DIY Climatic Chamber - Need a voltage controller / buck converter - Page 1
Forum section: electronics / projects

author: roberthartung
author_rank: Newbie
timestamp: July 29, 2017, 04:04:04 pm
content: Hello everyone,I have bin a long-time fan of Dave and watching all of his videos. Now, that he always advertises his forum, I hope to find some help here. I have built climatic chambers for research purposes, that are based on Peltier Elements (TEC). For my new system I seek a solution where I can adjust the voltage between 0 and 12V with an accuracy of at least 0.1V. The problem is that I need some significant amount of power (The TEC elements used are 50 or 70W elements, resulting in a maximum current of about 12 Amps for chambers with two elements).That was the short story. If anyone knows of a nice solution for this, let me know. If there is none, maybe one can help me with the my PCB design:I talked to several experts and they said that giving a PWM directly on the elements is not what they would do, they would rather control the voltage (this is really important because it's a cost factor and the TEC elements shouldnt be stressed too much, as the experiments will go on for a long time - several months!)Therefore I designed a PCB with a nice PC power supply connector (those can deliver 12V at a lot of amps), a buck converter, an H-Bridge and some other features (e.g. connecting to a raspberry pi as a controller and connectors for fans etc).However this design never worked really well. The main problem was that I was missing a capacitor at the input first (overlooked it somehow in the design process), but after adding one, I still had small negative overshoots (below 0V/GND), resulting in some of the transistors of my H-bridge to give me a short circuit on the PCB. Therefore when increasing the power output with currents > 2A, I had a problem with the design and the power output broke down (due to a short circuit of the overshoots).I had a nice excel calculation where I trying to calculate the capacitors I need. However I think the wide range of voltage might be a problem here... not sure yet!Additionally I chose a wrong digital potentiometer, but that's another problem that should be solved easily. So ... does anyone have an idea about such Buck converters with such a high range of output voltage?I attached a quick picture of the schematics and the EAGLE files can be found athttps://gitlab.ibr.cs.tu-bs.de/lctc/hardware/tree/master/platineThanks to all comments and help. I really appreciate it!

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: July 30, 2017, 09:58:43 pm
content: In order to avoid headhaches, use a simple/unipolar buck converter, and switch the polarity with your H bridge. Plenty of chips are suitable, just use the search engine of manufacturers like linear tech, ti... And since you won't change the polarity very often, you can also use a mechanical relay instead of a MOS based H-bridge.Plan B : Buy a high current programmable power supply. Use a DPDT relay to change the polarity.Plan C : LTC1923Some hints :- High current TECs are not adapted to your thermal chamber application. They're designed to move a lot of (thermal) power, not to reach low temperatures easily. Use a small current TEC! If you insist... You'll need to inject a lot of (electrical) power in the TEC, and to dissipate the tremendous amount of heat generated by the joule effect on the hot side with an enormous heatsink. And when I say enormous...- The "optimum" current written in the datasheet is measured for a perfect heatsink on the cold side, and a perfect heatsink on the hot side. When you add a real-world heatsink on the hot side, the optimum current for cooling effect decreases.A present for you, attached below : A plot of the maximum delta T reachable with a TEC (TEC1-1206, 12V 6 Amp) with a real-world heatsink on the hot side. You'll notice the ~70°C written in the datasheets is far far away in a real-world application. The most the TEC is powerful, the steepest the curve after Rth = 0.

author: roberthartung
author_rank: Newbie
timestamp: July 31, 2017, 07:40:05 pm
content: First of all, thanks a lot for your answer. See my answers inline.Quote from: EmmanuelFaure on July 30, 2017, 09:58:43 pmIsn't that exactly what I am doing? I just searched for a suitable chip and build it up. Problem here is that I was looking at 12V 6Amps peak, where no integrated chip was suitable.Quote from: EmmanuelFaure on July 30, 2017, 09:58:43 pmI have 8 of these chambers, so cost is a big factor here. The power supply solution works fine right now but is a big cost factor.Quote from: EmmanuelFaure on July 30, 2017, 09:58:43 pmLimited to 6V output from what I can tell?Quote from: EmmanuelFaure on July 30, 2017, 09:58:43 pmSo far the cooling works fine - that's what I thought. But maybe you're right and I'd be better suited with a different element than the 50W element - e.g. 30W or even less? From what I can tell the Delta_T is the same for all of them, but the current is different. I guess using a lower power/amp version will result in a slower cooling/heating. But is the minimum temperature also influenced? Can you give any insights about that?If I can just use a smaller TEC element, that'd be fine, and I can use a different chip - that'd be a different game!Quote from: EmmanuelFaure on July 30, 2017, 09:58:43 pmThe Delta_T is where for the cold side and only happens when no thermal energy has to be transported. I am aware that I will never reach -70K

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: August 02, 2017, 11:59:00 am
content: Quote from: roberthartungYup.Linear tech, 54 matches, 4 monolithic :http://www.linear.com/parametric/step-down_(buck)_regulators#!cols_1038,1036,2167,1107,1034,1105,1367,1033,1032!s_1105,0!gtd_!t2_0!1033_>=12!1107_>=6!1105_<=0.8!1034_>=12TI, 48 matches :http://www.ti.com/lsds/ti/power-management/step-down-buck-products.page#p238max=12;100&p634min=0;1&p634max=12;92&p451max=6;420&p32=1;1Analog, 5 monolithic matches :http://www.analog.com/en/parametricsearch/11001#/p4666=Switching%20Regulator&p3972=12|60&p4689=6|800&d=sel|0|3104|3103|s8|3916|3971|3972|4689|4680|4688|s14|s3|s5Quote from: roberthartungDatasheet page 24 : Application information/Higher Voltage ApplicationsQuote from: roberthartungDelta T is the same for all of them with 2 perfects heatsinks. When you have a real-world heatsink on the hot side, the most powerful/current-hungry the TEC is, the less the real max Delta T will be.With a higher power/current-hungry TEC, the cooling power will always be greater initially when delta T = 0, but it may slow down after because of the effect of non ideal heatsinks, Joule heating and heat conduction between the two sides. TEC are non linear devices, and their behaviour is not always intuitive.For each combination of thermal chamber rth and hot side heatsink rth, there's a TEC geometry that maximizes the Delta T. You can find which one by modeling your system and doing simulations. An analytic equation for the temperature on the cold side of a TEC + non ideal heatsink on the hot side exists, but there's no analytic solutions to it. The most straightforward method is to make your calculations numerically. It's still feasible to do that using excel and an iterative method (Calculate T cold with the simple equation for a perfect heatsink with T hot = T amb, then replace T hot with T amb + Rth * Power hot side... It converges very well in 1 or 2 steps).Simple equations for a perfect heatsink can be found here :https://www.electronics-cooling.com/2008/08/a-simple-method-to-estimate-the-physical-characteristics-of-a-thermoelectric-cooler-from-vendor-datasheets/Another present for you : A plot of the max Delta T reachable for a real-world heatsink (0.5°C*W) and a real-world cold box (20°C/W, it's like a cubic box with an 15cm edge length and 5cm of polystyrene insulation). You can see that in this case Imax = 2 Amps is what's maximizing the delta_T, and that it's quite sensitive. It can be computed using only excel by the method I've explained above.