Different kinds of peltiers in series

Discussion in 'The Projects Forum' started by abhaymv, Feb 6, 2014.

  1. abhaymv

    Thread Starter Active Member

    Aug 6, 2011
    104
    4
    Hello everyone,

    Quick question. I have three peltier (thermoelectric) elements, two of them rated 6 A, 8.8 V and 33.2 W and one rated 3.4 A, 7.8 V and 16.5 W. Due to the limitations of the components I have available, I need to use all three peltiers (doing otherwise would cause power dissipation problems in the driving circuitry). Now I want to know if there is a possibility of a problem arising from the fact that the three peltiers that I aim to connect in series are not of the same rating? All three peltiers are from the same company, supercool.
    I'm using an op amp PID controller to drive the peltiers. The op amp in question is OPA549.

    If you guys think there might be a problem, I'll go into details of the circuit I'm using and hopefully this can be sorted out... :)
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,753
    760
    I have never tried peltier in series.
    But I think if you never exceed the current rating of the lower one, all should be good.
    Ofcourse this would lead to lower operating temperatures of the higher rating ones.

    But you can always beef up the driving element if it is a MOSFET or transistor

    In series u are limited to current but with higher voltage.

    Still I am not sure if it safe to or even possible for them to work correctly
     
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  3. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Connecting elements with different current ratings is not a good idea.
    You can not go higher as 3.4 ampere as that is the rating of the weakest element.
    The 6 ampere element will not reach the power it is rated for in this case.

    Bertus
     
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  4. abhaymv

    Thread Starter Active Member

    Aug 6, 2011
    104
    4
    I am not planning to exceed the current rating of the lower one. Peltiers can work in series (atleast from what I learned of the subject), but I am unsure about doing it with ones having different ratings...

    I do know that, but is that the only limitation?:confused:

    I mean, I already bought these peltiers and as the peltiers are not cheap, I do not have many other options... I already ordered a custom made transformer to fit the voltage rating of three peltiers plus some overhead for op amp swings...

    I'm controlling the current through the peltiers by adjusting the voltage supplied. So I'm worried about how well the voltage division will work amound pelters... I have a swing of 23.4 V for three series connected peltiers (I initially assumed I'd get three peltiers of the same type, that is, the 3.4 A, 7.8 V and 16.5 W).
    So my question is, at 23.4 V, will thw voltage be divided equally between the peltiers? Would the 8.8 V peltier be more responsive to voltage changes than the 7.8 V one? If so, I can't give them a full 23.4 V supply since they will take more current than the current limited case...:eek:
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    You need to adjust the current supplied, not the voltage.

    And the max current for the series string will be the max current of the smallest device (as people have already said).
     
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  6. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    Unfortunately, life is never simple. The peltiers are not Ohmic devices. When you first the voltage, the current is higher than when there is a temperature difference (see the info from Tellurex). So if the voltage at 23 volts, you may have over 3.5 A flowing in the peltiers. Also, the 3.4 A is the current for maximum efficiency, not the maximum current that the peltier can handle.

    Info: http://www.tellurex.com/technology/peltier-faq.php
    Set of 10 devices ($26): http://www.newegg.com/Product/Produ...C5632&ef_id=UtsokgAABDXF2Rd3:20140207042951:s
     
  7. TANDBERGEREN

    Member

    Jan 20, 2014
    74
    4
    There is no way this setup will work. Sorry.

    Peltier elements is best handled one for it self at a time.
    Only when having identical elements one could use them in PARALLEL.
    Never in series.

    The voltage will never divide it self proportionally between theese different elements. Sorry.
    Put theese out for sale at Ebay, and get others that will suit Your construction better.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,106
    3,038
    The OP has already said that total current will not exceed the spec of the weakest link. As long as that is true, it'll work fine. The bigger ones will actually operate more efficiently at less-than-max current.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Yeah it will work fine with a constant current driver.

    All it needs is a two-transistor constant current driver where one transistor is large enough to handle the current and heatsunk well enough to handle the transistor linear dissipation.
     
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