Need a Cool-only Peltier Temp Controller

Discussion in 'The Projects Forum' started by frrrosty, Oct 24, 2010.

  1. frrrosty

    Thread Starter New Member

    Oct 24, 2010
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    Hi,

    I don't know why it's so difficult to find this. I need to use some kind of thermostat / temp controller to regulate a pair of Peltier devices (225 watts @ 15 volts each).

    I DO NOT want BOTH cooling AND heating (who wants a hot peltier?), I need it to COOL ONLY. Now, this shouldn't be too tough to make, but I cannot build from schematics, so I need one that ACTUALLY EXISTS.

    I am not going to pay $2,700 for a thor, this device isn't going to the moon or into a nuclear war. I simply want to have the peltier turn on when the air temp rises above 15 degrees C and turn off when it's below 15 degrees C. How tough could that be???

    I DO NOT want it to control a laser diode.

    It can't be wimpy. It needs to handle 2 loads of 20 amps at 12-15V (most controllers are in milliamps, or 3 amps, etc and at totally the wrong voltage for most peltiers, most peltiers work at 12 v).

    It doesn't need to control the temp to within a millionth of a degree, within a few degrees is fine.

    Am I on drugs or is this impossible to make or what? No one makes one, or at least if they do, it's 3000 dollars. Rediculous. Its a freekin thermostat!!!!!! It shouldn't cost 4 figures, or even 3. it should cost $50.
     
    Last edited: Oct 24, 2010
  2. bertus

    Administrator

    Apr 5, 2008
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  3. frrrosty

    Thread Starter New Member

    Oct 24, 2010
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    possibly, i wouldn't know. I cannot build from schematics so i certainly can't modify one, and he doesn't even list the specifications of the circuit.

    This is what i mean. everyone grasps the idea, but no one actually makes one. and, if he is making this circuit, he doesn't give any specs, so it's totally useless. thanks anyway.
     
    Last edited: Oct 24, 2010
  4. Wendy

    Moderator

    Mar 24, 2008
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    It's not hard, it could be done with a power transistor to ground, but you do need a voltage spec, and need to have some idea of a amperage limit (for the transistor). Since this will be an analog application I'd go with a BJT.

    Have you considered how you'd dump your waste heat? I had a TEC circuit that was malfunctioning at one of the companies I worked for where the heatsink was under a table with a large skirt (for mechanical reinforcement). The skirt trapped the hot air over the weekend (hot air floats), and the TEC would run away thermally. The solution was simple, put a fan under the table and plug it in for a couple of minutes. The normal movement of people around the testing station kept it cool for the rest of the week.

    Most TECs (same name) are bidirectional, reverse the current reverse the heat flow. The OP has made it simple, one direction only.
     
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  5. frrrosty

    Thread Starter New Member

    Oct 24, 2010
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    of course they do. anyone who knows anything about peltiers knows that.

    however, one uses a peltier to cool something OR heat something, but generally not at the same time (except TEC dehumidifiers, which alternate).

    Obviously, as common sense would suggest, the heat is considered exhaust and will be dissapated by a heat pipe OUTSIDE the chamber. I say "not both" because many TEC controllers have 2 outputs, one for limiting heat to a specified maximum, and one for maintaining heat. I don't want to pay for a heating output when all i want to do is cool with it. That would be like buying an air conditioner that is also a room heater. not very useful and you'd be paying for something you wouldn't use.

    I may not be an EE, but i know how to use heat pipes and heat sinks, and about deltaT. My implementation is not the issue. I simply need a thermostat to turn the peltier on when the temp rises above 15 degrees C, and off again when the air temp inside the chamber is at or below 15 degrees C,and the peltier i need to use is 225 watts at 15 volts.

    and, yes, i would be putting the cold side INSIDE the chamber, the hot side OUTSIDE the chamber, relative to the voltage potential.

    I will use heat-pipe heatsinks with fan-forced air to achieve heat transfer on both sides of the peltier. PWM is the best way to control the heat back-slide, so that would be nice, but probably too expensive for me. On-Off is the only other real option, which is satisfactory for my application. I'm cooling an air chamber approx. 16 cubic feet that will have components inside generating approx 400 watts of heat. The 225 watt peltier (times 2) should be sufficient, but i can add more peltiers and heatpipes if needed. The delay in transfering heat in or out that large a volume of air makes the heat backslide something I can live with.

    and again, i appreciate the schematics and specs, but i need a controller that ACTUALLY EXISTS IN THE REAL WORLD AT THIS TIME. I will not be making a controller, as i'm not an EE and cannot build from schematics.

    is 15 volts / 225 watts (and thus 18 amps) NOT a voltage and amperage specification? and i thought a peltier IS a bjt? forgive my ignorance, thats why i'm asking here!

    This is really blowing my mind, there are literally hundreds of peltier wafers out there, most operate at 12 volts. yet no one make a 12 volt controller! are all the people using these devices NOT controlling them? or, are they all so rich they can pay $2,700 for a Thor temperature controller? besides, the Thor is way too large to be useful. this needs to be a small circuit board just a few inches square (plus a heatsink for the board, probably). it doesn't need any LED readout, networking or PC-based software. all i need it a pot to vary the temperature setpoint. 12 volts, 20 amps, potentiometer, thermistor. thats it. it's not going to the moon or into a nuclear war. I just want one that actually exists, and at a price that isn't triple of the entire chamber and it's contents!
     
    Last edited: Oct 24, 2010
  6. Wendy

    Moderator

    Mar 24, 2008
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    The TEC controller that company used was expensive, very expensive.

    Have you tried Omega? One of their many specialties is thermal controllers, they have an excellent online catalog, and probably have many off the shelf options for your problem.

    You're big problem is what kind of sensor you want, RTD, thermocouple, or thermistor, and where to place it.

    www.omega.com
     
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  7. DonQ

    Active Member

    May 6, 2009
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    Two words:

    1) Power-supply.
    Get one capable of 15 volts / 225 watts, available at almost any electronic parts outlet.

    2) Thermostat.
    Available at almost any home improvement center.

    If you are really harsh with the people at the store, maybe they will understand the difference between what you say and what you mean.
     
  8. frrrosty

    Thread Starter New Member

    Oct 24, 2010
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    thought of that, but it won't work. the thermostat would have to be capable of switching on and off a 12v/20 amp current, not what they are designed to do. the current they handle is on the order of milliamps, not 20 amperes. that means employing some kind of relay, and thats where i get out of my depth.
    Also, they only turn ON to HEAT if BELOW a setpoint, i need it to turn ON to COOL if ABOVE a setpoint, but thanks




    The thermistor is not a problem, I have several, all with temp ranges applicable to this project. Placing it is also not a problem. The problem isn't even finding TEC controllers. They do exist, all be it at $3000, and they do much more than i need. The problem is finding a basic, cheap controller, minus the NASA grade specifications and features the Thor controllers have. I don't need or want that. But i will check out omega, thats the best real-world suggestion yet. thanks
     
    Last edited: Oct 24, 2010
  9. frrrosty

    Thread Starter New Member

    Oct 24, 2010
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    sorry for the dp, the edit option is not available for some reason.

    for those interested, i found this:

    Johnson Controls Refrigerator Thermostat

    it functions by turning off whatever you've plugged into it's 120vac outlet at a certain temp, set by an analog dial. it's small and cheap, but not quite the solution i was looking for. This will shut off EVERTHING connected to the power supply. I was planning on running the fans off the same power supply, but not connected to the thermostat. That way the fans would keep circulating air even when the peltier is off. With this solution, I will need a second, small 12v power supply for the 10 92mm fans on the heat pipes (4 on the cooling side, 6 on the hot side) on the two 225w peltiers.

    This should work if the JC unit can handle the amps, but I'd still prefer a semiconductor-based PWM temperature controller with a 12-15v/20-40A rating, if i can afford it. Any pointer are appreciated.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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  11. ifixit

    Distinguished Member

    Nov 20, 2008
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    Dear Mister Snowman:),

    An electric brake controller might work for you. $300-400 new... much less used. Google it.

    They work with 12V at high current using a MOSFET, and you can even modulate the drive level with a slide switch (PWM). They come with instructions.

    I know you don't like schematics, but have a go at the attached anyway.

    Have fun,
    Ifixit
     
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  12. wayneh

    Expert

    Sep 9, 2010
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    I've just built myself a TEC controller, and I've posted the circuit here. I believe it would meet the OP's specs, perhaps with minor modifications. If you want, I can share the perfboard layout in addition to the schematic.

    This was my first experience with TECs, and I learned the hard way that the warnings are correct: The real design problem is how to get rid of all the heat. Step one is to NOT run the TEC at capacity. They're much more efficient at something like 75% of max current. The max ∆T drops with current, but your ability to get cold is going to be influenced more by your cooling arrangement than by the TEC performance. I looked to overclocker techniques to cool the hot side of my TEC. Aluminum heat sink with big fins, a big fan, thermal grease, etc.
     
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  13. DonQ

    Active Member

    May 6, 2009
    320
    11
    A relay is exactly what you need (so make it 3 words instead of 2). A relay can easily be found to handle 20A, and a relay often (usually) has two sets of contacts. One closes when the coil is energized, the other opens, and vice-versa. This would turn a "on when below a temperature, off when above" into a "off when below a temperature, on when above." This seems to be what you need and would be available for a fraction of the price of some of the other methods.
     
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