[sdiy] Ways of simulating thermal drift and stabilization?

Roman Sowa modular at go2.pl
Mon Apr 19 13:01:37 CEST 2021


Without diving into depths of thermodynamics theory let me just say you 
don't want heating or anything oscillating with high current at 80Hz 
anywhere near VCO. It eventually will bleed into tuning. It should 
stabilize to a DC. May oscillate a bit but with damped oscillation but 
then calm down to very slow motion within a minute after powerup.

If Spice does not alow thermal simulation, and I would be surprised if 
it didn't, you could approximate temperature influence by feeding heater 
current to RC filter (2nd order possibly?) with long time constant, like 
couple of seconds, which then controls VCA insetred on the way of your 
temperature sensor.

Roman

W dniu 2021-04-18 o 11:37, Spiros Makris via Synth-diy pisze:
> Hello list,
> I know this is far fetched, but is there a method to simulate 
> temperature effects in circuits? Specifically, I would like to look 
> closer to oven stabilization, which I have implemented before, but 
> designed on a completely empirical level.
> That is, I first tried on/of control (opamp with no feedback), then I 
> tried simple proportional control and finally added a capacitor to 
> reduce the loop bandwidth, all on the breadboard, with 15-20 minutes 
> required for each experiment, and it was fairly hard to visualise the 
> data (looking for a 30-50mV signal on a scope over the course of 20 
> minutes...).
> Are there any realistic alternatives to trial and error? Could SPICE do 
> such a thing? Or perhaps are there some thermal models I could load up 
> in Matlab and design a controller around it? I don't know enough about 
> control to extract these by experimenting, but I think I can handle it 
> if there's some transfer functions already available for me to play with.
> 
> Just for reference, I attach the circuit I came up with last time, and 
> its measured output during power up. To calibrate, the circuit is first 
> powered with heat enable disconnected, and the voltage of the diode 
> connected transistor is measured. The voltage of T_ref is set 50mV or so 
> above that, then heater enable is connected. This sets the operating 
> point at 50C or so.
> This module has a couple of rookie mistakes, such as using the rails 
> directly as voltage references, but I think they are irrelevant to the 
> question itself so let's try to ignore them 😂
> 
> image.png
> image.png
> 
> On off control
> image.png
> Opamp output at startup
> image.png
> opamp output at steady state
> image.png
> 
> it's interesting that this waveform was 80hz or so, and the duty cycle 
> was depended on the temperature reference. Touching the chip (therefore 
> changing its thermal properties) changed this frequency.
> 
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