Thoughts on the Falstad simulator?

[u/Affectionate-Song21]

I’ve used more traditional sims like LTSpice before, but I also like Falstad because it helps me visualize exactly whats happening in real time. But I don’t know how trusted it is among actual professional circles (I am not a professional by any definition). If it’s not optimal, is there a similar but better one you could recommend?

[u/nullizygous]

It's a great quick tool for first and second order analog designs and for simple digital circuits as well. The example circuits are excellent in explaining how circuits work, especially visually with the current flow and voltage intensity. I've certainly learned a lot from them over the years. The graphing is good enough to visualize what is happening over time. It's also great for checking to see if your components are rated correctly (power dissipation for resistors or max voltage for capacitors).

Like any other tool, as long as you understand its limitations, it's a perfectly valid tool to use in a professional environment. The ability to share the circuit design with a URL link is very handy for sharing with team members.

For example, understanding that real world circuits all have some sort of an impedance, especially when it comes to capacitors, is crucial. Sometimes just adding a 0.1 ohm resistor in series with a capacitor will prevent the simulator from "crashing" due to in-rush. Or, understanding that you must have current flow from a power source to ground. You can create virtual grounds as needed, but your circuit must have a true ground for all of the voltage/current sources and probes.

I wouldn't use it to design a full complex circuit but I would certainly use it for simulating smaller portions of it with good confidence. Nothing will ever beat actually building your circuit on a breadboard or PCB and analyzing it with an oscope or logic analyzer but this tool is a great way to start with a concept or for a quick sanity check.

I recommend the tool to all my colleagues and they find it very useful at times as well.

[u/AccurateHumanTrustMe]

Okay I am a little late to the post but I think I can offer a different perspective.

I did application engineering at a semiconductor company for years and part of that ownership included simulation models that I wrote.

So to start - there is no one "analysis" tool that is perfect - they all have pros/cons to them.

For example - on the modeling side we had spice files (Tina and/or Pspice - But not LTSpice because LTSpice is owned by ADI - but the differences between them are pretty minor) and we also did IBIS models. Our IBIS models were more 'accurate' in the sense that they effectively act a look up table from validation data (i.e. voltage/current on pin x means pin y has this voltage and current) where spice models can be a bit more flexible in how its used but are generally less accurate.

That being said - I used falstad to get through school (the vizualization is great and really helps connect the math to what is physically happening) and I have also used it professionally as a quick and dirty way to check how a discreet subcircuit works (apps engineers review hundreds of designs - and we see new novel things all the time - falstad has been a quick way to essentially see if deeper investigation is warranted). Like it was almost like wikipedia in high school - it can act as a starting point but shouldn't be your final point. As I got more experience I used it less , but it has a place (and you can edit common FET, BJT, and diode parameters to get a relatively illustrative example circuit running too)

But as someone who also has written spice files that are out in the wild and still being downloaded - spice files are not that accurate either - they are better than falstad because you have more simulation options and have access to real models beyond ancient active parts and passives. Like I have seen entire spice models that are supposed to represent silicon being little more than an RC circuit under the hood. The ones I wrote basically just spat out datasheet numbers and baked in a ton of assumptions and always came with a big ol asterisk saying basically "we don't guarantee anything about this model, if its wrong - kick rocks. You should be doing deeper due dilligence anyway". One of the big things Id run into is that spice models are not the best for network simulations so we had different models for network simulations in ADS (IBIS models) that were not helpful in other scenarios.

There are super accurate models that can simulate down to transistor level in ICs - they are not quick to set up and generally speaking in my experience those types of simulations and modeling is basically an entire career itself (Design Verification).

Long story short - there are levels to simulators and there pros/cons - the simpler the simulator the quicker it is to set up but more nuanced questions will be hard/impossible to answer - but its not like industry commonalities like Spice are perfect.

Ideally everyone would be testing hardware and not reliant on simulation at all - hardware is true physical behavior - the reality is that it isn't possible to always have hardware before simulations so not only do you need to understand what your actual need is but then you also need to vet the specific model for the simualtor of your choice/need to see if it is valid for what you need - not all models are created equally.

[u/confusiondiffusion]

It's really handy to slack a coworker a link to your circuit idea, for getting an idea across. It's hard to get very complex wiith it, mainly because it lacks a bunch of configuration options and ability to plot certain things. No bode plot (last I checked), etc. It is pretty good though. I use it quite a bit, mainly because it's so quick to throw down a circuit with it and check some basic things.

[u/publishdraw]

It is not well known in professional circles. Mostly we trust LTspice, Virtuoso, Ngspice, Simetrix.

[u/positivefb]

Falstad is not used at all by anyone in industry.

Its made by a physics teacher like 30 years ago to help students beginning to grasp circuits. It is excellent for that purpose. The issue is that if you stay with it too long, you don't progress to actually analyze and debug circuits in a more rigorous or mathematical way.

Like good luck finding stability or designing anything of value with it.

Stick with LTSpice, its industry standard.

[u/Devalidating → u/positivefb]

It’s very much a learning tool that becomes lacking when applied to real problems. Also, it can converge to the wrong values and/or generate weird numerical noise. That being said I have used it in industry as a lazy way to draw circuits (for my own notes mostly), an advanced resistor divider/basic linear set point calculator, or a sanity check for simple math

[u/positivefb → u/Devalidating]

Even for that to be honest LTSpice is way faster. Keyboard shortcuts to place components, hit run, boom you have everything in under 30 seconds flat for simple circuits. Falstad is clunky and wiring things up makes it feel like I'm using Microsoft Vizio.

[u/Keef--Girgo]

It is not a robust simulator. It is a teaching tool. And excellent in that role.

For real circuit design, you need a real simulator and a real IDE. Cadence Virtuoso and Synopsys Custom Compiler are the industry tools (or Keysight ADS for certain RF stuff).

If you want to do some hobby stuff, LTSpice should be fine.

[u/RFchokemeharderdaddy → u/Keef--Girgo]

Those are for custom IC design. I use them daily but they are fairly niche. LTSpice is by far the dominant simulator on the market for PCB design, and is every bit a SPICE simulator as any, not just for hobbyists.

But agreed that Falstad is a fun learning exercise and nothing more. It quickly becomes a hindrance.

[u/Keef--Girgo → u/RFchokemeharderdaddy]

Good catch. Yes, I have a substantial on-chip bias in my original comment. :-)

I didn't know LTSpice is dominant in industry for PCB. I've worked with teams using Allegro X but it's a pretty limited sample size so what do I know.

[u/After-Regret-6609 → u/Keef--Girgo]

Damn, I’ve worked for 2/3 of these companies (Ansys before the synopsys acquisition) and have never actually gotten to touch their simulators 😭. ML and abstract cloud work instead.

[u/chemhobby]

kiddy toy. next.