use some element with time delay because then it hangs on some windows DLL which performs hyperbolic tangential (I think) but that can be easily fixed with replacement DLL.

      LTSpice also runs under Wine, even the new version.

      Funny sidenote – as Qucs is GPL, QucsStudio likely violates its license by not being open source. Also the reason why it’s Windows only could be that since it’s one man projecy, it might be easier to make a monolithic executable which always works instead of fiddling with constantly breaking dependencies.

      1. No, the original author of Qucs stopped being part of the project then created QucsStudio. He released the original as GPL, so being the original author, he is free to take his original code and make a new program.

    1. Accuracy has been an option in SPICE ever since. So play with the corresponding parameters (abstol, reltol, …) and see what you get.

      Maybe you actually meant convergence? This is often as well a question of picking the appropriate parameters.

      And when nothing helps you can always fallback to the backward euler integration (MAXORD=1).

  3. Hi everyone, with these circuit simulators, how do you tell if you’ve wired things incorrectly or have current flowing in the wrong directions or too much through a resistor that would result in melting or exploding components? Do you have to measure the components individually once the simulation is running and determine it yourself? Thanks.

    1. You can ask it to print out values at almost any point in the circuit. It’s up to you to measure whatever you need to figure out if your circuit is working. It only calculates what you tell it, so if you wanna tell if a circuit is wired correctly you have to make whatever measurements you need to verify.

      If you want something a little more visual/interactive you probably wanna play around with Falstad’s circuit simulator.

      1. Actually it calculates everything (because it has to) but only displays what you tell it to for transient simulations. For operating point simulations it shows you everything. And there’s a new kid on the block- QSPICE.. free but not open-source, from the author of LTspice.

        1. @Spehro Pefhany said: “And there’s a new kid on the block – QSPICE.. free but not open-source, from the author of LTspice.”

          Yes, Michael Thomas Engelhardt was the creator of LTspice back before Linear Technology Corporation (LT) was eaten alive by Analog Devices Incorporated (ADI) in 2016.[1][2][3][4] To be honest, I have been pleasently surprised by how well ADI has treated LTspice after buying-out Linear Technology Corp. From experience I never considered ADI to be an Engineer-friendly company. That feeling lives on today, especially after ADI simply bought-out most of their competition; Linear Technology Corporation [2016] and Maxim Integrated [2021].[3][5] At least QORVO (initially formed by the merger of Triquint Semiconductor and RF Mico Devices) survived the semiconductor mass consolidation brought on by low interest rates due to the U.S. Government’s greedy Quantitative Easing program (QE or Money Printing as it is commonly known).[6]

          * References:

          1. Mike Engelhardt

          2. Introduction of QSPICE Simulation Tool, video with Mike Engelhardt

          3. Linear Technology

          4. Analog Devices

          5. Maxim Integrated

          6. Qorvo

  4. There’s also microcap that’s available as a freeware and probably a bit better (also works with Wine). There’s also QUCS-s that’s really open source and available on Linux.
    And there’s Xyce from SNL too that’s open source, able to simulate very large circuits.

  5. Please double check things before publishing. QucsStudio is NOT open source.

    QucsStudio is closed source though it uses several open source programs and libraries in the background:

    The original Qucs project was released under the GNU General Public License (GPL). Does QucsStudio violate the GPL?
    No, the closed-source parts of QucsStudio are completely written and owned by the author. All programs that contain GPL code by other authors are published on this website (see download section).


    Qucs is free and open source:

    From the Qucs ReadMe:
    — README

    — Copyright (C) 2003, 2005, 2011 Stefan Jahn

    — This is free software; you can redistribute it and/or modify
    — it under the terms of the GNU General Public License as published by
    — the Free Software Foundation; either version 2, or (at your option)
    — any later version.

    1. The author explicitly says QucsStudio is a fork of qucs. How separate from original source code could be the additions of QucsStudio? Surely some modifications of the original code were done. Modification of a GPL software, if published, should be released under GPL as well.
      Not saying GPL is the way to go or that it is the best type of licensing in all cases, but the choice of licensing of a software should be respected.

      1. He was the original author and wrote most code. It’s quite conceivable most code was still his, and other parts were additions he didn’t like and therefore moved on with his own parts only.

        Things he modified himself and released under the GPL dont force him to publish new things *he* did under the GPL as well, since the base project is non-GPL code.

  6. The linked article say both Qucs-S and NgSpice engine was not updated, but infact are both open source and last release is past month and work very well. So we do not understand why hackaday sponsor a closed source app that say something not true. Please fix that

  7. Spice wasn’t the first. ECAP, the electronics circuit analysis program was available for the IBM 1620 in 1967, and on the IBM 360 around 1970. Reportedly Spice was derived from ecap

  8. I have used Qucsstudio for years and found it amazingly capable. The math section is full of many functions that you would ever need. The author was open to suggestions and he did incorporate a few that I submitted. It runs 100% perfect on Wine/ Linux, not one issue. I also use Qucs (open source) and did Qucs “packaging” for one of the Linux distros. But Qucsstudio did forge ahead, especially with capabilities such as E/M sim ( works great!), a more functional harmonic balance simulator and there are fewer issues with it compared to Qucs. Recall all those Jacobian Singularities? Aaack!
    I find it best to keep all of these in your toobox. LTSpice, QUCS-S and a raft of the others.

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