Power single device from two circuits

Thu Dec 07, 2017 1:30 am

Background for question
My father likes beer, so I built a box from 5cm thick Styrofoam with outer dimensions 60x55x80cm. I wanted to use it as a fermentation chamber for home brew purposes. The reason why I did not choose regular (some used cheap) refrigerator is I need to have it in a small chamber with floor dimensions of 80x130cm. Also door of that chamber is only 55cm wide. So getting inside some refrigerator would be most likely impossible.

I decided to buy 2 peltier modules - TEC-12715 and attached them to a "hot side heatsink" of dimensions 23x17x4cm and "cold side heatsink" of dimensions 10x17x5cm. I am powering each separately from the dedicated ATX power supply which each can handle 18A max at 12V, so there is a reserve. Hot side is being cooled by 5 fans removed from old cases and atx power supplies. The smaller, cool heatsink is inserted into the top part of the box and sealed. On the cold heatsink there is one 12cm fan.

Unfortunately I can only get max of approx 18 degree Celsius inside (35L barrel with approx 20L of a beer inside + 4x 1.5L bottles with water just as a accumulation of the "cold"). Hot heatsink has most of the time 50 degrees Celsius (I have a temp sensor sticked on it's middle part (near to the peltiers which are also in the middle)). Ambient temperature is now 29 degrees Celsius. So it is a difference of max 11 degrees of Celsius.

Should I consider this to be "normal" when considering inefficiency of peltier modules? Or should I be able to squeeze more cold inside? I read somewhere on the internet the peltier's efficiency is terrible once the hot side is over 40 degrees, but I am not sure if this is true. I planned to do some temp regulation as well, but as I wanted to get closer to 15 degrees Celsius, this does not have the reason now. I don't know.
Based on the above unsatisfactory experiment, I decided to design a circuit by myself. I am creating a closed loop gycol heating/cooling system for beer fermentation and lagering. The system uses a ITC-1000 temperature controller and two power supplies to feed a heating circuit and a cooling circuit with peltier devices attached to a water block and pump. See below:
I need to have a single pump(to pump gycol) and fan(to cool internal components) powered when either circuit is active. The circuits will not be active at the same time. How do I wire these to not create a feedback loop and not supply power to both circuits? Do I need to use diodes http://www.kynix.com/Detail/106000/DIODES.html , and if so how/where do I wire them?
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Joined: Thu Dec 07, 2017 1:08 am

Re: Power single device from two circuits

Mon Dec 18, 2017 6:16 am

Peltier coolers are quite inefficient. It has a lot to do with how cool you can keep the hot side, more cooling there might help you, but you might be close to the limit.

For the pump, it depends on what kind of pump you have. If it is a 12v pump you could use diodes ( or a bridge) to make a logical OR of the heating and cooling signals. If it is 120v ac, then you would want to use a solid state relay, the. Use the two 12v drives to the heat/cooler diode ORed into the control input. You might also put a little ballast resistor (1k ohm or something) into each leg going to the diode OR for the control signal. You probably don't need that, but it might give a little protection in case the diode breaks ( unlikely).
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"Risk of failure should be no deterrent to trying"
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