Please help me get LTspice to run an active load simulation - Page 1
Forum section: electronics / projects

author: Davidwasalreadytaken
author_rank: Contributor
timestamp: August 20, 2025, 03:43:24 pm
content: I am obviously missing something. Can you give me the nugget I need to get me from "relatively n00b" to "can make this work in sim"? I got as far as getting rid of errors so bad it won't complete a run, but I can't figure how to get a sweep of voltage at the Vset node, to drive the rest of the circuit!I am back to thedrawing boardcomputer designing my next load, this time aiming to understand instead of bodging parts together and crossing fingers. But before I put actual part numbers and finalize values, I want to have a model that will function!p.s. if there are GOOD tutorials you could point me to that would be appreciated. There is so much aislop online I got discouraged

author: ledtester
author_rank: Super Contributor
timestamp: August 20, 2025, 04:27:24 pm
content: I modified your schematic with these changes:- Vset voltage source change to PWL and grounded.- used .tran for transient analysisThe Vset voltage changes from 0 to 10 over the course of 10 seconds.If you plot the current through Rsense you will get the current as a function of time and thus as a function of Vset.

author: Davidwasalreadytaken
author_rank: Contributor
timestamp: August 20, 2025, 05:16:11 pm
content: Okay, thanks for that! Now I am playing with the various component values and it is responding more like I expected. The initial sweep gave me a rise of 2mA over the range of Vset, but I have found a few ways to get that up into the amperes range. And I found a few spots where I had typos that were significant!Vset was ungrounded because I was changing it trying to make it work and deleted a previous ground node

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2025, 07:47:28 pm
content: The basic NMOS model is for a small signal transistor on an IC, not a power MOSFET. It has a saturation current of something like 2mA, so is no good for power.What current vs voltage do you require?Only one op-amp is needed.

author: Davidwasalreadytaken
author_rank: Contributor
timestamp: August 20, 2025, 08:16:44 pm
content: Zero999 eventually I am going to have a dozen IXFK55N50 in here, each with her own drive circuit. There are several things on this schematic which are not strictly necessary to have a load that can function, but thanks very much for your input.I found a bizarre behavior in the model due to having picked universal parts...UniversalOpAmp was swinging the gate drive to a few kilovolts! That would be a neat trick on the breadboard. Changing the opamp model has helped quite a bit in that regardEventually this is going to look much more realistic, especially if I can find the right models.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2025, 09:22:29 pm
content: UniversalOpAmp doesn't simulate the power rails. It's a voltage controlled current source, in parallel with an RC circuit, to convert it to a voltage, with the appropriate cut-off frequency characteristics. Use UniversalOpAmp2, as per my attachment, which does model the power supplies.I used a trick in my simulation to map the 3 pin op-amp to the 5 pin universal op-amp model. Starting a node name with $G_ makes it a global node, which can be referred to within a model. In this case, it's $G_oa+ which is connected to UniversalOpAmp2 +V rail, inside a new model called opamp, which is referred to by the symbol..subckt opamp +in -in out Aol=1Meg GBW=10MegX1 -in +in $G_oa+ 0 out level2 Avol=Aol GBW=GBW Slew=GBW.lib UniversalOpAmp2.lib.endsI'm plan to create a new thread tutorial about this, but it's getting late now.