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| Very stable temperature control |
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| 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 --- I think I will add a capacitor, and implement this. Using a much lower power / error temp ratio. The situation is well suited for it and I know now already how it behaves without. It will probably add some ringing, but that will than slowly die out. (I guess) It is also nice to lose the needed offset. |
| HendriXML:
Here's is then the new schematic. I'll try to analyse how to get a good value for C3. As said, in this setup it is most easy to start with a too high temperature. This ensures everything is in balance, when the sensor measures the target temperature. Before that no power is delivered, so no influences of the heater. Until that time the opamp U2A outputs 0V. However I know the stable power is around 2.5W, thus 0.32V at the U2A output. The circuit is setup to deliver 7.73 W/V. (Limited at 17W, zener voltage 2.2V) This means that C3 will need drop some of its voltage (0.32V) , this will be done while the amplification is in "slow mode" (undergoing a strong, time related C3 feedback). In slow mode a temp difference of 0.5 grad would be needed to get the required difference of 2.5W power. (That would be the very max temperature it would drop, if C3 was infinite) If the time it takes to have C3 drop half of its voltage is much shorter than the time it takes to loose half of 0.5 grad, I think the system will stabilize just fine. I know the temp drops roughly, 1 deg per 180 sec. So taking 30 sec for the 0.25 grad is on the safe side. This would be at 43 RC. When using a 680K resistor, this will lead to a 63uF capacitor. So with a 10 uF there's nothing to worry about. Thats is the largest bipolar capacitor I've lying around. Whether it is too little? It might if the power is high compared to the thermal mass, but with the turned down reactivity? We'll see. Maybe it is a good experiment to try no amplification feedback at all, and see how that oscillates. (I know the calculations are very rough, discarding some meanwhile changes in voltages) |
| joeqsmith:
I havn't had an eBay account for at least 10 years now but thought I would have a look on eBay for cheap used bench meters. Now I'm glad I closed my account as I would have been in a world of trouble buying some this old vintage gear. May have had to join that TEA thread to recover. :-DD There are some decent meters out there, and some very expensive junk. |
| David Hess:
I have hacked together small temperature controllers a few times and always got by with a bridge circuit using a diode as the temperature sensor (1) and an operational amplifier implementing just the integration part of a PID configuration. Frequency compensation using the normal methods is only time consuming because it takes a couple minutes to run each test. Temperature regulation of better than 0.1C is easily achievable this way but actual accuracy will depend on physical construction and how it limits frequency response. (1) One of the various PTAT IC temperature sensors will work just as well as a diode. |
| max_torque:
--- Quote from: joeqsmith on June 28, 2019, 05:05:45 pm ---My room temperature varies way to much to be useful. --- End quote --- Does it? OK, if you have a room that gets lots of sun, or leave the windows open, but the OP asked for "20min" stability, and over that time i suspect the actually air temperature doesn't change that much. Find a shady corner of your house, put the device there, and i see no reason to chase complex PID controllers etc, just put on a couple of watts of heating, leave it on till the temperature reaches stability and you're done! |
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