Electronics > Projects, Designs, and Technical Stuff
Very stable temperature control
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HendriXML:
I build a apparatus to do a temperature controlled measurement on a simple diode:
https://www.eevblog.com/forum/testgear/using-a-awg-and-a-scope-do-a-low-voltage-level-characterization-of-a-1n4005/msg2514567/#msg2514567

The measurement takes more than 20 min. So I needed a way to stabilize a certain temperature for at least that period. The accuracy of the temperature was less of a concern.

Using this setup the temperature will fluctuate less than 0.1 deg. This is done by:

* Using a relatively large thermal mass (heatsink and +/- 400 ml water)
* Good thermal capacity (water)
* Good thermal conduction (heatsink and water)
* Good thermal isolation (Isolating can and covering the setup with foam)
* Controlled heating (2 x TIP120) symmetrically near the diode under test
* Measuring near the diode under test (LM335Z)
The best way to use this setup is to start with boiling water and let it then cool down. The regulation will than eventually kick in. To keep the temperature stabile it needs about 2 W, but the setup can deliver 16 W if needed. Without any heating, the temperature would drop with some 1 deg per 3 minutes. So very slowly.
The regulation oscillates only a little (which is inevitable) but this is in the range of 1 hz. (So a minor climb/drop every second)
HendriXML:
Here's the schematic. The working is as follows:

* One part is responsible for dividing the voltage, thus power evenly across the two TIP 120's (Q1/Q2)
* One part amplifies sensed current
* One part amplifies the difference between the measured temperature (LM335Z) and a reference voltage (BT1), I use my signal generator for the reference voltage
* One part compares that difference against the delivered current/power, and "requests" more power is the temperature difference becomes higher. Zenerdiode D2 limits this power request to 16W
dzseki:
If you'd add an integrating part to your control loop, then you could get rid off the small ripple as well.
blackdog:
Hi HendriXML,  :)

You cant say that you build a good themperature controler and also say the loop is not stable!  :-DD

The regulation oscillates only a little (which is inevitable) Ooooh...
I think you have to work on the loop control, it must be stable, don't fool yourself.

Normaly you need only a reference voltage, a bridge circuit and a power stage and one opamp to maak a good controler.(and ofcource a tuned loop)
Search for oven controlers and jim williams from LT, and you can also search for oven controlers and blackdog  ;)

It never have come to my mind to use a plastic isolated cup, i wil buy one en wil test its performance.

Kind regards,
Bram
HendriXML:

--- Quote from: dzseki on June 28, 2019, 10:35:00 am ---If you'd add an integrating part to your control loop, then you could get rid off the small ripple as well.

--- End quote ---
Thanks for your response. In a controlled (no external temperature fluctuations) environment (which this setup is) that would even be better, I agree. But when there're would be fluctuations, it would respond a bit slower to changes. I choose for mainly for heat buffering. If without any heating (0 W) the temp drops 1 deg / 180 sec. Then this would be less than 0.01 deg per second. However I can see the power delivery oscillate between 2.4W and 3W.  So my rough guess is that the temperature fluctuation is even 5x smaller.
I keep it this way because one of the driving questions was whether this method would be stable enough.
It seems to work quite well. If one influences the temperature (by blowing at the sensor for example) it does have a little overshoot -as one expects- but still very little.
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