Inexpensive Peltier Temp Controller? - Page 1
Forum section: electronics / general

author: eas
author_rank: Frequent Contributor
timestamp: June 03, 2015, 02:05:38 am
content: I'm looking for a TEC controller to use to retrofit a used Cuisnart thermoelectric "wine celler" I bought to use as a controlled temperature chamber for testing lithium ion batteries, and misc devices.The unit has a TEC1-12706 module in it. The existing controller supports a reasonable operating range given the original intended use, which includes the temperature region I'll be doing most of my testing, but it would be nice if it could cool a bit lower, and could also run the current in reverse to allow heating above ambient.I'd also like tighter and more accurate control of temperature. As it is now, it seems to use a 20 minute cycle and runs the cooler about 50% of the time. During the cycle, the temp swings <0.5C. Set to 15C, with a couple of pounds of gel packs inside for thermal mass, but no DUT, the chamber holds to a 1-1.5C band over a day, with a clear daily cycle to the drift.Can anyone suggest an inexpensive temperature control module for TECs that supports heating as well as cooling, ideally one that can vary the current rather than just switching it on and off? Barring that, can anyone suggest a generic heating/cooling control module + supporting components/modules that could be easily adapted to analog control of a TEC? I am up for wiring together a couple modules and doing a bit of configuration, or putting together a well-enginered kit.I foundone controller, specifically for TECs. The $22 price is right in the ballpark, but it seems to use an internal relay, so it can't control current to maintain efficiency. I'm surprised there aren't more solutions out there, given the number of people selling TEC kits with heat sinks, fans, even the DC power supplies. It makes me think there a suitable generic solution that also works well for TECs.Thanks!

author: Andreas
author_rank: Super Contributor
timestamp: June 03, 2015, 04:17:13 am
content: Hello,there are several issues when you want to reverse polarity on your TEC.With a temperature difference the TEC will act as voltage generator.So if you reverse polarity immediate a large current will flow (may overload your power stage or your TEC).Usually you would have to wait about 5 minutes cooling down with TEC off before you switch polarity.It might be more easy to add a small heater (foil) to achieve higher temperatures.https://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/msg462298/#msg462298With best regardsAndreas

author: awallin
author_rank: Frequent Contributor
timestamp: June 03, 2015, 05:17:54 am
content: Depends on what you see as inexpensive... these are quite good and start at around $100http://www.teamwavelength.com/products/temperaturecontrollers.phpI think most controllers use PWM for current control. Keeps the controller cool, but in some cases might cause unwanted spikes/noise at the PWM frequency.I've built two linear bipolar current controllers, one based on a TI app-note. It uses 4 transistors in a H-bridge and a sensing-resistor + instrumentation amplifier to measure the current:http://www.anderswallin.net/2013/08/tec-drive-prototype/my second try at this is simpler using a 2A op-amp OPA569:http://www.anderswallin.net/2014/09/tec-drive-v2/These are just constant current drivers. You also need a sensing circuit (thermistor in a bridge, with a voltage reference matching your price/performance goal?) as well as a PID controller (for an analog PID circuit the I-term might be challenging if your time-constants are long - if so use a microcontroller with e.g. LTC2440 digitizing the temperature reading).If you want to do credible science/serious-hobby, don't trust the in-loop temperature sensor. Build an independent out-of-loop temp. sensor, log that to computer, and use it to evaluate how good your PID-controller works.HTH,A.

author: eas
author_rank: Frequent Contributor
timestamp: June 03, 2015, 05:46:42 pm
content: Quote from: Andreas on June 03, 2015, 04:17:13 amAndreas, thanks for the warning about a potential trap. At this stage, it sounds like one more reason for me to look for purpose-built module, or well-understood combination of modules.Thanks too for the pointer to the description of your setup for resistor measurements. I've considered adding a resistive heater, but for now, for simplicities sake, I'd prefer it if I could use the TEC for heating and cooling. In the longer run, I may have the need for both greater precision and/or sustained operation at higher temperatures. When that time comes, I'll probably use a separate heating element.awallin. Thanks for the pointer to the teamwavength products. $100 is more than the $50 budget I had in mind, but not enough more that I wouldn't pay it if I was confident in it solving my problem. Unfortunately, it looks like the best they can do for $100 is a component (no user interface) that can do 5A. It looks like the lowest price for a module is $200, and one that would give me the headroom to upgrade to two TECs starts at $300.Odd that they use PWM. At typical PWM frequencies the thermal stresses probably aren't an issue, but the resistive heating inefficiency would be. I haven't read their app notes, maybe they recommend a filter to smooth the current?Thanks too for the links to your linear driver designs. Its great to have a design that has gone through a couple iterations to start from, if it comes to that, but I think I'd probably up my budget before things get to that point.I'll definitely be using a separate sensor for verification and logging of experimental conditions. Right now I have a cheap USB temp logger, but the foundation of my measurement setup is a Keithley 2000 or 2700 with a switcher card. My plan is to take 3 readings per cell under test -- Cell voltage, current shunt voltage drop, and temperature. That should leave me with some extra channels for chamber and ambient temp.I think for now, I'd be pretty happy if I can just manage to get it to hold to a 0.5C temp band. Ideally that would also eliminate obvious daily drift.

author: macboy
author_rank: Super Contributor
timestamp: June 03, 2015, 07:01:03 pm
content: I only know a little about temperature controllers, but I do know that you want aPIDcontroller, not aThermostat. A thermostat will only cycle on/off as the temperature goes above/below the set temperature. This results in the temperature swinging up and down a little bit. That swing is often adjustable to prevent rapid cycling in systems like those using a conventional compressor-based heat pump. A PID controller aims to maintain the set temperature tightly, with minimal overshoot, by responding to the delta between set/actual (P term), the rate of change of the actual temperature (D term), and the long term error (I term). A PID usually cycles the heater (or cooler) rapidly; I would stop short of calling it PWM but it may cycle on/off roughly once per second or so. This should work fine with a thermoelectric device, but make sure to keep the fan running constantly.

author: eas
author_rank: Frequent Contributor
timestamp: June 05, 2015, 12:50:51 am
content: From what I've learned so far, PID control isn't limited to things that can be controlled with on/off, and indeed, there are PID controllers on the market that can output an analog voltage.Assuming I find one in my price range, I guess the next step is figuring out an inexpensive yet robust way to use the output voltage to control up to 5-10A of current from a ~12V DC supply.

author: Bassman59
author_rank: Super Contributor
timestamp: June 05, 2015, 05:19:25 pm
content: Quote from: eas on June 05, 2015, 12:50:51 amGet a microcontroller with a DAC, an op-amp, a MOSFET, and there ya go.

author: eas
author_rank: Frequent Contributor
timestamp: June 05, 2015, 07:04:21 pm
content: Quote from: Bassman59 on June 05, 2015, 05:19:25 pmNo. That puts me in the position of knowing enough to know I don't know what hidden traps my befall me. A quick look at people building analogous devices, dummy loads, suggests that while some people can knock one off quickly, getting it to the point that its robust takes some revision. I do know enough to know that the MCU is superfluous if I'm already using a PID controller and content dealing with some heat though.I thought that this would be a common enough problem in industrial control that I could integrate a couple of commodity-priced modules to get what I was after. Perhaps my guess was wrong.

author: PlainName
author_rank: Super Contributor
timestamp: June 06, 2015, 08:57:11 pm
content: QuoteNot necessarily. Nowadays, a MCU is often cheaper to use as a simple timer than a 555 with associated caps and stuff. Doesn't suffer from drifting tolerances either. Saying a job is too trivial for a MCU, or you already have one so don't need another, is just displaying elitistic bias that should have been left behind years ago