Best Relay to control high current devices (Peltiers)

Discussion in 'The Projects Forum' started by BackyardBrains, Oct 15, 2010.

  1. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
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    We are designing a low-cost DIY polymerase chain reaction (PCR) device to be used in high schools. PCRs allow you to amplify a single piece of DNA to millions of copies of that DNA sequence so you can use it in the lab.

    The DNA is amplified by thermal cycling. Heating up, then cooling down over periods of minutes. We designed a simple circuit that uses an Arduino to 1) turn on the Peltiers and 2) to switch the direction of current (switching from heating to cooling). See attached schematic.

    We are trying to figure out what would be some options for the relays. Can we use MOSFETs or other Transistors? Our current solution is using a bank of relays to handle the high current load (~9-10A), but each one makes a clicking noise when it cycles. We are looking for a single relay, or some solid state alternatives.

    Any suggestions would be appreciated.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    Yes, I've been using a MOSFET to switch a TEC with great success. I'm driving a 12709 at 5v (3-4A) and switching that on and off with a IRF540 under the control of a comparator. With care to the gate voltage - I pull it up to 12v when the circuit goes "on" - the MOSFET stays nice and cool. If you need to pull it up to just 5v, choose a logic level MOSFET that will be full on at 5v. If you use a normal MOSFET at 5v, it'll get too hot with your current load.
     
  3. someonesdad

    Senior Member

    Jul 7, 2009
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    You might want to consider some kind of thermal cutout on the heating circuit (look at a dryer or a home heating furnace for typical examples), depending on how hot things could get if the heating power stayed on. And, for sure, test it to see how things work when components fail -- or your "customers" will have to do it for you... Sorry if that's obvious, but it's all part of engineering design.
     
  4. debe

    Well-Known Member

    Sep 21, 2010
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    Hi BackyardBrains. I wouldnt expect very long life out of your Peltier devices as they dont like being cycled at all it stresses them causing early failure. Daryl
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    Then why are they so widely controlled using PWM - ie. rapid cycling? Perhaps they don't THERMALLY cycle so much under PWM control.

    Wouldn't the threat of failure be highly related to the RATE of thermal change and the ∆T developed? So operating well below peak current might mitigate the threat? Power efficiency is also much better below peak current.
     
  6. debe

    Well-Known Member

    Sep 21, 2010
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    184
    Hi Waneh, Quite right to regulate temp you use PWM. but they dont like cooling then switching to heat it shortens there life stresses the P&N junctions. Daryl
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    Over time they get used to working in one direction and not the other. To properly cycle one you need a cooling off period inbetween.
     
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