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Help Peltier elements

Discussion in 'Electronic Design' started by :: Marcolino ::, Jul 14, 2004.

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  1. Hi, I've a question:
    I build a termosensor circuit with termosensor (lm335) and peltier elements
    (68W 15V) for cooling my box.
    The circuit job is:
    - tx > 20°c relais turn on so peltier is on so cool my box
    - tx <20°c relais turn off so peltier is off so stop the cooling
    regulated with a trigger to avoid micro oscillation of relais.

    the problem is after 3 days the circuit go well but my peltier elements
    broken... I think than peltier element suffer on/off continuos switch so
    have you same ideas to resolve problem?

    Thanks

    Marco

    p.s.
    sorry for my english but I'm foreign
     
  2. legg

    legg Guest

    Running the junction with a bang-bang controller and a lot of
    hysterisis means running it with maximum thermal stress. If the
    attachment method (source and sink) is too rigid, cracking may result.

    If you simply ran it linearly, with degenerated temperature regulation
    (ie allow 2degrees rise between 0 and 15V applied range), you'd be
    operating at a minumum stress cycles. Reducing the rigidity of the
    attachments would reduce the actual stress, regardless of cycle count
    and rate of change.

    RL
     
  3. colin

    colin Guest

    peltiers that ive seen are fairly robust all that goes wrong usualy is when
    they get too hot the solder melts and they fall apart. i asume you have a
    big enough heatsink on the cold side of the peltier?

    Colin =^.^=
     
  4. Peltiers are not intended for such kind of operation.
    And you need a fan on both sides to move the warmth or the cold.
    What was the switching periond ? 30 seconds, 1 minute, 10 minutes ?

    Rene
     
  5. The 'maximum working temperature', of most Peltier elements like this,
    unless higher temperature units are deliberately used, is about 80C, so if
    the heatsink is poor, it could easily explain an accelerated failure. They
    often have insulation that fails before the slder...
    The other question would be how the supply is arranged. If (for instance),
    it was going overvoltage when the Peltier module is disconnected, then a
    very high current could be flowing through th module, when the relay
    switches back on.
    Personally, I'd prefer to see a PWM regulation system, limiting the
    current through the Peltier, and controlling the temperature as well.

    Best Wishes
     
  6. Bill Sloman

    Bill Sloman Guest

    Bang-bang control is a very bad idea for Peltier elements. The problem is
    that the heat transferred by the Peltier effect is linearly proportional to
    the current through the junction, but the heat generated in the Peltier
    junction by its ohmic resistace increases as the square of the current, so
    running a junction at half the current all the time works a lot better than
    running at twice the current for half the time.

    There is a formula you can use to calculate the nett heat transferred by a
    Peltier junction as a function of current - I've put it in print twice (once
    in Measurment Science and Technology in 1996, and last year in Review
    Scientific Instruments) and much the same formula was published in EDN
    around 1998, but I've not yet seen it on the web.

    When you are using a Peltier junction as a cooler, the cooling effect
    reaches a maximum at a specific current - the manufacturers data sheets
    specify this for junction on essentially perfect heat sinks - and if you put
    any more current through the junction you actually get less cooling effect.
    Rather higher currents are known to melt the (low-melting) solder used to
    put the things together - I've not managed this, but one of my bosses
    claimed to have unintentionally achieved such a disassembly.
     
  7. period is 2 mins.

    thanks,
    Marco


    "Rene Tschaggelar" <> ha scritto nel messaggio
    Peltiers are not intended for such kind of operation.
    And you need a fan on both sides to move the warmth or the cold.
    What was the switching periond ? 30 seconds, 1 minute, 10 minutes ?

    Rene
     
  8. I think now to reduce cell current to half manner to reduce switch timing
    and power cell.

    Thanks

    Marco
     
  9. 2 minutes ?
    Within these 2 minutes, the applied power creates a temperature
    difference which makes some warmth to flow against the usual gradient.
    Then when switched off the lost heat spreads and again the gradient
    between ambient and to be "cooled" settles. This kind of operation
    gives the lowest possible efficiency. Much lower than the 2 minutes
    average current as constant DC. Get or make yourself a controller.
    A manual on how it is done is downloadable :
    http://www.ibrtses.com/products/tec.html


    Rene
     
  10. Marcolino,
    I remember Melcor, one of the manufacturers of Peltiers has
    a rather good brochure
    http://www.melcor.com/

    Rene
     
  11. Tom Seim

    Tom Seim Guest

    One project I was involved with had problems with Peltier coolers
    because an unregulated DC supply was used to drive them. The ripple
    was reducing their cooling capacity for the reason you mentioned. We
    switched to regulated supplies; problem solved.
     
  12. Rich Grise

    Rich Grise Guest

    Don't reduce it by exactly half - reduce it by an adjustable amount,
    and don't switch it at all. Simply use regulated DC, and make
    the control loop's response time really slow, so you avoid overshoot.
     
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