Temperature stabilised enclosures Heating and or Cooling - Page 1
Forum section: electronics / general

author: beanflying
author_rank: Super Contributor
timestamp: November 30, 2017, 05:32:20 am
content: Starting to look at some options for just heating enclosures for some Voltage References but also potentially cooling for other applications as my shack gets to well over 30 degrees.These little 30-40W 12V cartridge heaters from evilbay are a bit over $1 each so hooked up in series 4 of them would give circa 10W driven from 12V using a linear reg to keep the noise at bay. The jackets of the elements are isolated from the electrics too btw. Stuff the rear of them with kevlar thread or similar or make some teflon plugs to keep the heat in?Anyone had a go at Peltier devices strapped direct to cases or maybe better running them on water cooled loops to remove the electrics/noise away from the case?Had a good search around and apart from the recent posts in the KX thread didn't find anything.External monitoring/control of a 3 or 4 wired RTD to PIC or Arduino solution would be the most accurate option for control.

author: TiN
author_rank: Super Contributor
timestamp: November 30, 2017, 11:58:33 am
content: I think 10W of heat flux would be waaaay to much. I usually see around 1.5-3.5W numbers peak with my TEC setup (simple noname TEC strapped to bottom of the box) when there is ~30C differential).Benefit of the TEC setup is that you can have heatflux in both directions, not only heating.

author: Andreas
author_rank: Super Contributor
timestamp: November 30, 2017, 01:45:37 pm
content: Hello,I fear that the cartrige heaters will give a unequal temperature distribution between the corners and the center of the enclosure.Thats why I use always (mirror) heater foils.For cooling I use a car cooling box. (with fan to distribute the "cold").And do not forget to mount a over temperature switch if you leave your Arduino/PIC unattended.You can also use a NTC for controlling the temperature.It can be connected directly to the ADC input of a microcontroller.A 10 Bit ADC will give you around 0.1K temperature resolution.With a 12 Bit ADC (e.g. PIC24FV32KA302) resolution is 0.025 K.I use 33K NTCs with 27K pull-up resistor which is a compromiseof self heating of the NTC and the max source resistance for a unbuffered ADC input.I am using e.g. 100Hz PWM for the heater which is suppressed by the multimeterbecause it is a multiple of the (50 Hz) line frequency.setup is usually similar to here:https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg462298/#msg462298with best regardsAndreas

author: timb
author_rank: Super Contributor
timestamp: November 30, 2017, 01:49:29 pm
content: You could always use two power op-amps in a bridge configuration for linear drive of a TEC as that’s pretty low noise. (Though I wouldn’t do that for more than a 5W TEC unless you have room for a large heatsink for the amps.)That said, TI has a nice inexpensive H-Bridge that can be used for PWM based TEC drive up to 15A, with a couple of added inductors and filter capacitors. (So long as you keep the ripple below 10% you’re OK to drive a TEC with PWM in the hundreds of kHz range).A third option would be to use “time proportional” control, coupled with a pair of relays, SSRs or any off the shelf H-Bridge. This is basically really, really slow PWM. Essentially, you set a time window (say 10 seconds) and turn the TEC fully on for a portion of that window (50% would be 5 sec on 5 sec off).I’ve currently got a small TEC based cooler (designed to hold 6 cans of soda) that I converted to a temperature control chamber for testing voltage references. It uses the last option mentioned above. It’s got a range of 5c to 65c (at 25c ambient) and can maintain the setpoint +-0.25c, which is pretty good considering it uses nothing but parts from my junkbox.

author: Andreas
author_rank: Super Contributor
timestamp: November 30, 2017, 02:00:36 pm
content: Hello,you could also let the Peltier running at full load and heat against the (slow reacting) cooler to get quick to the wanted temperature.(a Peltier ages faster when there are frequent temperature changes).I also use several temperature sensors (minimum one at the heater and one at the DUT) to get a faster control loop.With best regardsAndreas

author: Echo88
author_rank: Frequent Contributor
timestamp: November 30, 2017, 02:04:55 pm
content: I suggest MTD415-module + high power bridge IC like OPA2544 for bigger Peltiers.https://www.eevblog.com/forum/metrology/mtd415-a-low-cost-rs232-controllable-bipolar-1-5a-tec-controller-module/http://www.ti.com/lit/ds/sbos037/sbos037.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: November 30, 2017, 02:14:07 pm
content: Jim Williams suggested this circuit.It is stable to 3.5mK with a 20K change, so probably quite good. Also, it is linear and Linear.http://cds.linear.com/docs/en/lt-journal/Thermoelectric_Williams_Mar02_Mag.pdf

author: beanflying
author_rank: Super Contributor
timestamp: November 30, 2017, 02:44:38 pm
content: Thanks for the ideas so far. I have a couple of Peltier modules as well to play with as well.I have a few (16)of these enclosures so some experimenting is in order to see how they heat or cool.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 30, 2017, 04:07:38 pm
content: The efficiency of peltier coolers is best if the current is reduced proportional to the temperature difference. Thus for running them for a 10-20 K temperature difference, one would ideally not use the full rated current but more like 1/3 to 1/2.Still the power density is quite high and thus a good thermal contact (and maybe a fan) is need at the outer side, especially for cooling.For temperature control it is a good idea to not only measure the temperature of the inside, but also the temperature of the other side of the peltier element, as this will be a mayor path for external perturbations.

author: timb
author_rank: Super Contributor
timestamp: November 30, 2017, 06:52:33 pm
content: Quote from: NANDBlog on November 30, 2017, 02:14:07 pmJust FYI, that circuit is designed for laser diode control and isn’t suitable for control for what the OP wants without a lot of modifications.The TECs integrated into these lasers are relatively small and run at lower voltages and currents (5V@2A or less), which is OK since they are cooling a relatively small thermal mass. (The laser, TEC and thermistor generally come as a complete, integrated package.)When used for scientific, medical and imaging applications these lasers require very, very, very high temperature stability (0.01c or better) to stabilize their output.The enclosure the OP is using is much larger than the type laser diode assembly that app note is targeting. Therefor the thermal time constants will be much greater, making it much more difficult to stabilize the loop gain bandwidth of the error amplifier. You’d also need dual supplies for that circuit (+-7V for a 5V TEC or +-15V for a 12V TEC) which is a pain.You’d be better off getting two power op-amps (TI makes a nice one capable of over 2A and up to 36V) and driving them with a DAC from a micro in a bridge configuration.For the temperature sensor, if you don’t need better than 1% absolute accuracy or 0.125c stability, you can’t go wrong with using the venerable DS18B20. A lot easier than fiddling with thermistors and it’s relatively low speed, so you don’t have to worry about noise. Also, it only requires two wires (data and gnd) so it’s easy to wire up.Then you’d simply implement a PID algorithm on the MCU and away you go. (There are plenty of existing PID controller libraries, written for every platform and every language, some of them pretty good, too!)

author: beanflying
author_rank: Super Contributor
timestamp: November 30, 2017, 10:51:06 pm
content: In my stash of bits I have a Willhi Peltier controller but a more dedicated one can get made at some stage. I haven't used this model before but their standard 240V fridge controllers work well. I will rig a couple of RTD sensors across the enclosure and do some playing on the bench. Bigger Peltier modules most likely needed?http://willhi.com/product_detail.php?product_id=115&lang=en

author: tszaboo
author_rank: Super Contributor
timestamp: November 30, 2017, 11:59:32 pm
content: Quote from: timb on November 30, 2017, 06:52:33 pmYou obviously haven't read it. loop compensation leads to AN89, where it revealed that the laser used has a temperature time constant measured in minutes. Also, the analog method is 2-3 orders of magnitude better at the temperature control. I have no idea, why would you need a micro with all the extra work, instead of changing an RC value, and looking at it with a scope. For worse results.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: December 01, 2017, 03:06:32 am
content: In a pinch you can try the ILX Lightwave LDT-5910(B). I have one that works well and uses standard thermistors like the 44006/44007. It can be controlled via GPIB and is a good match with the low power peltier modules. Don't expect Keithley 2510 performance but it can be bought in the $100 range.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 01, 2017, 10:50:20 am
content: Quote from: NANDBlog on November 30, 2017, 11:59:32 pmWether it makes sense to use a digital control with an µC instead of analog control depends on the time constants involved. The still rather fast Laser with a few minutes time constant might still work better with analog control, but for a larger and thus slower system it gets more an more attractive to use digital control, as the long time constants are difficult to to implement in analog hardware. Also with digital control anti windup is usually standard because it is easily implemented, while analog anti windup is tricky, especially if low drift is needed. Anti_Windup can be important if the limits of the heater / cooler are reached. Even for time constants in the minutes range, it takes quite some extra effort for the analog implementation - µCs and ADCs got better since AN86, so things have changes since than.p.s.:Another reason to use digital control is that peltier elements are nonlinear. Digital correction for this is easy, analog nonlinear functions tend to be temperature sensitive and thus drift can be a problem.

author: timb
author_rank: Super Contributor
timestamp: December 01, 2017, 11:13:37 am
content: Quote from: Kleinstein on December 01, 2017, 10:50:20 amThanks! That sums up part of what my reply was going to be.I have read AN86, but like Kleinstein pointed out, things have gotten better since that app note came out. Essentially, it takes a lot of trial and error to stabilize an analog control loop over long time constants. It’s much easier to modify the Kp, Ki and Kd values in firmware on a uC. Not to mention the fact that you can get PID libraries with auto tune functionality that will, at the very least, give you sane starting values to hand tune.If I needed precise 0.005c control of the temperature of a laser in a $500,000 piece of equipment I was building, sure, I’d go with a fully analog solution (both the control loop and the TEC driver) and use a DAC to control the setpoint.Or, if I only needed 0.01c control, I might use an analog control loop and filtered PWM drive of the TEC. (In fact, Linear makes a part that essentially takes an analog voltage on the input and drives four MOSFET’s in an H-Bridge configuration with high speed PWM on the output. It’s designed specifically for TEC control.)However, you can get easily get 0.1c stability with a MCU and off the shelf H-Bridge using filtered PWM drive of the TEc. 0.05c or better is achievable if you throw in a 12 or 14-bit DAC and go with a two power amps in a bridge configuration.Keep in mind, analog drive isn’t really feasible for TECs over 5V@2A. That’s OK for laser diodes, since most TECs coupled to them fall into that range. However, the OP may want larger TECs for his enclosures, in which case analog drive becomes harder. (You really need big heatsinks and forced air cooling for the power amplifiers or discrete transistors.)Also, for what the OP is doing 0.1c stability should be more than enough. So yeah, I’d much rather use a digital solution, as I could implement it a lot quicker. It would also scale to different sized enclosures a lot better than a purely analog solution would.

author: timb
author_rank: Super Contributor
timestamp: December 01, 2017, 11:17:32 am
content: Quote from: beanflying on November 30, 2017, 10:51:06 pmKeep in mind that’s not a temperature controller but a thermostat. It uses a relay and Bang On/Bang Off type control. Taking into account the built in hysteresis and overall accuracy, it’ll keep the temperature +-2c of the set point, absolute best case.

author: beanflying
author_rank: Super Contributor
timestamp: December 02, 2017, 01:02:57 am
content: Thanks TimB, I got it at the time more for interest as I have a couple of Peltier based fridges I use for my Coffee 'day job' but I have gone down a Arduino/RTD path for thoseHysteresis is adjustable down to 1C and an Offset to the NTC is built in to the firmware. But as you say commercial fridge applications only.The driver Chip is badged WH7016K and from memory the front board is the same as their other controllers so simple plug in to their stock controller/display board and a different output board to allow for the invert switching relays.

author: timb
author_rank: Super Contributor
timestamp: December 02, 2017, 07:34:43 pm
content: Quote from: beanflying on December 02, 2017, 01:02:57 amYeah, they’re great for basic temperature control, they’re just not meant for precision. And you’re right, that one does specify 1c hysteresis (though you still have to add thermal lag between the TEC and sensor causing overshoot into the stability figure). I must have been thinking of the specs of a similar looking unit from eBay that I’ve worked with in the past.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: December 03, 2017, 12:11:55 am
content: For heating-only application You may consider using self adhesive heater which may result in better temperature homogenity:https://www.tme.eu/gb/details/fg12v_35w/heating-elements/or similar

author: beanflying
author_rank: Super Contributor
timestamp: December 03, 2017, 12:55:42 am
content: Depending on whats in the box low power heating pads internally make good sense I think. To my way of thinking the benefit of a stable temperature internally somewhere above a 'normal' ambient is a more simple option than needing to heat and cool. Also means no need for fans or heatsinks for Peltier modules.In my case around 30-35 C fixed would do nicely, any hotter and it's time for a beer

author: cdev
author_rank: Super Contributor
timestamp: December 03, 2017, 01:00:39 am
content: Whatever you do to increase insulation and thermal mass around your application (you could use foam to create a cooler like box, the outer of perhaps two thermally insulation layers) might be an easy thing thats not likely to create any new problems. So much preferable to heating and cooling its worth increasing insulation and thermal mass to a substantial amount before adding minimal amounts of heating, IMHO.Thats just my thought, not an expert in this stuff.

author: beanflying
author_rank: Super Contributor
timestamp: December 03, 2017, 01:37:55 am
content: Plastic Pelican style case with the Aluminium heated boxes inside surrounded by pick and pluck foam might do the insulation job or even pluck some more and run blue foam outers for the aluminium?I have quite a few 5&6S 5000 LiPo's (beyond R/C use) around to run the box too.

author: timb
author_rank: Super Contributor
timestamp: December 04, 2017, 12:47:54 am
content: Quote from: beanflying on December 03, 2017, 01:37:55 amThat pick and pluck foam isn’t a great thermal insulator. I’d look for small styrofoam coolers (designed for shipping stuff with the addition of ice packs) which you can find cheap on Amazon in a myriad of sizes.You can also buy styrofoam sheets of various sizes that could be cut and glued together to make a form fitting insulated case for your metal boxes. (You really need a hot wire cutter to cut them properly, however you can rig one up with some PVC piping, metal fittings and nichrome wire; I made one up for $10 in plumbing supplies! Just hook it to a bench power supply and you’re good to go.)

author: beanflying
author_rank: Super Contributor
timestamp: December 04, 2017, 01:13:45 am
content: I have a hot wire cutter and blue foam from building R/C wings so a custom foam box is easy. Depron in 2,3 and 6mm is worth a look for circuit board insulation as it cuts clean with a sharp knife and is a more closed cell than white styrofoam.I have seen EPP or similar being used on some of the threads here which isn't a good insulator but it is certainly the most abuse proof and mechanically shock proof to keeps boards away from mechanical shock.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 04, 2017, 05:12:27 pm
content: When using a peltier element for control, there is no need to do a lot of insulation. The TEC will give quite some thermal coupling from the outside to the inside. It still makes sense to have the rest of the case with a thermal resistance that is lower than the TEC internal one, but no need to go very much below that. Some insulation is good, but no need to overdo it.As much of the heat-flow in will also be through the TEC, it really makes sense to measure the temperature on both sides of the TEC and not just the inside.

author: timb
author_rank: Super Contributor
timestamp: December 04, 2017, 11:58:39 pm
content: Quote from: Kleinstein on December 04, 2017, 05:12:27 pmHe was considering using heating pads, cylindrical elements or power resistors for heating instead of a TEC. In which case good insulation can really help improve set point stability.Quote from: beanflying on December 04, 2017, 01:13:45 amI’ve also made custom insulated enclosures by taking a larger plastic project box and filling it with that expanding spray foam stuff, then placing a smaller metal project box inside (using small shims to elevate it). Once the expanding foam dries, it can be easily cut and sanded.

author: beanflying
author_rank: Super Contributor
timestamp: December 05, 2017, 12:59:08 am
content: No problem at all hijacking the thread into a more general discussion on ideas and solutions. I partly started the thread as the site search only yielded some odd posts.I will drill and tap out a test enclosure in the next few days for sensors and power in. Also make up a few dummy thermal blocks from foam covered sheet for mock boards.

author: beanflying
author_rank: Super Contributor
timestamp: January 09, 2018, 10:49:08 am
content: Minor change of plan for my internal needs. Still looking at methods of making a portable stable Reference/s as well.Took delivery of this 'former' 6 bottle wine cooler today. Less than $60 delivered. Waeco branded but available around the world it seems under the part number MF-6W. It runs a removable (clip in) Laptop type power supply and there is already a 12 input plug so swapping out this one for either a linear or 12V battery based supply is really easy.Typical Peltier cooler/heater module on the back and the smarts above the door in the front. 5-20 degrees C rated and driven by a Samsung S3C9454B 8 bit micro.https://www.insidegadgets.com/wp-content/uploads/2014/09/3F9454BZZSK94.pdfTemperature Hysterisis seems nearly 2 degrees with the stock controller and fitted sensor and driven via an on off relay like most of these types.Even though it supports PWM and a bunch of other nice features I plan to junk the controller board rather than learning about Samsung's programming/assembler etc. The Plan is an Arduino based controller, 4 wire RTD, Peltier Cooling and non Peltier warming pads or elements. Locally it will spend 75%+ of it's time needing warming to a set point of 25 to maybe 30 degrees (haven't decided yet) so as Peltier efficiencies SUCK heating by other means makes sense but using the Peltier for cooling. Generally sticking with 12Vdc Linear control methods to minimize the noise to the DUT's.This isn't meant to become an environmental test chamber much more of a stable reference enclosure be it Volts, Temps or Resistance. Internal Airflow is very low but internal protection from that will be done and at this stage laying it on it's side in the rack at the top (peltier hot end outside the rack), 34970's below it, Mini ITX PC Rack mount, UPS and spare batteries at the bottom.Any thoughts are welcome

author: cellularmitosis
author_rank: Unknown
timestamp: January 09, 2018, 06:22:10 pm
content: Quote from: beanflying on November 30, 2017, 10:51:06 pmI picked up one of cheap ebay knock-offs of that controller and was really disappointed with the results. Every time the relay switch on, I could see the temperature readout jump by several degrees (instantly). I suspect the ground current of the relay is influencing the NTC reading (i.e. they didn't pay attention to their ground current paths).Hopefully the Willhi unit behaves better!https://www.ebay.com/itm/DC-12V-F-Fahrenheit-Temperature-Controller-Thermostat-Control-1-Relay-NTC-Sensor/141777090187?epid=595237246&hash=item210292b68b:g:lOgAAOSwQPlV-i0Y

author: beanflying
author_rank: Super Contributor
timestamp: January 10, 2018, 03:18:48 am
content: If anyone buys a second hand peltier cooler full strip and clean inside and out. Major fluff magnet!Temporary 2k series NTC Bodge gets the stock controller to 25 while reading 20

author: cellularmitosis
author_rank: Unknown
timestamp: January 10, 2018, 07:25:00 am
content: So, if you were to roll your own h-bridge to control a TEC using PWM, and you were to use an LC filter to protect the TEC, do you need two filters, as in the diagram below?Initially I thought you could get away with just one filter (in series with the TEC), but wouldn't it only act as a low-pass filter in one direction?

author: cellularmitosis
author_rank: Unknown
timestamp: January 10, 2018, 07:33:44 am
content: Some additional googling indicates that yes, two LC filters are required.http://www.ti.com/lit/an/slua202a/slua202a.pdf

author: beanflying
author_rank: Super Contributor
timestamp: January 10, 2018, 07:48:03 am
content: Still thinking about the basics with long term power consumption being fairly important. Even for cooling to drop the power consumption I could run a scavenge loop from my couple of commercial fridges and freezers in the shack using water/coolant and pump and remove the peltier from use all together. For reasons of trans portability and independence I will still likely use it as the number of sustained 25 degree weather we get is minimal.Heating will almost certainly be as I put a few posts back with Linear controlled 12Vdc elements. Providing the noise is manageable the gear under it should help limit the heating needs. Managing the thermal drift of the 34970's is another issueA couple of internal temp sensors and an ambient external to help set the PID settings and minimise over and undershoot at this stage.

author: beanflying
author_rank: Super Contributor
timestamp: January 11, 2018, 06:37:27 am
content: Clearly the designer of this was deluded as to the needs of Beer Drinkers 6 pack minimum required. 30-36+C in the shack today.Waeco holding 25 +-1 and a bit with ease

author: cdev
author_rank: Super Contributor
timestamp: January 22, 2018, 02:57:46 pm
content: Flat foam core board is extremely easy to cut neatly to any size you want, it can also be layered, and it also is styrofoam so it insulates.But its not fire safe. Styrofoam is very flammable.

author: cellularmitosis
author_rank: Unknown
timestamp: January 22, 2018, 06:12:16 pm
content: My little peltier-based chamber for measuring resistor tempco's worked out fairly well:https://www.eevblog.com/forum/projects/mini-tempco-characterization-rig/I'm currently working on a resistive (heating only) version of a similar box, which is intended solely for keeping components or a small board at exactly 25C.It consists of 12 10R resistors affixed to the bottom of a Hammond 1590BB using Arctic Silver thermal epoxy, insulated with 1/4 inch "craft foam".I hooked it up to a 12V supply (actually, at 95mA, to simulate 12V - 0.6V for a 2N3904, so about 1.1W) and let it run unregulated overnight, and it looks like it maxes out around 28 or 29C (in 72F ambient), so it should be able to regulate at 25C.

author: cellularmitosis
author_rank: Unknown
timestamp: January 23, 2018, 01:11:09 am
content: Here's a useful trick from anold threadto apply both amplification and offset to a linear temperature sensor (e.g. theMCP9701A), using just an op amp and three resistors. Thanks again, T3sl4co1lHere, I have 20C to 30C mapped to 0 to 5V, which interfaces well with an Arduino.I've included the LTSpice circuit with this post, so that you can dial in the values for your temperature range of interest. R1 and R2 control the amplification, and R3 controls the offset.To demonstrate how to modify the circuit, I've also included a screenshot of values which map 20C to 50C to the 0 to 5V range.

author: cellularmitosis
author_rank: Unknown
timestamp: January 23, 2018, 06:47:57 am
content: The Arduino's 3.3V rail is a bit less noisy than the 5V rail. Here are values for a 3.3V version.

author: TiN
author_rank: Super Contributor
timestamp: January 23, 2018, 10:10:30 am
content: Quote from: cellularmitosis on January 22, 2018, 06:12:16 pmI'm sorry, but it just looks like you don't value your time and going bit wrong direction, gluing resistors not designed for heatsinking to the box.I'd go proper TO-220 packaged ones (example), or at least heatsinked cement ones (e.g.). Yes, yes, it's 20$, but at least you not guessing will it work or not...

author: cellularmitosis
author_rank: Unknown
timestamp: January 23, 2018, 11:00:24 am
content: Quote from: TiN on January 23, 2018, 10:10:30 amActually, I’m waiting on my polyimide heaters to arrive from China. I have three weeks to kill, so I thought I’d give this a shot in the mean time.All I need to do is maintain my current source shunt resistors within 0.1C, which is 100x smaller than the 10C ramp I’ll use to test the tempco of resistors. This should be adequate.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 23, 2018, 02:42:16 pm
content: The alternative to using resistors as a heat is to use transistors not only for control, but also as the heater. So no more loss at an external transistor and no PMW that might produce EMI problems.

author: Magnificent Bastard
author_rank: Regular Contributor
timestamp: January 23, 2018, 03:33:04 pm
content: Quote from: Kleinstein on January 23, 2018, 02:42:16 pmAlso, perhaps series high-wattage Zener diodes as well as the heat from the control transistor. This would change the square-law power term (with only a resistor heater) into a more linear one-- resulting in better temperature control.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 23, 2018, 04:00:30 pm
content: Clearly this is an electronics forum, with many comments on circuit configuration, loop stability and the like, and only one comment on actual thermal modeling of the environment, plus Kleinstein's comment on heat distribution. Admittedly thermal modelling software is every bit as complicated as EDA software, and generally just as pricey so most of us have little access to it, but that isn't an excuse for ignoring key parts of the problem.There needs to be attention paid to the fundamentals of the problem. Which elements (if any) need to be isothermal? Which need to be stable over time, but not necessarily isothermal? What are heat paths to and from those elements? What are the heat capacities and thermal conductivity of those pieces? What are the overall control loops, including those elements? Note that making heater power much greater than heat flow out makes control inherently non-linear and complicates stabilization. This is one advantage of TEC as you can get better balance to these flows.For a reference oven, where there is much less time pressure to reach the operating temperature, no variation in the thermal characteristics of the controlled item, and little or no need to vary that temperature the inherent reliability of a mechanical solution (insulation, nested boxes etc) combined with a really simple outer control loop will be the best choice. Very different case than an environmental test oven.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 23, 2018, 05:06:22 pm
content: Quote from: Magnificent Bastard on January 23, 2018, 03:33:04 pmOne can still get a linear control with a resistor, if PWM is used. However with voltage or current control the resistor is square law, while a transistor at a constant voltage and with current control (e.g. with a small emitter resistor) is about linear, which makes control easier.For power distribution one could still use transistors in parallel or just normal diodes in series. It is also possible to use a 2 nd transistor in series (with a base voltage at about 50% or the supply) - which could also act as an emergency off.A reference heater only needs to be constant and might thus be a rather simple linear analog controller. For a test chamber one might want faster control and a programmed ramp - thus more like a digital control. This could compensate for a nonlinear heater (a peltier element is also nonlinear, when used at relatively high currents).

author: Andreas
author_rank: Super Contributor
timestamp: January 23, 2018, 10:42:52 pm
content: Quote from: cellularmitosis on January 22, 2018, 06:12:16 pmHello,I don´t know your room temperature in summer.But I have changed the reference temperature to 30 deg C to have some headroom even with somewhat higher room temperatures.with best regardsAndreas

author: cellularmitosis
author_rank: Unknown
timestamp: January 23, 2018, 11:35:46 pm
content: Quote from: Andreas on January 23, 2018, 10:42:52 pmI am a spoiled American. My room never gets above 72F (22.2C). I am curious though how thin of a margin I can pull off and still maintain regulation!

author: bertik
author_rank: Contributor
timestamp: January 24, 2018, 02:33:01 am
content: Have a look at my Arduino project of a while ago:http://hololaser.kwaoo.me/electronics/Arduino_TEC/Arduino-TEC.html

author: cellularmitosis
author_rank: Unknown
timestamp: January 24, 2018, 03:51:01 am
content: Quote from: bertik on January 24, 2018, 02:33:01 amwow, stable within one millikelvin!

author: Pipelie
author_rank: Regular Contributor
timestamp: January 24, 2018, 04:45:04 am
content: Have a look at this one, they have very nice software run on windows.https://item.taobao.com/item.htm?spm=a230r.1.14.18.6985ef74GxyII7&id=535913852209&ns=1&abbucket=17#detail

author: TiN
author_rank: Super Contributor
timestamp: January 24, 2018, 04:50:07 am
content: I have no problems for 5 mK stability over long time with my crude setups and fancy Keithley 2510. Thermal sensors are either Honeywell HEL-705 RTD or YSI 44006.Setup with ILX 5910B and YSI 44007 shows simialr performance too, with much smaller cost then 2510.

author: Pipelie
author_rank: Regular Contributor
timestamp: January 24, 2018, 05:16:07 am
content: Quote from: TiN on January 24, 2018, 04:50:07 am10 mK stability over an hour period is enough for me, find some test results, temperature measure by Shimaden FP23 with a Pt100 RTD.

author: Pipelie
author_rank: Regular Contributor
timestamp: January 24, 2018, 05:23:17 am
content: the Ambient temperature is around 15C, all the resistor under test are in the chamber expect SR-104.

author: cdev
author_rank: Super Contributor
timestamp: January 24, 2018, 02:24:13 pm
content: TimB,I would be really interested in hearing just a bit more about your project.Those cheap coolers look like a perfect start for that.Quote from: timb on November 30, 2017, 01:49:29 pm

author: ramon
author_rank: Regular Contributor
timestamp: January 24, 2018, 03:14:30 pm
content: Quote from: CatalinaWOW on January 23, 2018, 04:00:30 pmTo add some example to those questions I will add the info about patent 3,028,473 - Temperature stabilized oven, by George A. Dyer and Roy J. Patterson (Filed March 12, 1959) North American Aviation'simple, highly reliable, and extremely compact device which is capable of regulating temperature within .01º F'1x Thermistor 51A3 (Victor Engineering Company)1x Thermistor D054 (General Electric Corp.)1x NPN 690T11x NPN 2N5471x Zener 911D181x Heater resistor 120 ohms1x Heater resistor 30 ohms1x Pot 25K1x Pot 2K1x Res 4.22KI think it is an interesting (20 minutes, 4 pages) reading.

author: cellularmitosis
author_rank: Unknown
timestamp: January 24, 2018, 06:56:09 pm
content: Thanks ramon!For the lazy reader:

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 24, 2018, 08:55:46 pm
content: Might not be obvious to all, but the heater in the patent application uses cordwood packaging. Well suited to this purpose.

author: Vgkid
author_rank: Super Contributor
timestamp: January 24, 2018, 11:40:46 pm
content: Quote from: bertik on January 24, 2018, 02:33:01 amThat is your site , cool. That was the site that got me learning about tec temperature controlling.

author: cellularmitosis
author_rank: Unknown
timestamp: January 26, 2018, 01:20:44 am
content: Quote from: cellularmitosis on January 22, 2018, 06:12:16 pmI hacked up a quick proof-of-concept on protoboard, which worked, but was noisy, and followed this up with a "proto shield" for Arduino. The results are okay: stable within about 0.05C (verified by using a Si7021 as the DUT)Unfortunately there is a large measurement offset which occurs when the 12V power is switched on. I had attempted to create a bit of a star-ground, but there must be some current somewhere which is lifting up either the op amp or the MCP9701A.Nope, the op amp is being powered by the 12V rail, so naturally cutting its power will disrupt things...Also, the ~1W max output turned out to be not quite sufficient. We had a cold front come through, and my apartment dropped from 72F to 67F overnight, and that was enough to cause the box to fall out of regulation.Also, I haven't tuned the PID yet at all.This is probably sufficient for what I needed it for, but I'd definitely like to improve upon it.https://github.com/cellularmitosis/logs/blob/master/20180124-25c-chamber/README.mdedit: I suppose it is also possible the measurement offset is being caused by a magnetic field generated from the loop which the heater resistors form.

author: cellularmitosis
author_rank: Unknown
timestamp: January 26, 2018, 05:21:21 am
content: Here's the circuitedit: forgot the cap on LM358 pin 6

author: cellularmitosis
author_rank: Unknown
timestamp: January 26, 2018, 05:58:36 am
content: omg, the op amp is being powered by the 12V rail... of course there is a "measurement offset" when you cut the 12V supply...

author: Andreas
author_rank: Super Contributor
timestamp: January 26, 2018, 03:04:25 pm
content: Quote from: cellularmitosis on January 26, 2018, 01:20:44 amno wonder:I use usually 2 heater foils 12V/12W so 24 Ohms in series from a 17V unstabilized supply.So I have 12W to heat up from room temperature (around 20 deg C) to the 30 deg C of my reference resistor heater.(ok I need only around 20-25% of the power).The 12W can go up only to around 50 deg C in my cooling box. (but that is sufficient for the 40 deg C that I need).with best regardsAndreas

author: ramon
author_rank: Regular Contributor
timestamp: January 27, 2018, 01:21:49 pm
content: Search google for 'MCH'. It's a high efficiency heater.I bought a 10x10mm MCH with 20 ohms or 40 ohms resistance (I bought two with different resistance and didn't remember which one was. In the worst case you can get +75C with only 1.25W).This is the temperature test with PT100:Code:MCH 10x10 with PT100 : (ambient temperature around 20-22ºC)@ 0.0V -> 108.62 ohms 22.11ºC@ 0.1V -> 108.62 ohms 22.11ºC@ 0.2V -> 108.63 ohms 22.13ºC@ 0.3V -> 108.72 ohms 22.36ºC@ 0.4V -> 108.82 ohms 22.62ºC@ 0.5V -> 109.03 ohms 23.16ºC@ 1.0V -> 110.63 ohms 27.29ºC@ 1.5V -> 113.29 ohms 34.16ºC@ 2.0V -> 116.58 ohms 42.67ºC@ 2.5V -> 120.25 ohms 52.20ºC@ 5.0V -> 142.10 ohms 109.48ºC@ 7.5V -> 165.00 ohms 170.61ºC

author: cellularmitosis
author_rank: Unknown
timestamp: January 28, 2018, 08:39:13 am
content: Made a few adjustments (reduced the heater from 10 to 8 resistors), hooked up a second Si7021 to track ambient temperature, and spent the evening fiddling around with the PID parameters.I got it stable within +/-0.01C across a 1C ambient excursion!details:https://github.com/cellularmitosis/logs/tree/master/20180126-25c-chamber-tuning

author: Andreas
author_rank: Super Contributor
timestamp: January 28, 2018, 07:03:46 pm
content: Hello,not bad.I get similar performance for a PC-based PWM algorithm.Switching every 1 second either on or off depending on the integrated measurement deviation.And all via RS232 (RTS) signals with a simple FET.for the 1 minute averaged values I see 0.012 K deviation (without noise) for a 2 deg C environment change.within 1 minute the typical change is another 0.01K (0.35mV at NTC with 55mV / K sensitivity)picture:green: environment temperature (measured by ADC3 temperature sensor so contains 2-3 deg C self heating)red: tempeature at reference resistor (setpoint 30 deg C). Lagging somewhat due to low amplification to avoid oscillation.blue: heater power in % (from 12W).With best regardsAndreas

author: texaspyro
author_rank: Super Contributor
timestamp: January 28, 2018, 07:31:29 pm
content: Lady Heather has a temperature control pid routine that is used to stabilize a GPSDO environment. It uses two serial port modem control lines (for enable and heat/cool) to PWM a fan at 1Hz (could also drive a heater/cooler). The usual implementation is to mount the GPSDO in a box (usually with some added thermal mass... I use a simple cardboard box) and use a fan to blow air in. The temp control point is set below what the self-heating of the GPSDO is.Attached is a plot of Heather stabilizing a Trimble Thunderbolt over 72 hours. It uses the temperrature value that the Thunderbolt reports. Over 72 hours the RMS error was below a microdegree. Instantaneous temperture change is in the low millidegree range.I have an SR-104, a Thunderbolt, and a 10V ref installed in a box with the temp control point set to where the SR-104 is at exactly 10000 ohms. I also use the same algorithm on an ATMEGA-2561 touch screen micro to control the temp in my reflow toaster.

author: cellularmitosis
author_rank: Unknown
timestamp: January 28, 2018, 10:03:44 pm
content: I worked up an LTSpice simulation using 10k thermistors rather than the MCP9701, and spun that up as an Arduino shield Kicad project. This design uses mostly "jelly-bean" parts, and also has an on-board thermistor for ambient temp. Note that the same resistor values work for a 5V supply as well.github:https://github.com/pepaslabs/isothermalOSHPark link:https://oshpark.com/shared_projects/cw8b4XGmhttps://oshpark.com/shared_projects/9wWvXwMwWhen the boards arrive I'll solder it up and post here again.I also ran my current prototype (using MCP9701) for 8 hours and it (mostly) stayed with +/-0.01C the entire time. Settings: kp = 10, ki = 0.2, kd = 0, period = 3000ms, setpoint = 25C (406 ADC counts).I've also attached the current Arduino sketch.Edit: Ah, shit. I accidentally swapped the position of the thermistor and fixed resistor. Luckily I can just invert the value of the ADC in software.Edit2: attaching schematic.

author: cellularmitosis
author_rank: Unknown
timestamp: January 29, 2018, 05:58:06 pm
content: So I thought about this a bit more and realized that the Arduino's 3.3V reference (and ADC) do not have a zero tempco.So when the temperature of the Arduino changes, it effectively changes the temperature reading, which will introduce (undetected) error into the temperature control.I epoxied an additional thermistor into the enclosure and hooked it up to my HP 34401A in 4-wire mode, and used a python script to convert from resistance to temperature.I then logged the Arduino's notion of its error against the HP's notion of the error (see chart1.png). Turns out they disagree!However, the smoking gun I need to is compare the difference in those two errors to the change in ambient temperature. If those are correlated well, that's evidence that the tempco of the Arduino directly causes additional undetected error in the temperature control loop. And a smoking gun it is indeed! (see chart2.png).Edit: looks like the Arduino tempco introduces a control error of 0.02C per ambient C.Edit2: (and that's assuming my HP 34401A has a zero tempco

author: Andreas
author_rank: Super Contributor
timestamp: January 29, 2018, 09:12:04 pm
content: Quote from: cellularmitosis on January 29, 2018, 05:58:06 pmIf you have a ratiometric measurement then it should be only the drift of the amplifier.Thats why I use my 24Bit ADCs also for temperature measurements.(although the noise of local air drafts results in much less than 24 Bits).In this case I have a full ratiometric measurement. And the drift of the ADC itself is negligible.With best regardsAndreas

author: PartialDischarge
author_rank: Super Contributor
timestamp: January 31, 2018, 03:53:56 pm
content: I'm currently working on a 10V heated reference totally homebred. It is based on the AD588KQ and is 'double' everything:The power supply is linear and has double 78xx 79xx pairs to improve line regulation.The whole enclosure is metallic and the AD588 will be enclosed in a small copper box too (double shielding)Also the inside of the enclosure will be covered with heat insulating material, as well as the small copper box for the AD588 (double thermal insulation) (https://uk.rs-online.com/web/p/thermal-insulating-sheets/7248909/)Finally the temperature regulation is based on the TMP01 for its simplicity of use. A resistor switched to the mains rectified voltage serves as the heater. Also a DC fan running at low speed on the inside of the enclosure will try to keep the temperature constant.

author: cellularmitosis
author_rank: Unknown
timestamp: March 15, 2018, 08:09:28 am
content: Quote from: texaspyro on January 28, 2018, 07:31:29 pmThanks texaspyro, I just picked up a second 34401A and I’ll be trying the box + PWM fan method to see if I can give one of them a thermally regulated ambient environment, while I leave the second 34401a flapping the in breeze. Hopefully I’ll be able to see if my resistance measurement noise floor is due to the drift of my meter, or some other source.

author: cellularmitosis
author_rank: Unknown
timestamp: March 20, 2018, 05:46:18 am
content: Quote from: cellularmitosis on March 15, 2018, 08:09:28 amFirst attempt: a small appliance light bulb inside of a cardboard box. There is an inlet hole and on the outlet there is a small fan affixed inside of a cardboard (toilet paper roll) tube. An arduino reads the temperature of an Si7021 (suspended in the middle of the box by another cardboard tube) and controls the duty cycle of the fan (on a ~400ms period).Even with a crudely tuned PID loop, it was easy to hit +/-0.1C, in an environment which showed a 1C swing (measured outside of the box the day before).Next, a box big enough to fit a 34401A into...The first plot is the ambient temperature as measured the night and morning before this run. Second plot is the first 30-minutes (warmup) of the PID control, last plot is an all-day run.(arduino code also attached)edit: the fan is one of these 38mm units:https://www.ebay.com/itm/222273911362searching for "video card fan" seems to produce these "shroudless" fans which can be mounted inside of a tube.

author: cellularmitosis
author_rank: Unknown
timestamp: March 20, 2018, 06:08:14 am
content: (bonus analysis of the first plot, because it's fun

author: beanflying
author_rank: Super Contributor
timestamp: March 20, 2018, 07:20:58 am
content: Finally getting back to this after a busy Summer.Finished the rack ears and joiner for my 34970's today, 700W UPS fitted with a full load of new AGM's and a soon to be added temperature controlled fan (this model didn't have a fan), PC is a work in progress as are the extra Ref's and Batteries.While this will be home to the References and my couple of Weston Cells and Resistors I am still playing with a portable stabilized enclosure which should be able to sit inside when at home. The more full the fridge is within reason the more stable it will be.The plan is to keep the Rack Case in the region of 25C using heat scavenged off the UPS and other gear with a few fans if needed to minimise the power the fridge consumes so a small string of sensors placed around the Rack driving the fans as needed. As the Rack is going into a wheeled bench it can be moved around to help keep it cool or warm depending on the seasons.The Fridge gets Ears tomorrow

author: bsdphk
author_rank: Regular Contributor
timestamp: April 02, 2018, 10:04:30 pm
content: The word "fridge" has been used a lot of times here, but without going all the way.What you really want *is* a fridge, and then you want to do some serious case-modding.The first thing to realize about temperature stabilization is that you need to think about both the energy-flow and the time constant.Energy-flow first: How much electricity do you feed into your circuit? - You'll have to get rid of all of it again with your cooling system.If you want stable temperature, the smartest thing you can do is to have stable energy-flow.If you really want to go hard-core with this, you do not have anything which switches on/off, only things which send the same exact amount of energy in or or the other direction to keep the total energy-flow is constant.Think in terms of running idle current through your calibration resistors also when you do not measure them, think in terms of Class-A amplifiers for anything regulated (ovens around Xtals, Rb's etc.)Time-constant is a thing people almost always overlook: You can either go high or low.If you go low time constant, you have as little (thermal) mass as possible in your controlled zone, and you use an aggressive airflow to keep the temperature stable.This is far more tricky than it sounds, but is nice if you want to open the door and experiment with things. (This is why professional temperature chambers are expensive.)If you go high time constant, you add (thermal) mass in your controlled zone, to help keep the temperature stable in time, just like you add big capacitors to keep voltages stable in time.There are many ways to think about that time constant, there actually is something called "Thermal impedance" and you can get far by using Spice to simulate it.So back to the fridge thing: If you want a cheap large enclosure with a stable temperature, find an old fridge.Get rid of the compressor but first get somebody to remove the gas properly, we need that Ozone layer.Now use the cooling circuit piping to circulate water instead, this allows you to remove quite a lot of heat from the interior, if you need to.(You will want to replace the thin high pressure tubing, you cant get enough water through that.)You probably want to control the inlet temperature of the water. Start with a variable speed fan on the external radiator and see where that gets you, upgrade to peltier if necessary.Finally, if you do not plan to open the door all the time, add thermal mass *and* impedance. Some slices of cinderblock works great for that.And obviously: Drill some holes for the test-wires, but remember copper is a very good thermal conductor.Other hints: A small camera can save a lot of door-openings if you need to monitor indicator lights./Poul-Henning

author: cellularmitosis
author_rank: Unknown
timestamp: April 03, 2018, 03:58:43 am
content: Glad to see you on the forum Poul-Henning, I was wondering when you would make the jump from the volt-nuts mailing list

author: beanflying
author_rank: Super Contributor
timestamp: April 03, 2018, 07:09:56 am
content: Re loading and thermal stability. Agreed a space with more mass is more stable. Had a customer who complained to a fridge mechanic about a fridge that never 'stayed cold" 300L of commercial fridge holding 30-50L opened 10 times plus an hour

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 03, 2018, 03:44:57 pm
content: Thermal mass is good. In cases where you have non-linear inputs (doors opening) a non-linearity in the control can help. The heat of a phase change simulates an enormous thermal mass, and depending on the temperature you are aiming for is fairly easy to implement. Water is great for temperatures below freezing. There are waxes that can help at high temperature.

author: texaspyro
author_rank: Super Contributor
timestamp: April 05, 2018, 01:13:30 am
content: I recently (mostly) finished adding external environmental sensor support to Lady Heather. You can use the sensor as the primary "receiver" device or in conjunction with any of the "receivers" that Lady Heather supports (except currently the HP-5071A which uses the same plot queue entries as the environmental sensors). Heather supports humidity, pressure, and two temperature values.I am currently using a dogratian.com USB-PA sensor with temperature, humidity, and pressure. I am also designing a Heather specific board (BME280, two thernistors, temperature controller interface, maybe a couple of ADC channels, etc). Are there any recommendations for other off-the-shelf sensors worth looking at? BTW, you can poll data from the USB-PA anytime, but the readings only update every 5 seconds.The main requirement is that the sensor should send data over a serial port or virtual serial port or maybe ethernet. Ideally it would stream readings at 1 Hz, but a polled device (like the dogratian.com devices) can be accomodated. Also, it would be very nice if the temperature sensors are small, responsive, and on leads that could be attached to whatever is being monitored.Attached is a screen dump of the USB-PA running. Can you spot the furnace cycling and sunrise?