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Very stable temperature control

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Rerouter:
Its a bit of a weird circuit, just thrown it into a simulator,

I would say simplify back to a simple output constant current load style circuit for the output, and some basic differential circuits for the regulation.

So something like this, It heats up quickly but backs off the heating significantly when the heatsink starts getting too much hotter than the measurement point, and vice versa turns back up the heat a little when it gets too far below it.

The 100K under the second op amp from the left adjusts how aggressive this back off strategy is, the lower the value the more careful it is to not dump too much heat into the heatsink,

And the 100K under the first op amp from the left controls how agressive the heating rate is for the measurement point vs the set temperature, lower is slower and more careful. the higher the value the hotter the heatsink will generally get on startup.

Final note, your heatsink temperature would ideally be something like direct to the heat source,

HendriXML:
Maybe the current amplification is less conventional, I found it in the LM358 data sheet.

But otherwise I don't think I went to crazy.

I like that it can be tuned with scientific meaningful parameters. The power is linear with the error temp. My guess that the setup would lose less than 5W of heat, resulted in matching values right away.

It works how it supposed to, with not much deviation from calculated values so I'm confident that it is not a "lucky shot circuit". Using the current limiting zenerdiode is a bit sensitive to tolerances. But if it was off too much I would than upped or lowered the supply voltage to get a 17 W max output.

On the first iteration I used two TL071 to be able to do some input offset. But those opamps cannot have inputs near ground (V-), so it didn't work.

joeqsmith:
I assume you are within +/- 0.1 Celsius.   I would normally want a well insulated container with some decent airflow inside.   It looks like you actually mount the parts outside in open air and try to heat the mass to a temp.   Beach towels work well. 

When I have matched transistors and such, I have used cardboard boxes with foam and a resistor for a heater.   I am not sure how tight the current version of this is but I suspect it's under +/- 0.005 degrees C.
https://www.eevblog.com/forum/projects/poor-mans-thermal-chamber-meat-packing-box-ii/msg2454591/#msg2454591

This chamber was just for playing with some ICs. 
https://www.eevblog.com/forum/projects/waveforms-in-a-74ls04-ring-oscillator/msg646512/#msg646512

The internals on this project are controlled to about a tenth, which dominates the error.     
https://www.eevblog.com/forum/testgear/playing-with-delta-sigma-converter-poor-mans-meter/msg2041111/#msg2041111

HendriXML:

--- Quote from: joeqsmith on June 28, 2019, 04:06:30 pm ---I assume you are within +/- 0.1 Celsius.   I would normally want a well insulated container with some decent airflow inside.   It looks like you actually mount the parts outside in open air and try to heat the mass to a temp.   Beach towels work well.

--- End quote ---
I don't have the MM to do high resolution measurements. But looking how the power fluctuates I think the temperature  fluctuates around 0.002 deg. This mainly because of the thermal capacity/heat loss ratio. Water is cheap and does its work well. Much better than alu.
The active setup is with a 10 cm foam on top. Not the best, but insulating enough. The most stable setup is the one that doesn't lose any heat and needs almost no heater.

Damianos:
In the past, I played with a circuit very similar to the one at page 16 here: http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=snoa748&fileType=pdf&keyMatch=AN-460&tisearch=Search-EN-Everything
I did also a variation, using a second sensor, to make a differential controller for a setting "above the environment temperature" (for testing heatsinks).
Except of the "original" parts (LM35, LM10, LM395), I tried also other types with similar results.
Unfortunately I have not access to the lab yet, so I can't give more information.

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