Thanks, THOR73 for reading my post.
On to your questions/comments:
Tempting to try it on an MTH unit (at a lower duty cycle, to get the same average power), but I'm guessing it might choke on the ~8 ohm element, as that would be in the neighborhood of 1.5A while powered on.
I think you can do this by using the 5V powering scheme you described in your post and use the HTR- as the MOSFET switch control: "high-side" as you posted or "low-side" as I have used. That was what happened when I had to power the heater resistor when testing with the Profi board.
Were you able to tell while experimenting if the temp sensor is actually in the control loop for the heater? For instance, does the heater run near 100% duty cycle while first heating up, and then drop to the 30% steady state once hot?
The duty cycle did indeed change with time. For instance, on first power-up, the duty cycle appeared to be 50% and then changed to the roughly 30% I mentioned. I never saw the duty cycle exceed 50%, but I noticed some slight jitter on the steady-state duty cycle, suggesting feedback. It's difficult to know if the cheap digital scope I used was doing a credible job of estimating the duty cycle. I have no real way of knowing if that was feedback, but since the sound file setting for the ESU smoke unit list temperatures, I surmise the decoder must be measuring the TMP- voltage and converting the voltage to temperature.
If you set Rfixed and Voffset well in the voltage divider, then the T vs. V curve is approximately linear over a limited range, say from 100C to 250C. I won't bore anyone with the quantitative details (which I have) unless there is interest.
I'm a little surprised you had to dig into the sound file like you did. That option is not available on the Loksound Selects, and they clearly are supposed to support the smoke unit as well.
Funny you should mention that. I was just on the phone with Tony's Trains, where I purchase the LokSound L's, in regards to a Cab Forward sound file. It's only available as a Select file, but they were concerned that it might not have the ESU smoke activated. I indicated that they should go ahead and send me a LokSound L Select loaded with the Select version of the Cab Forward, and I will determine if smoke is activated. If not, then I will contact ESU USA to get that corrected.
Thanks, THOR73, again for taking the time to read my original post.