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Help with upgrading a minifridge

Discussion in 'General Electronics Discussion' started by Des, Aug 19, 2012.

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  1. Des

    Des

    3
    0
    Aug 19, 2012
    Hi, first time here. I am an extreme novice, I know enough to not hurt myself and I have good soldering skills, beyond that...

    I have a mini fridge that uses a Peltier chip to lower the temperature. Said chip has burned out, and the unit which used to reliable cool to 40f now stays at 78f. I replaced the chip with one from a smaller, lower power cooler and could get the temps to 50f but no lower.

    The original peltier in the fridge I am salvaging was 12v 50 watts. I bought a 12v 90 watt one a as replacement. The power supply for the fridge is 12v 6 amps and 75 watts. I have also used a converted PC psu at 12v 10 amps and 250 watts. The new peltier chip consumes 12v 5 amps 90 watts, the original peltier in this is 12v 5 amps 50 watts.

    When the 90 watt peltier is installed the relays for the fridge (it has a thermostat in it) click continuously and will not fire up the fans or peltier. When the lower wattage chips are in the fridge starts up fine. So, obviously the 90 watts is drawing more than the fridges thermostat circuitry can supply. Here's where I get lost: Now, do I need to increase voltage or amperage or wattage, or replace a component on the circuit board?

    I prefer having the thermostat as it switches the unit on and off thus somewhat increasing efficiency. Also the built in system is much more elegant than anything I can come up with.

    Hooking the cooling fan, peltier and circulating fan directly to the psu everything works fine, but runs continuously.

    I suppose I can also remove all headaches by buying the correct chip which are under $10, but I am trying to work with parts now on hand.

    Any insights or suggestions are greatly appreciated.

    Des
     
    Last edited: Aug 19, 2012
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,449
    2,809
    Jan 21, 2010
    I suspect that your 90W peltier device draws more that 5A at 12V.

    12V 5A is 60W
    12V 10A is 120W

    It is quite likely that the power supply in the fridge is being overloaded by the higher current demand and is switching itself off to prevent catastrophic failure.

    You *may* be able t get away with a higher current power supply, however there are other issues, especially the ratings of wires and especially the device which turns the peltier on and off.
     
  3. Des

    Des

    3
    0
    Aug 19, 2012
    Hi Steve,
    Thank you for your reply. I think I misunderstood the spec on the peltier chip I bought,and have looked it up based upon the manufacture number:
    TEC1-12709 Thermoelectric Cooler Peltier 12V 90W
    Model number: TEC1-12709
    Voltage(V): 12V Umax (V): 15.4V Imax (A): 9A
    QMax (W) : 138.6W
    Dimensions : 40mm x 40mm x 3.6mm

    So, I am seeing now that it is a 9 amp chip?

    I have tried running the fridge with a converted PC psu which is 12 volts, 10 amps and up to 250 watts, and the relays just click continuously. Actually there are 2 relays.
    This fridge has a thermostat that can be raise up to 120f and down to 36f so it will warm or chill.

    The peltier hooked directly to the PC psu with a fan and heat sink gets frosty very quickly.

    I have a feeling that the simplest, and fo me perhaps wisest thing to do is to purchase the correct 50 watt chip then. I don't mind upgrading some components, but it sounds like it will take more than just that to get this working without looking like a Frankenstein mini-fridge.

    Des
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,449
    2,809
    Jan 21, 2010
    OK, so that's nominally 12V, 15.4V max, at a maximum of 9A.

    It suggests that it draws 9A (or close to that) at 12V, which is 108W, so clearly it required some form of current control.

    Possibly. Do you have the manufacturers specs?

    Sorry, 12V 10A is up to 120W, not 250W. Maybe it has other outputs that bring the total up to 250W.

    Tell me what the two relays do, which one is clicking, and how fast.

    Chances are that the fridge is detecting a fault (current in the peltier too high).

    Well that tells you it's working, but have you measured the current flowing through it?

    It may be a simpler solution.
     
  5. Des

    Des

    3
    0
    Aug 19, 2012
    Hi Steve, sorry for the delay in responding, tonight was my first chance to get back to the fridge.

    I did order the correct chip, and have installed it this evening, 30 minutes the fridge has dropped from 79f to 63f, so far so good. I also ran the chip on a fan cooled heat sink for a few minutes before installing it. It gets cold, but not as cold as fast as the 90watt one, which makes sense. I wouldn't mind perusing the upgrade, but I feel it may be beyond me.

    Steve, Thank You very much for your fast response and the info, I wish I could give you more information.

    Best regards,

    Des
     

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