How to simulate thermal processes in LTSpice? - Page 1
Forum section: electronics / projects

author: MegaVolt
author_rank: Frequent Contributor
timestamp: April 28, 2023, 03:07:41 pm
content: Maybe you have seen instructions for modeling thermal processes in circuit simulators?This is when current = power, voltage = temperature, capacity = heat capacity, etc ....Something similar to the one shown in the figure, only in more detail and deployed.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 28, 2023, 03:20:44 pm
content: Not sure why you'd want to. Download the student edition of Quickfield-https://quickfield.com/

author: MegaVolt
author_rank: Frequent Contributor
timestamp: April 28, 2023, 04:08:41 pm
content: Quote from: Conrad Hoffman on April 28, 2023, 03:20:44 pmI would like to build a thermostat model.

author: IanB
author_rank: Super Contributor
timestamp: April 28, 2023, 04:13:25 pm
content: Quote from: MegaVolt on April 28, 2023, 03:07:41 pmDoing this is basically the same as using an analog computer to solve differential equations. You would need to convert your physical model to dimensionless form, and then map it onto an equivalent electrical circuit. You can do it, but it requires a bit of work. To succeed, you will need to have a good understanding of the physics involved, and how to create an appropriate model.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 28, 2023, 06:10:22 pm
content: Quote from: MegaVolt on April 28, 2023, 04:08:41 pmYou can use arbitrary voltage/current sources with expressions depending on other voltages/currents/parameters in your circuit, and time.With that you should be able to simulate a thermostat.

author: iMo
author_rank: Super Contributor
timestamp: April 29, 2023, 08:26:31 am
content: https://www.hackster.io/alainstas/ltspice-simulation-of-thermostat-with-transistors-and-ntc-4b50c1

author: MegaVolt
author_rank: Frequent Contributor
timestamp: April 29, 2023, 09:03:11 am
content: Quote from: iMo on April 29, 2023, 08:26:31 amYes. I'm looking for a tutorial that explains well how to build such circuits.Here's a little tutorial I found:

author: iMo
author_rank: Super Contributor
timestamp: April 29, 2023, 09:25:05 am
content: In this thread I made some experiments with simulating thermal processes with 399https://www.eevblog.com/forum/metrology/lm399-heat-loss-within-a-vacuum/msg4394029/#msg4394029https://www.eevblog.com/forum/metrology/lm399-heat-loss-within-a-vacuum/msg4387291/#msg4387291

author: MegaVolt
author_rank: Frequent Contributor
timestamp: April 29, 2023, 09:38:54 am
content: Quote from: iMo on April 29, 2023, 09:25:05 amThank you, I will definitely look into this topic.I'm surprised that there is no such instruction on paperHere's another instruction.P=I PowerQ=R thermal resistanceT=V themperatureCt=C Thermal Capacity

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 29, 2023, 09:44:00 am
content: Quote from: IanB on April 28, 2023, 04:13:25 pmIndeed, a better description of SPICE is a nonlinear numerical integration engine; it's dimensioned for circuits (amps, volts, seconds..) but you can rename a voltage or current as any complementary unit in any other problem: velocity and acceleration in mechanics, temperature and power in thermal; etc.So it's not an unusual practice to incorporate models in exactly this way. You occasionally see thermal models for transistors where there's an extra output pin corresponding to its power dissipation, which you connect to an RC circuit* which models temperatures of junction, case, heatsink and etc., as the case may be.*Thermal problems obey the heat equation, which only has real poles or fractional power solutions in the resulting transfer functions**. We can approximate the latter as the former for some desired degree of accuracy.**For linear materials, of course. Nonlinear materials could exhibit thermal shock waves... but I'm not aware of any system that exhibits such. It would have to have a negative tempco of thermal conductivity I think? Or maybe it's more mundane, like, chemical reaction fronts can move faster than heat transfer in the material, explosives for example; but nah, that's a nonlinearity of the material's heat capacity (you could model a kinetic reaction as a one-time discontinuity in heat capacity: that is, on heating the substance, its temperature starts accelerating then suddenly rises stepwise for zero additional external heat input), nothing to do with its heat transfer. Anyway, needless to say this is nothing you need to worry about in practice.Tim

author: iMo
author_rank: Super Contributor
timestamp: April 29, 2023, 09:55:09 am
content: In early 90ties a guy from russia or ukraine came to a CAD conference held in poland and presented a complete model of pneu/hydro/electro control of an aircraft they produced (and he said they used the model for actual design of the airplane), made in Spice (I saw it in person). In spice you may simulate almost everything provided you are able to write the transformation equations between "something" and R/L/C/I/V/time etc..

author: rstofer
author_rank: Super Contributor
timestamp: April 29, 2023, 03:06:42 pm
content: Out on the Internet there is a group of analog computer components for LTspice. These include the integrator and a summer, at least.https://www.edn.com/a-virtual-analog-computer-for-your-desktop/Attached is a Simulink model of the Mass-Spring-Damper problem which results in damped harmonic motion. The 1/s blocks are integrators and the one that integrates velocity has an initial condition of -1. That's where the program starts with an initial position of -1.I have done a similar thing with the LTspice components above. Analog computing is fun!I also attached a photo of the Mass-Spring-Damper problem on a Comdyna GP-6. The initial conditions and spring/damping factors are not the same as those I used with MATLAB/Simulink but the objective is the same. The patching is overly complex because there are two different projects patched for my entertainment. Figure about half that patching is actually required. Those 'gain' blocks (the triangles) in the Simulink model correspond to the potentiometers in the Comdyna version.

author: coppercone2
author_rank: Super Contributor
timestamp: April 29, 2023, 03:54:52 pm
content: I won't be happy till there is a thermal inductor

author: mawyatt
author_rank: Super Contributor
timestamp: April 29, 2023, 04:10:21 pm
content: We developed some thermal modeling with PSpice back in the late 80s, later published in EDN in 1994. This was originally done to help understand the thermal behavior of various power devices under complex waveforms, and proved useful in designing heat-sinking methods.ttps://www.edn.com/edn-access-08-18-94-spice-runs-thermal-analysi/Best,

author: MegaVolt
author_rank: Frequent Contributor
timestamp: April 29, 2023, 07:00:05 pm
content: Quote from: mawyatt on April 29, 2023, 04:10:21 pmThank you! Old EDN articles are losing pictures( But you provided the right keywords.

author: mawyatt
author_rank: Super Contributor
timestamp: April 29, 2023, 07:38:33 pm
content: Don't know how to dig up the old images of the stuff we were allowed to publish in EDN (most of our work was prohibited from publishing), we do have some paper copies of some articles but not this....sorryBest,

author: RoGeorge
author_rank: Super Contributor
timestamp: April 29, 2023, 07:39:31 pm
content: Quote from: coppercone2 on April 29, 2023, 03:54:52 pmHeat flowing from cold to hot without external intervention by using a “thermal inductor”https://doi.org/10.48550/arXiv.1804.06405Happy now?

author: RoGeorge
author_rank: Super Contributor
timestamp: April 29, 2023, 07:54:28 pm
content: Quote from: MegaVolt on April 29, 2023, 09:38:54 amSearch for 'analogy between electric and thermal circuits', e.g.:http://www.ingaero.uniroma1.it/attachments/2176_Cap_3%20Thermal-electrical%20analogy.pdfhttps://www.rcet.org.in/uploads/academics/rohini_59825600069.pdfhttps://nvlpubs.nist.gov/nistpubs/jres/61/jresv61n2p105_A1b.pdf

author: MegaVolt
author_rank: Frequent Contributor
timestamp: April 29, 2023, 08:14:01 pm
content: Quote from: RoGeorge on April 29, 2023, 07:54:28 pmThank you!! I found all the answers.

author: RoGeorge
author_rank: Super Contributor
timestamp: April 30, 2023, 12:30:21 pm
content: Quote from: mawyatt on April 29, 2023, 07:38:33 pmThey were so concerned about copyrights that they lost all the old pics, and thus rendered useless most of their archive articles. There are no snapshots on the Wayback Machine either.

author: coppercone2
author_rank: Super Contributor
timestamp: April 30, 2023, 02:17:43 pm
content: Quote from: RoGeorge on April 29, 2023, 07:39:31 pmits a start. I was expecting something with a MCU but if its just a D-I circuit thats interesting. They did have to use super conductors though