Peltier modules - Different voltages in parallel?

Discussion in 'Electronic Basics' started by Rob Salkin, Mar 30, 2005.

  1. Rob Salkin

    Rob Salkin Guest

    I'm a computer scientist and know very little about electronics. After
    a while of google searching, I haven't been able to find a solution to
    my problem.

    I'm building a simple DC device that uses 1 to n Peltier modules to
    generate electricity to power a motor. I heat one side of the module
    and cool the other to get ~.3-.6v and 60-120mA output. I've found that
    using 1 peltier module is more efficient than multiple ones in parallel
    (or series). From my research, I think that the dynamic nature of the
    output due to the uneven heating/cooling of the modules makes it a
    different voltage in parallel problem (or different current in series).
    I think that the highest voltage module is leaking power into lower
    voltage modules and therefore reducing the output to the motor. I
    understand that I could add a diode in series after each peltier module
    in order to stop that backflow, but that would effectively be
    eliminating the usefulness of all but the highest output module. So,
    what can I do to utilize all of the power being generated from these n
    modules?

    Using the water analogy -- I'd like pour power into the top of a
    reservior after each module to allow the pressure to build up enough so
    that there would be enough force to overcome the pressure of the
    backflow through a pipe at the bottom of the reservoir. Would I use a
    capacitor for that? How could I use one without a real ground?

    Thanks!

    -Rob
     
    Rob Salkin, Mar 30, 2005
    #1
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  2. Rob Salkin

    Guest

    Commercial Peltier modules are built up of lots of elements, apparently
    connected in series.

    http://www.marlow.com/TechnicalInfo/introduction_to_themoelectric_co.htm

    which suggests that you should be able to connect several modules in
    series.

    If this generates more volts than you want at a lower current than you
    want, you could always use a DC-to-DC converter to transform the high
    voltage to a lower, or vice versa.

    The Farnell catalogue lists a couple of pages of such devices -
    admittedly, mostly aimed at converting 5V, 12V, 24V and 48V DC into
    something in the same ballpark.

    All of them use some kind of switch - these days it is usually a power
    MOSFET or two - to chop the DC input to produce AC, which is then put
    through a transformer to get a higher or lower AC voltage, which can
    then be rectified to give the DC output you want.

    This can be quite efficient - 90% or better.

    There are ways of performing similar tricks with capacitors, but
    transformers can shift a lot more power per unit volume.

    With a bit more ingenuity you can build a circuit which can handle a
    moderately wide range of input voltages - modern "universal"
    switch-mode power supplies accept mains voltages from 85V rms to 265V
    rms.

    Your voltages sound more like the output from a solar cell, and you
    might find that you could use something intended for that application.

    ---------------
    Bill Sloman, Nijmegen
     
    , Mar 30, 2005
    #2
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  3. Rob Salkin

    Mark Guest

    interesting... sounds like a similar problem with solar cells, perhaps
    you can look for some info on how that is done...

    in order to get the most power out of each module and combine them if
    each has different V and I, you would need a small switching power
    supply for each module then coordinate and combine the output of the
    switchers, not simple.

    Mark
     
    Mark, Mar 31, 2005
    #3
  4. Rob Salkin

    Luhan Monat Guest

    Rob Salkin wrote:
    > I'm a computer scientist and know very little about electronics. After
    > a while of google searching, I haven't been able to find a solution to
    > my problem.
    >
    > I'm building a simple DC device that uses 1 to n Peltier modules to
    > generate electricity to power a motor. I heat one side of the module
    > and cool the other to get ~.3-.6v and 60-120mA output. I've found that
    > using 1 peltier module is more efficient than multiple ones in parallel
    > (or series). From my research, I think that the dynamic nature of the
    > output due to the uneven heating/cooling of the modules makes it a
    > different voltage in parallel problem (or different current in series).
    > I think that the highest voltage module is leaking power into lower
    > voltage modules and therefore reducing the output to the motor. I
    > understand that I could add a diode in series after each peltier module
    > in order to stop that backflow, but that would effectively be
    > eliminating the usefulness of all but the highest output module. So,
    > what can I do to utilize all of the power being generated from these n
    > modules?
    >
    > Using the water analogy -- I'd like pour power into the top of a
    > reservior after each module to allow the pressure to build up enough so
    > that there would be enough force to overcome the pressure of the
    > backflow through a pipe at the bottom of the reservoir. Would I use a
    > capacitor for that? How could I use one without a real ground?
    >
    > Thanks!
    >
    > -Rob
    >


    Mount all of them on the same pair of heatsinks (one on each side).
    That will tend to equalize the voltage/current outputs and allow for
    either series or parallel operation.

    --
    Luhan Monat (luhanis 'at' yahoo 'dot' com)
    "The future is not what it used to be..."
    http://members.cox.net/berniekm
     
    Luhan Monat, Mar 31, 2005
    #4
  5. Rob Salkin

    Rob Salkin Guest

    Mark,

    What sort of switching power supply do you mean? I don't have much
    knowledge in the area...

    -Rob
     
    Rob Salkin, Apr 1, 2005
    #5
  6. Rob Salkin

    Rob Salkin Guest

    Bill,

    The area I'm working with is a few sq inches, so each module doesn't
    produce the same V or mA output, due to differences in temperature.
    Series and parallel circuits work worse than 1 alone. Futhermore, the
    heat/cold aren't constant, so I can't even guarantee that the
    individual modules will have a different, yet constant output.
    Basically, all I know is that I have n modules that will put out a
    range of voltages and currents, at any given time. I *wish* my problem
    was that I have too much voltage!

    -Rob
     
    Rob Salkin, Apr 1, 2005
    #6
  7. Rob Salkin

    Rob Salkin Guest

    Luhan,

    I thought about that, but the area is too big. I have about a 5"x2". I
    already have thick sheet metal on either side that seems to be evening
    things out more than direct heating/cooling. I doubt I'd be able to get
    it much more even. Even if the temps were similar, wouldn't even minor
    differences cause a loss in useful power?

    -Rob
     
    Rob Salkin, Apr 1, 2005
    #7
  8. Rob Salkin

    Guest

    The temperature difference across each Peltier junction sets up a
    potential difference across that junction. If you stack up the
    junctions in series, the potential differences just add up.

    If the junctions are in series, you have to have the same current
    running through all of them. Because each junction has an associated
    resistance you lose some of that potential difference as a resistive
    drop in each junction. This also just adds up over all the junctions,
    but it doesn't have to distributed in the same way as the thermally
    generated potential differences.

    This won't be as efficient as drawing the optimum current from each
    junction, but if the junctions aren't seeing vastly different thermal
    environments, such that you'd be better off by-passing some of them, my
    guess is that a single series string of juctions driving a single
    pulse-width modulated DC-to-DC converter is going to be the most
    practical way of handling your job.

    Putting modules in parallel would be much more difficult - the high
    voltage strings would just drive curren through the low voltage strings
    as well a your load, as you've already said.

    ---------------
    Bill Sloman, Nijmegen
     
    , Apr 1, 2005
    #8
  9. I read in sci.electronics.design that wrote (in
    <>) about 'Peltier
    modules - Different voltages in parallel?', on Fri, 1 Apr 2005:
    >If the junctions are in series, you have to have the same current
    >running through all of them. Because each junction has an associated
    >resistance you lose some of that potential difference as a resistive
    >drop in each junction. This also just adds up over all the junctions,
    >but it doesn't have to distributed in the same way as the thermally
    >generated potential differences.
    >
    >This won't be as efficient as drawing the optimum current from each
    >junction, but if the junctions aren't seeing vastly different thermal
    >environments, such that you'd be better off by-passing some of them, my
    >guess is that a single series string of juctions driving a single
    >pulse-width modulated DC-to-DC converter is going to be the most
    >practical way of handling your job.
    >
    >Putting modules in parallel would be much more difficult - the high
    >voltage strings would just drive curren through the low voltage strings
    >as well a your load, as you've already said.


    I agree entirely, and I was tempted to post an article advising this,
    but I wasn't sure that there was some subtlety involved that affected
    the advice.
    --
    Regards, John Woodgate, OOO - Own Opinions Only.
    There are two sides to every question, except
    'What is a Moebius strip?'
    http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
     
    John Woodgate, Apr 1, 2005
    #9
  10. Rob Salkin

    Mark Guest

    switching power supplies chop the DC input and can convert the voltage
    and current by changing the duty cycle of the switching. They are also
    called DC to DC converters. Also called flyback, boost or buck
    converters.

    I don't know if there is something off the shelf for what you need.

    I'm thinking a bunch small boost type supplies whose outputs are
    similar to current sources so the outputs can simply be put in
    parallel. You need a power supply designer to help you with this.

    Mark
     
    Mark, Apr 1, 2005
    #10
  11. Rob Salkin

    Fred Bloggs Guest

    wrote:
    > The temperature difference across each Peltier junction sets up a
    > potential difference across that junction. If you stack up the
    > junctions in series, the potential differences just add up.
    >
    > If the junctions are in series, you have to have the same current
    > running through all of them. Because each junction has an associated
    > resistance you lose some of that potential difference as a resistive
    > drop in each junction. This also just adds up over all the junctions,
    > but it doesn't have to distributed in the same way as the thermally
    > generated potential differences.
    >
    > This won't be as efficient as drawing the optimum current from each
    > junction, but if the junctions aren't seeing vastly different thermal
    > environments, such that you'd be better off by-passing some of them, my
    > guess is that a single series string of juctions driving a single
    > pulse-width modulated DC-to-DC converter is going to be the most
    > practical way of handling your job.
    >
    > Putting modules in parallel would be much more difficult - the high
    > voltage strings would just drive curren through the low voltage strings
    > as well a your load, as you've already said.
    >
    > ---------------
    > Bill Sloman, Nijmegen
    >


    In the usual application, the thermoelectric generators are arranged
    into identical series strings that are then paralleled so as to achieve
    an equivalent resistance equal to the load for maximum power transfer.
    Anything else is either less than optimal in power delivery or a waste
    of thermoelectric modules. Note that deviation from less than identical
    parallel string voltages is not so important when they are all driving a
    node that is at 1/2 their open circuit potential. The OP is quoting
    ridiculously low output voltages and unconcerned with telling us whether
    this is open circuit or loaded, so either he has a gross resistance
    mismatch or a pathetically unworkable temperature differential. Either
    way- as usual- the thread is a boring waste of time.
     
    Fred Bloggs, Apr 2, 2005
    #11
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