Peltier regulation with a potentiometer

Discussion in 'The Projects Forum' started by bradleym, May 1, 2012.

  1. bradleym

    Thread Starter New Member

    May 1, 2012
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    OK, I know Peltiers have been touched on many a time, but I need personal assistance anyway. I tried reading first and I just couldn't sort out an answer.

    I am building a cooler for a 12 gallon aquarium. The peltier will attach directly to the glass and have a heatsink and fan on the other side. The unit I am planning to use runs 80w at 12v. I am looking to cool this water no more than 15(F) degrees below ambient. In fact, it will most likely need to set only about 5 degrees below ambient.

    I have 2 power supplies available, one is a standard wall pack that runs at 3A, or 36w. I would prefer to use this one and use a potentiometer to reign in the peltier, IF the unit will be capable of cooling the water as described while running at basically half it's full power.

    If this will not be powerful enough, I also have a desktop PC power supply that can run up to 12A, or 144w. This would allow me to run at full power but would add more noise than I like.

    In both cases, I want to be able to control the voltage or amperage on the peltier, turning it up a day at a time until I get the temperature down low enough, but I can't afford to overshoot it by too much or I freeze out the livestock. I had planned to use one of the setups described above and start with the potentiometer at full resistance, gradually increasing power to the peltier daily over a period of a few weeks as necessary.

    So question 1 (if anyone knows) is whether or not I could use the 2A source and expect a usable result.

    Question 2 is the same as the first using the PC power supply.

    And question 3 is, assuming I could use one of these 2 setups, is there a potentiometer that can handle 80w? Or am I coming at this all wrong by using a potentiometer?

    Thanks in advance.
     
  2. P-MONKE

    Member

    Mar 14, 2012
    83
    5
    Potentiometer?

    I would have thought a thermostat would be more appropriate for a temperature control application.
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    High power pots are crazy expensive and a very poor solution here. You just need a thermostat, simple on/off. You could consider a PWM controller, but it would be overkill for this.

    Here's a completed project that might interest you.

    Before you go much further, do some math. I think you'll find that to maintain a 5°C drop across the total surface (top, bottom, walls) of your aquarium will take more power than this TEC can provide, even at full power. And you don't want to use full power; it'll be more efficient below it's max rating.

    To make the calculations, you'll need an estimate the thermal transfer coefficient of water-glass-air, which you should be able to look up somewhere. The heat transferring IN to the aquarium (because it's cooler than the room) will be a function of area x ∆T x coefficient. The heat moved OUT by your TEC and fan will be about 10% of the power consumed by the TEC. They're very inefficient in this regard; they can only move about 1 heat unit for every 10 consumed. Your home AC moves 10 for every 1 consumed.
     
  4. bradleym

    Thread Starter New Member

    May 1, 2012
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    Thank you both for your suggestions. My concern with the thermostat is the temperature difference accross the glass.

    I should back up a bit as I see I made a mistake in my original post. The ambient temperature is not the issue. The water in the tank holds steady at approximately 10 degrees above ambient due to an internal pump generating heat. I would like to lower the temperature from approx. 86 to somewhere close to 80. However, if the peltier kicks on and drops to 20 degrees or something I am afraid it could crack the glass.

    However I get the impression from wayneh that this is not likely? I assume this by your comment about it not being powerful enough to cool the tank at all. All this said, I may be better off trying rig a fan to blow ambient air across the water's surface, I was just hoping to avoid this due to the amount of evaporation it causes.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    I'd definitely monitor water temp and run the thermostat off that.

    So, the root problem is too much heat added by the pump? Maybe there's something that could be done about that?

    I thought about that, and I think it could be a slight concern at full power. You know, since you are above ambient, the same fan and heat sink you would have needed for the peltier might remove just about as much heat without it. The peltier increases ∆T, but at the expense of 10X as much heat to dissipate.

    I think you'd need a darn big one and a lot of power to run it.
    Yes, a swamp cooler would do a great job, but the evaporation is an issue. That and filling your house with fish tank odor! :eek:
     
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  6. bradleym

    Thread Starter New Member

    May 1, 2012
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    The pump is in the water, not sure what I could do about the heat.

    So are you suggesting I could cool the water as effectively by putting a heatsink and fan directly on the glass?

    So the 80w would be a joke for this job?

    Agreed! Any better ideas? Seeing as how the peltier thing may not be worth it...
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    Pumping warm tank water through an external radiator comes to mind. And moving air over the walls of the tank would help too.
     
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  8. bradleym

    Thread Starter New Member

    May 1, 2012
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    OK, thanks for everything! I'll start brainstorming along those lines.
     
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