Very stable temperature control - Page 1
Forum section: electronics / projects

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 10:02:28 am
content: 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/#msg2514567The 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 testMeasuring 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)

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 10:15:06 am
content: 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 currentOne part amplifies the difference between the measured temperature (LM335Z) and a reference voltage (BT1), I use my signal generator for the reference voltageOne 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

author: dzseki
author_rank: Frequent Contributor
timestamp: June 28, 2019, 10:35:00 am
content: If you'd add an integrating part to your control loop, then you could get rid off the small ripple as well.

author: blackdog
author_rank: Frequent Contributor
timestamp: June 28, 2019, 11:12:03 am
content: Hi HendriXML,You cant say that you build a good themperature controler and also say the loop is not stable!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 blackdogIt never have come to my mind to use a plastic isolated cup, i wil buy one en wil test its performance.Kind regards,Bram

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 11:15:25 am
content: Quote from: dzseki on June 28, 2019, 10:35:00 amThanks 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.

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 11:45:26 am
content: Quote from: blackdog on June 28, 2019, 11:12:03 amThanks for your response!Heat traveling takes time, so whenever there's a distances between heater and sensor. There're will be some feedback time. One can eliminate that by using "prediction", thus using an average (integrator/electronic buffer) but when the temperature does change, it will be slower/more off.This setup is used in a stable environment, so I could make the loop entirely stable (slow). But that would also give it a less interesting behavior. The circuit is designed to give 50W of power / delta T (deg C).So in a way it even needs some compensation at the reference voltage. Which is thus dependent on the average needed power to sustain a temperature. (Heat leakage)If I where to improve this setup, I would target the total thermal resistance between the TIP120's and the sensor.The used silicone pads are rubbish, and the parts of the heatsinks where the TIP120 are mounted on are somewhat hollow. The heatsink was sold as high tech back in the days, but I think the used materials is rubbish regarding to thermal conductivity. It was never a good CPU heatsink.This means while controlling extra heat will build up, which will flow eventually and give a bit of overshoot.Using a better heatsink, more transistors and better pads, better mounted sensor, would be a nice experiment.

author: dzseki
author_rank: Frequent Contributor
timestamp: June 28, 2019, 12:27:02 pm
content: Quote from: HendriXML on June 28, 2019, 11:15:25 amSo you could look for a conventional PID control loop, with proper tunig you could have the fast response with the stable output at the same time. One can even realize a PID control with a single opamp, you'd not even have to overcomplicate your design.But of course it is up to you, and if you have already satisfactory results, all good.

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 12:45:39 pm
content: Quote from: dzseki on June 28, 2019, 12:27:02 pmI used what I had laying around. The TIP120 as heaters for example. Because they cannot dissipate a lot of power, it had to have a current limiter anyway.I"m pleased with the results. I'm a beginner, so the more components/complexity, the more there's to learn.

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 12:53:00 pm
content: Quote from: blackdog on June 28, 2019, 11:12:03 amFound your project, with the inner and outer oven, very nice to read. Targetting 40 deg, so you could also have use transistors as heaters.

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 01:21:45 pm
content: I always try to keep track of where the values and underlying information of the schema comes from. For this purpose I use a scripting tool.I then try to separate the values I've chosen, the values from datasheets, the values that where measured and the values that are calculated. The calculations are done running the script so it is easy to recalculate stuff.But because I know what information is provided or consumed by what I also know what the effect of design changes are. Using this modular approach much larger project can be kept under control.To know how the values are used, one needs to check the script. But that could also have been reported.The script fails when assertions haven't been met.Highly stable temperature controlVersion 1.0.34Choosen valuesprovidesvoltVCC : 24,00 VdegHeaterMax : 80,00 °CdegHeaterMin : 10,00 °CohmR3 : 100 mΩohmR4 : 100 mΩohmR6 : 214 ΩohmR7 : 5,1 kΩohmR11 : 1 kΩohmR12 : 1 kΩohmRV1 : 10 kΩwkControlPower : 50,00 W/KDatasheet values TIP120provideskwThermalResistance : 1,92 K/WdegJunctionMax : 150,00 °CfacHFE : 1000,00voltBEOnMax : 2,50 VEmperical values max powerconsumesChoosen valuesdegHeaterMax : 80,00 °CDatasheet values TIP120kwThermalResistance : 1,92 K/WdegJunctionMax : 150,00 °CintermediatedegJunctionSafeValue : 140,00 °CdegMeasuredHeatSink : 80,00 °CdegMeasuredTransistor : 122,00 °CwattMeasuredWatts : 17,00 WwattSafeValue : 18,75 WprovideswattPowerQ1Q2 : 17,00 WMax current determinationconsumesChoosen valuesvoltVCC : 24,00 VohmR3 : 100,00 mΩohmR4 : 100,00 mΩEmperical values max powerwattPowerQ1Q2 : 17,00 WintermediateohmR3sR4 : 200,00 mΩprovidesampMaxCurrent : 712,56 mAwattMaxPowerQ1Q2 : 17,00 WvoltMaxSenseVoltage : 142,51 mVEmperical values 1N4728AprovidesampCurrent : 5,00 mAvoltReverseVoltage : 2,20 VSense voltage amplificationconsumesChoosen valuesohmR6 : 214,00 ΩMax current determinationvoltMaxSenseVoltage : 142,51 mVwattMaxPowerQ1Q2 : 17,00 WEmperical values 1N4728AvoltReverseVoltage : 2,20 VintermediatefacAmplification : 15,43ampCalcR6R7 : 665,95 μAohmCalcR7 : 3,30 kΩprovidesvwPowerPerVolt : 7,73 W/VohmR7 : 3,3 kΩDatasheet values LM335ZprovidesvkSensitivity : 10,00 mV/°CampForwardCurrentNominal : 1,00 mATemperature deviation amplificationconsumesChoosen valueswkControlPower : 50,00 W/KohmR11 : 1,00 kΩohmR12 : 1,00 kΩSense voltage amplificationvwPowerPerVolt : 7,73 W/VDatasheet values LM335ZvkSensitivity : 10,00 mV/KintermediatevkControl : 6,47 V/KfacAmplification : 646,76ohmCalcR13 : 646,76 kΩohmCalcR14 : 646,76 kΩprovidesohmR8 : 18 kΩohmR9 : 1 kΩohmR13 : 680 kΩohmR14 : 680 kΩD1 series resistor (R10)consumesChoosen valuesvoltVCC : 24,00 VdegHeaterMax : 80,00 °CohmRV1 : 10,00 kΩDatasheet values LM335ZvkSensitivity : 10,00 mV/KampForwardCurrentNominal : 1,00 mAintermediatemaxSensorVoltage : 3,53 VampRV1 : 353,15 μAampRV1pD1 : 1,35 mAvoltCalcR10 : 20,47 VohmCalcR10 : 15,13 kΩprovidesohmR10 : 16 kΩDatasheet values LM358providesvoltPositiveOutToRailVoltage1mA: 1,40 VvoltPositiveOutToRailVoltage5mA: 1,50 VD2 series resistor (R16)consumesChoosen valuesvoltVCC : 24,00 VEmperical values 1N4728AampCurrent : 5,00 mAvoltReverseVoltage : 2,20 VDatasheet values LM358voltPositiveOutToRailVoltage5mA: 1,50 VintermediatevoltR16 : 20,30 VohmCalcR16 : 4,06 kΩprovidesohmR16 : 4,3 kΩQ1 (R18) and Q2 (R19) base resistor calculationconsumesChoosen valuesvoltVCC : 24,00 VohmR3 : 100,00 mΩDatasheet values TIP120facHFE : 1000,00voltBEOnMax : 2,50 VDatasheet values LM358voltPositiveOutToRailVoltage1mA: 1,40 VMax current determinationampMaxCurrent : 712,56 mAintermediateampBaseQ1_2 : 712,56 μAvoltCalcR18 : 8,03 VvoltCalcR19 : 20,10 VohmCalcR18 : 11,27 kΩohmCalcR19 : 28,21 kΩprovidesohmR18 : 11 kΩohmR19 : 27 kΩ

author: Rerouter
author_rank: Super Contributor
timestamp: June 28, 2019, 01:53:25 pm
content: 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,

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 03:48:50 pm
content: 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.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 28, 2019, 04:06:30 pm
content: 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/#msg2454591This chamber was just for playing with some ICs.https://www.eevblog.com/forum/projects/waveforms-in-a-74ls04-ring-oscillator/msg646512/#msg646512The 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

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 04:33:49 pm
content: Quote from: joeqsmith on June 28, 2019, 04:06:30 pmI 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.

author: Damianos
author_rank: Frequent Contributor
timestamp: June 28, 2019, 04:40:00 pm
content: 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-EverythingI 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.

author: max_torque
author_rank: Super Contributor
timestamp: June 28, 2019, 04:41:31 pm
content: This sort of thing is really best done open loop tbh. The total heat flux drives the temperature, so just put a known and fixed amount of heat in, and wait for it to stabilise!Do it in a large room, where the thermal mass of the room and air is large, and there is nothing to drive any oscillation...

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 04:55:57 pm
content: Quote from: max_torque on June 28, 2019, 04:41:31 pmI understand your reasoning, but the thermal mass and conductivity of air is extremely low. (That's why we use heatsinks.)

author: joeqsmith
author_rank: Super Contributor
timestamp: June 28, 2019, 05:05:45 pm
content: My room temperature varies way to much to be useful. Simple beach towels over the device is a huge improvement.Quote from: HendriXML on June 28, 2019, 04:33:49 pmIt may be worth investing in a decent used DMM. If you are looking at characteristics of diodes, I would have thought you would have been all set with this. In the case of the meat packing box, I want to be able to run at different temperatures. Using the PC to control and record the data as much as possible just makes things easier.If you like watching paint dry and grass grow, this is about 10 minutes of data collected real time as the box is starting to settle at 23.0C with the stacked Peltiers.https://youtu.be/j9YcKQ-2uU8

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 07:51:09 pm
content: Quote from: Damianos on June 28, 2019, 04:40:00 pmFrom what I've understood is that K-type sensors are in fact differential temperature sensors. With a hot and cold junction. It should than be possible to use just a pair of those wires to do differential measurements, by reading only one voltage. This might eliminate some errors. Don't know whether this is used in practice.

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 08:20:27 pm
content: Quote from: joeqsmith on June 28, 2019, 05:05:45 pmI've gone to so much effort precise measurements with a scope and awg that I indeed earned my right to buy me a bench MM. I will explain this right in the upcoming years to my spouse. To her all these boxes on the bench are probably all the same and some of them are quite recent.Until than I've to be creative.It is also very nice to see how others build comparable "contraptions".

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 09:01:54 pm
content: Quote from: dzseki on June 28, 2019, 10:35:00 amI 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.

author: HendriXML
author_rank: Super Contributor
timestamp: June 28, 2019, 11:21:57 pm
content: 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)

author: joeqsmith
author_rank: Super Contributor
timestamp: June 29, 2019, 03:55:23 am
content: 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.There are some decent meters out there, and some very expensive junk.

author: David Hess
author_rank: Super Contributor
timestamp: June 29, 2019, 07:11:24 am
content: 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.

author: max_torque
author_rank: Super Contributor
timestamp: June 29, 2019, 09:42:51 am
content: Quote from: joeqsmith on June 28, 2019, 05:05:45 pmDoes 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!

author: HendriXML
author_rank: Super Contributor
timestamp: June 29, 2019, 10:56:35 am
content: Quote from: max_torque on June 29, 2019, 09:42:51 amYou know how it goes, first fluctuations within a degree are OK. But then engineers want to go better. For most people it is: if it aint broken, then don't try to fix it.For engineers it is: if it aint broken, it hasn't got enough features.

author: Rerouter
author_rank: Super Contributor
timestamp: June 29, 2019, 11:12:38 am
content: If we haven't reached our budget or a hard limit, why stop now.In reality the stacked peltier system joeqsmith has built is approaching what I am hoping for with one of my projects, but he really has it locked down to laboratory reference level of stability.

author: HendriXML
author_rank: Super Contributor
timestamp: June 29, 2019, 01:16:03 pm
content: So I did some testing. With the 10uF capacitor it kept oscillating slowly, never reaching a balance. I could have then turned down the reactivity, but I went for just wacking 5 more capacitors in parallel. Nearing the 63 uF I "analyzed". Now its stable after just a few ups and downs. The wattages varies only a few % in less than a second. Not having the proper tools to measure the temperature variations, I need to make a guess.With no power the temp drops 0.006 deg per sec. If the average power is stable, the 1% wattage change would correspond with 1% of 0.006, thus 0.00006 deg.It won't have this low variation in reality, mainly because of the used components and reference voltage stability.However I think it will do well under better testing. Not because anything special in the PID, but having a lot of thermal mass. Water is special. Also I found out the heatsink consists copperplates (if the specs are correctj, so the heat transfer to water might actually be very good.https://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html

author: HendriXML
author_rank: Super Contributor
timestamp: June 29, 2019, 02:00:34 pm
content: Quote from: joeqsmith on June 29, 2019, 03:55:23 amBuying an uncalibrated (used) one would lead to the continuous question whether the meter is accurate or not, if one doesn't own another accurate device. So I don't even check eBay stuff. Maybe in the future I'll buy something like the Siglent SDM 3055. But 6.5 digits would be nicer..

author: joeqsmith
author_rank: Super Contributor
timestamp: June 29, 2019, 03:01:20 pm
content: Quote from: HendriXML on June 29, 2019, 02:00:34 pmThis is why we have cal houses. I bought two brand new HP bench meters for home. Based on my experience with them, if I ever have a need to upgrade (doubt) or replace one (possible after that last lightning strike), I will buy one from Keysight. Say I spent $1000 USD. I've have had the 34401A for well over 20 years now, so $50/year. That's what, a trip to the bar, dinner, wife's trip to having her nails and hair done ...I've always looked at my hobbies as an investment in myself.Quote from: max_torque on June 29, 2019, 09:42:51 amOP's title is "Very stable temperature control". I'm just offering my perspective.Your "change that much" is subjective. For you, maybe that means a degree. Trying to control down to 0.001 C, small air currents and body heat can be a problem. Again, a basic beach towel can combat a lot of that but its not going to get you to the level I am showing. Automating the system can be helpful for running experiments.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 29, 2019, 03:10:23 pm
content: Quote from: Rerouter on June 29, 2019, 11:12:38 amMy advice is BIG heatsinks and more Peltiers and lots of copper spreaders. You can never have enough.

author: max_torque
author_rank: Super Contributor
timestamp: June 29, 2019, 06:49:07 pm
content: and how are you measuring this "0.001" degC ?

author: David Hess
author_rank: Super Contributor
timestamp: June 29, 2019, 07:25:31 pm
content: Quote from: HendriXML on June 29, 2019, 01:16:03 pmThat indicates to me that a zero needs to be added to the transfer function like a resistance in series with the integration capacitor.QuoteThe output of the error amplifier can be measured to tune the transient response.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 29, 2019, 08:04:23 pm
content: Quote from: max_torque on June 29, 2019, 06:49:07 pmFrom the pictures in the links I provided, you may notice a sensor located pretty much in the center of the box. There is a fair bit of air flow inside and it should have little dead volume. The sensor is a platinum RTD that is read through my HP 34401A bench meter. The PC reads the meter and performs the conversion. The conversion is supplied by the manufacture. Keep in mind that my goal is not to have absolute accuracy but rather short term stability, followed by repeatability and then accuracy. If you have specific questions, feel free to ask.***I forgot to mention that normally when I am running experiments where I need stable temperatures, I will normally have other test equipment near by controlling and collecting data. All of that equipment also creates air currents and heat which can cause problems with my measurements. I expect OP may have a similar problem.***A link showing some of the basic differences between sensors:https://www.omega.co.uk/temperature/z/thermocouple-rtd.html

author: HendriXML
author_rank: Super Contributor
timestamp: June 29, 2019, 08:56:50 pm
content: Quote from: joeqsmith on June 29, 2019, 08:04:23 pmOP needs only a square cm for his diode, so his task is easy. I've put a 10 cm foam on top, so above the heatsink there's only a tiny room of air. The diode and sensor are glued to the heatsink, so most of the heat will be taken from that. I guess the air might be a tiny bit cooler.If I where to scale this setup to a larger box, I would probably have a box in a box where the inner box would be surrounded by water. The outerbox would then have styrofoam isolation on the outside. Then have maybe the outerbox made of alu or something with heaters to keep the water at the requested temperature. Maybe with dual heaters, one water boiler 2300W for fast heating, and one much less powerful to keep it at the target temperature. Maybe throw in a waterpump as well, pumping water trough copperpipes mounted on a isolated alu lid. Inside the box the temperature will eventually be the same as the water temperature.In the inner box, there also should be a fan to move the air around.Maybe one day I'll have a purpose for this idea.I assume that the peltier aficionados know the following vids/channel:https://youtu.be/aSGwaQWA05UI really like the stuff they're doing.

author: joeqsmith
author_rank: Super Contributor
timestamp: June 30, 2019, 05:14:37 pm
content: I was asked about the control system in a PM. Would rather keep discussions in a public forum so others may benefit.I posted about how I monitor the temperature in my previous post. The PC uses this feedback to run a simple PI controller (D set to zero). Output from the PI drives a power supply in voltage mode which in turn powers the Peltiers. The voltage is limited to a safe operating area. Similar to the video posted, the Peltiers are different for the two stages. Lower power on the cold side. It's nothing too complex.If the goal is to run below -40 with any sort of volume and loading, I would still suggest a more conventional system. Faster ramp times, more efficient, maybe longer life depending how you plan to use your system. The datasheets for the Peltiers talk about their life and how they are cycled.

author: HendriXML
author_rank: Super Contributor
timestamp: July 01, 2019, 10:30:40 am
content: Quote from: 3roomlab on June 29, 2019, 06:25:00 amI had to think about this for a while. I see the usage of the capacitor as adding a way to store state. In this circuit it should store the avg difference voltage that is needed to counterbalance the temp leakage. By oscillating a few times it will approximate that voltage. This will happen without problems because the heating power is low to the thermal capacity and the cooling power is even lower. Except when the capacitor gets near its max and min voltages too soon while heating and cooling. When that's the case it's "memory function" is not entirely effective anymore.So I guess the calculation of the value of C should mainly take that into account. It is like the rough analysis I did before, but without the conclusion is should be much less than 68uF. It should be around 68uF, so the the capacitor is kept in a more effective range of it's charging and discharging curve.So I think half the capacitor voltage at half of the temp difference would be a good rule of thumb, when doing exact calculations -due to missing parameters- is impossible. There is for example unknown thermal resistance to the thermal mass. (And the most power will be used to heat the mass a bit more.)If my reasoning seems broken, please let me know.

author: HendriXML
author_rank: Super Contributor
timestamp: July 01, 2019, 12:51:53 pm
content: Also I think the circuit can be improved by hooking up C3 after R16. Charging C3 with more voltage than what will ever be used by the following stage is not helpful in getting the balancing voltage in that cap.It wouldn't surprise me if that would make a 10uF as C3 stabilizing as well.

author: HendriXML
author_rank: Super Contributor
timestamp: July 01, 2019, 05:02:55 pm
content: The modification did not mean only a 10uF cap could be used. It wasn't the heating that unbalanced things. It is the slow spread of heat to thermal mass in the cooling down cycle part. That part of the cycle is also the part that was taken in the RC analysis. Don't know whether the analysis was correct, but it seems to work.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 01, 2019, 05:52:48 pm
content: Moving the capacitor to the other side of the resistor makes sense. It is a way to implement anti-windup. It helps for the initial approach, but no more in the final approach, when the heater no longer goes into saturation.The change should not effect the choice of the capacitor values.

author: HendriXML
author_rank: Super Contributor
timestamp: October 03, 2019, 09:45:30 am
content: I started this post with not much (formal) knowledge of PID control systems.In the meantime I found some good video's giving more insight into PID control.https://youtu.be/NVLXCwc8HzM

author: mzzj
author_rank: Super Contributor
timestamp: October 03, 2019, 02:19:27 pm
content: Look for INA330 peltier controller if you want stability.. noise level below 0.0001CThermistor is the way to go if you want maximum sensitivity but don't care about long-term stability.Platinum resistance thermometers(Pt100) or SPRT's have less sensitivity but better long-term stability.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 03, 2019, 02:50:55 pm
content: See if you can get a copy ofA microcontroller-based driver to stabilize the temperature of an optical stage to within 1 mK in the range , using a Peltier heat pump and a thermistor sensorA W Sloman, Paul Buggs, James Molloy and Douglas Stewart Measurement Science and Technology, Volume 7, Number 11Bill Sloman hangs out on sci.electronics.design (google groups) and is the principal author. He might just send you a copy.Steve