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Peltier elements

Discussion in 'Electronic Design' started by [email protected], Sep 19, 2012.

  1. Guest

    Hi

    I was playing around with a peltier element I got from a friend, and I did not get the result I was looking for. I wonder how they make those mobile coolers.

    By applying 12VDC it consumes 2-3A, stabilizing at app 2.4A = ~30W.

    I used to metal plates from a CD player there was around, but that was not enough to set of the heat - it got quite hot, 90 deg. C, and with a fan some 45C. That means, that the "cold" side was still warmer the otherwise inside temperature (26C at work, at home 21-22).
    Running it at 3V/0.5A it works better.

    As this was a simple test I was not expecting it to work the best, but I amsurprised then instead if getting a cooled part I actually got a heater intotal.

    I have a few options - running it at lower power, but the main thing would be to have a larger and more effective part for getting rid of the heat.

    My idea was to make some kind of small fridge, for the fun of it.

    What are your experiences with peltier elements?
    Electronics for controlling it?

    WBR
    Sonnich
     
  2. mike

    mike Guest

    Go lookup the specs/curves on a peltier of about the same size.
    You get most heat transfer at zero delta
    and not any heat transfer when the power in equals
    the power/heat moved.
    Bottom line, don't expect much deltaT if you want much heat transfer.

    Peltier refrigeration has its place when you can't have moving parts.

    Save yourself the hassle. Read up on it, then go buy a cheapo peltier
    fridge on sale at the local big box store.

    Or come by, I've got three in the attic I'll never use.
     
  3. Guest

    the hot side need to get rid of the power you put in plus the power
    you move from the cold side
    unless you have a large heat sink on the hot side it get hot and and
    the cold side will follow

    -Lasse
     



  4. With very few exceptions, Peltier coolers simply do not work and are an
    outright scam.

    The usual problem is that the temperature rise across the heatsink
    ridiculously exceeds the temperature drop across the cooler.

    See http://www.tinaja.com/glib/hack68.pdf for details.

    They certainly should not be used above a 500 milliwatt power level.



    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  5. Jamie

    Jamie Guest

    I bought a peltier chest cooler and warmer from our local club store and
    it never worked. At least from what I could see. I did notice however,
    the palette at the store had looked like it been pawed through and maybe
    some returns. I picked one that was an original package.

    I was going to toss it one day and then I decided to take it apart to
    see what made it tick. it has two fans in it that change direction via
    the switch, depending on what you're after heat or cooling. I did a
    little research and found that the fans were wired backwards with
    peltier unit.
    the peltier is 2 sided so you can select what you want. I switch the
    wires to the switch to change fan direction and the damn thing actually
    works now :)

    It cools the inside when you want it and heats up too. its not a
    supper cooler or heater but its fine for what it was designed for.

    I am kind of thinking that maybe there was a lot of them at the store
    that got returned.

    Jamie
     
  6. Bill Sloman

    Bill Sloman Guest

    They don't shift all that much heat, so if you are using them to
    regulate a large thermal mass you can't change the temperature all
    that quickly.

    For the work I wrote up in my 1996 paper, we compared three different
    sized Peltier junctions, and there wasn't any evidence of any thermal
    lag in the Peltier junctions themselves.

    I don't think that Peltier junctions are slow at all, but they are
    used in situations where you get to see the thermal time constants of
    the stuff whose temperature you are regulating
    Sounds crazy. The last thing you want to do with a Peltier is to shift
    any more heat through it than you absolutely have to.

    <snip>
     
  7. gregz

    gregz Guest

    I've played with them. Made led cooler for 45 watts, using good parts, CPU
    coolers, copper, etc.

    I worked on a mid sized lab cooler. As long as the fans keep spinning, your
    good.
    I own a small ice maker. It makes ice. What can I say?
    I was considering buying a dehumidifier for my trailer. Most don't have
    external drain lines.
    Somebody bought me a neck cooler from sharper image. It works, but...

    Keep the two sides and sinks separated and insulated, and use as much fan
    power as you can.

    Greg
     
  8. Hi Don, Please stop saying this. TEC's work great, But you have got
    to run the numbers and get rid of the heat on hot side if you want to
    use them for cooling! Designed with no thought they have thermal run-
    away. As the OP experienced.

    George H.
     
  9. Grin, my last TEC project was stabilizing the B field of a permenant
    magnet for NMR.
    The 'trick' is to lock it to whatever the local temp is and then
    change the frequency to match.

    George H.
     
  10. N_Cook

    N_Cook Guest

    Hi

    I was playing around with a peltier element I got from a friend, and I did
    not get the result I was looking for. I wonder how they make those mobile
    coolers.

    By applying 12VDC it consumes 2-3A, stabilizing at app 2.4A = ~30W.

    I used to metal plates from a CD player there was around, but that was not
    enough to set of the heat - it got quite hot, 90 deg. C, and with a fan some
    45C. That means, that the "cold" side was still warmer the otherwise inside
    temperature (26C at work, at home 21-22).
    Running it at 3V/0.5A it works better.

    As this was a simple test I was not expecting it to work the best, but I am
    surprised then instead if getting a cooled part I actually got a heater in
    total.

    I have a few options - running it at lower power, but the main thing would
    be to have a larger and more effective part for getting rid of the heat.

    My idea was to make some kind of small fridge, for the fun of it.

    What are your experiences with peltier elements?
    Electronics for controlling it?

    WBR
    Sonnich

    +++++++


    With all this global warming around these days, will I be able to make up a
    Peltier electric blanket for summertime use?
     
  11. Bill Sloman

    Bill Sloman Guest

    Were you trying to work with a constant gain around your thermal
    control loop? The Peltier output - in terms of heat transferred per
    amp through the junction - is heavily dependent on the temperature
    difference across the junction. If you are working with a simple
    analog control loop (no multiplier) this can force you to settle for
    slow settling over most of the range to avoid having the loop unstable
    at one end of the range. My 1996 paper spelt out how we avoided this
    in our digital control loop and my 2004 comment on Flaxer's 2003 paper
    spelled out the implications in some detail.

    Sloman A.W. “Comment on ‘Implementing of a precision fast
    thermoelectric cooler controller using a personal computer parallel
    port connection and ADV8830 controller’[Rev.Sci. Instrum. 74, 3862
    (2003)]” Review of Scientific Instruments, 75 788-9 (2004).”

    Using an extra heat source is much clumsier than using a sigma-delta
    converter and realising an adjustable control loop in a
    microcontroller. You do need to monitor the temperature of the exhaust
    side of the Peltier junction, but you don't have to do it particularly
    precisely, and it's a good thing to do in any event - you can even use
    the information to add a bit of feed-forward control as well, if
    you've characterised your system properly.
     
  12. Interesting, I've never looked at the time response of a Peltier.
    Do you have any 'rule of thumb' guesstimate of the time constant for a
    (say) one inch square element?

    Is it just the extra mass involved?

    A heater can be pretty small while a Peltier has to drag all that
    semiconductor mass around.

    George H.
     
  13. Thanks Phil, I'll file that away in the back of my brain. Lot's of
    times 'my' thermal loops are just plodding along with a time constant
    of seconds. So TEC response time is not so important.

    One of the fun things about instrument design are the 'onion' layers.
    It doesn't help to fix some problem that is two layers down. The
    'art' is being able to identify the top layer problem. (Not that I'm
    particularly good at it... I sometimes have to eat the whole onion to
    find the top layer :^)

    George H.
     
  14. Bill Sloman

    Bill Sloman Guest

    True - they are more mechanically complicated. Even single stage
    Peltiers consist of lumps of bismuth telluride sandwiched between two
    layers of alumina. This doesn't make them slow.

    My 1996 paper reports an initial 1.7sec exponential time constant on
    the cooling curve that preceded the 249, 414 and 589 second decay
    constants determined by the thermal mass of the block controlled and
    the thermal resistance to ambient.

    The thermistor we used had a thermal time constant of about one
    second, so the lag inside the Peltier junctions - which were
    respectively 16cm^2. 9cm^2 and 4 cm^2 - can't be more than one second.
    That's not long.
    We didn't see anything like that.
    <snip - I've never used a multistage Peltier - they are rather exotic
    beasts and it doesn't make much sense to lump them with regular single
    stage Peltier junctions>
    This doesn't make any sense to me. Thermal time constants are
    determined by mass of material and the geometry of its structure, and
    adding a heater isn't going to change that at all, nor move anything
    material closer to "the thing you care about".

    It may move the operating point into a region where the transfer
    function of the Peltier junction - in terms of watts moved across the
    junction per amp driven through the junction - is more nearly stable,
    but that transfer function is known, calculable and predictable, and
    it's a lot more sensible to calculate the amount of heat you need to
    move and supply the current that you need to move that much heat
    rather than adding extra heat to be moved to make the calculation
    easier.
    How does adding a heater make "the distances" shorter?
    Controlling the heat transferred - rather than the current moving the
    heat - does let you tune the control loop to be dead-beat over the
    whole range, rather than forcing you to use a proportional term that
    is too low over most of the range to avoid instability at one end.

    It's more computationally demanding, but thermostats don't need fast
    control loops, and the extra computation involved didn't come anywhere
    near overloading our micro-controller, in our application. Smaller,
    faster set-up could require more frequent up-dates, but there is a
    whole spectrum of cheap computational power available if you need
    something more potent than the Siemens SAB80C517A 8-bit
    microcontroller we used.
     
  15. gregz

    gregz Guest

    I was going to make a fridge once. You have to worry about condensation and
    freezing possibility. That ice maker I have is neat, and makes ice pretty
    fast. Makes hollow cylinder pieces, that stay wet. The ice builds up in the
    storage, and keeps melting. That reduces the total refrigeration needed,
    and keeps the ice from sticking together.

    I always had this thing at work on display. 2 foot copper probe, 1/2 inch
    thick, with heavy base. Must weigh 5 lbs. Experiment gone bad. They were
    attaching peltier devices on the base to cool the rod. I forget what the
    probe was for, but I don't want to get into that. Ridiculous.

    I've worked with tons of microscope heater/cooler stages. Newer ones use
    peltier for that. Some use water tubes for heat transfer. Switching
    supplies no good for things that need low electrical noise. The control
    voltage is a slow ramp DC. Going positive or negative. Usually the small
    working area is small, so external sinking is minimal

    Greg
     
  16. Bill Sloman

    Bill Sloman Guest

    Which I'd describe as feed-forward control. If you know what's
    happening on the exhaust - heat-sink - side of the Peltier junction,
    your scheme can change the amount of heat being generated in the
    controlled volume in such a way that the amount of heat that needs to
    be transported through the Peltier junction remains unchanged. You
    then need to know the temperature of the heat-sink almost as
    accurately as you know the temperature of the controlled volume - the
    thermal inertia of the controlled volume and the integral term of your
    PID control loop mean that you don't need to know the temperature of
    the heat-sink quite as accurately as you know the temperature of the
    controlled volume - but that's just making the circuit marginally more
    expensive.
    That's pretty much what we did to measure the time constant of the
    controlled block plus Peltier junction - though we did use turn-off
    rather than turn-on - and what we saw was monotonic - a quick small
    exponential decay with a time constant of about 1.7 seconds followed
    by the much slower exponential of the block cooling as a whole.
    Heater plus feed-forward control.
    And a temperature sensor that's got low thermal mass and is in really
    good thermal contact with volume whose temperature you are trying to
    control, which is to say a low thermal time constant.
    Okay. It's a separate local control loop, inside the the larger
    control loop that uses the Peltier junction to shift the bulk of the
    heat and generate the bulk of the temperature difference from ambient.

    Basically another version of your two-zone heat temperature control
    scheme
    No, it sounds more like two essentially independent nested control
    loops.
    But what you are now describing sounds more like two nested control
    loops, the outer one doing the heavy lifting, and the inner one
    providing the agility.

    To change the subject a bit, one scheme I've come across in precision
    calorimetry uses a Peltier junction as the sensor in a fast local AC-
    only local control loop.

    The Peltier - okay, Seebeck in this application - junction is
    sandwiched between the calorimeter and a solid, well-insulated block
    of copper - a thermal capacitor - which eventually thermally
    equilibrates with the calorimeter, so the Seebeck junction only
    monitors the transient temperature differences.

    The advantage of this approach is that the Johnson noise on the output
    of the Seebeck junction is much lower than that of a platinum
    resistance sensor and the sensitivity - in volts per degree - is very
    much higher.

    You are still reliant on some other temperature sensor to keep the
    controlled temperature stable in the long term, but the short term
    stability can be improved - if, of course, only up to the limit set by
    thermal diffusion times.
     
  17. Wow! That's great! The next project looks to be on the 'slow road to
    china' for exactly the above reason. But it's hard to get people to
    understand. It's 'just' a bit longer time constant isn't it..
    (I may copy and paste that, perhaps it will mean more coming from your
    mouth.)

    I sometimes think I'd like to build a thermal test-bed 'thing'.
    Different sensors, 'plants', hunks of metal of various lengths,
    loads ... Hook it up and control the T, and then beat on it in
    different ways. I'd learn a lot!

    I think I 'get' the TEC + heater idea. It's a bit specialized, but
    you keep the TEC cooling (a heavy idle.) and then control things short
    term with the heater. It's a 'class A' brute force solution... Just
    my style :^)

    Bill, you should give Phelans book a read. I got a copy for ~$15,
    delivered.

    Speaking of thermal diffusivity, I saw an ad for graphite thermal
    sheets.

    George H.
     
  18. Bill Sloman

    Bill Sloman Guest

    Then it's one feedback loop controlling two actuators.

    How do you partition the feedback between the slower Peltier junction
    doing the grunt work, and the little local heater providing the
    agility? Does the Peltier junction just get the integral term from the
    PID output?

    I don't want to try and re-invent that particular wheel - if you were
    prepared to be more explicit about what you do, it would be a waste of
    effort and bandwidth.
    Apart from the unspecified way you managed to develop to partition the
    output of the control loop into a control signal for the Peltier
    junction and a control signal for the local heater.
    One could do a more or less infinite amount of math on a more or less
    realistic physical model of the thermal mass being controlled,
    including the thermal lag from the mass to the sensor, the extra
    thermal lag from the local heater back to the controlled thermal mass,
    and the different - and somewhat larger - thermal lag from the Peltier
    junction back to the controlled thermal mass. For extra credit you
    could throw in the thermal mas of the heat sink on the exhaust side of
    the Peltier.

    Without real experimental feedback to test your choice of the
    simplifications required to keep the model tractable, it wouldn't be a
    particularly useful activity.

    Since I never saw anything that vaguely looked like your weird
    overshoot, I'd probably be doing the math on a rather different
    system.
    Of course it could help - the effectiveness of the Peltier junction
    depends on the temperature difference across it, and if you don't know
    that temperature difference you can't fully optimise your control
    loop.

    I can accept that you don't bother calculating exactly what the
    Peltier is doing for you, though I would see it as a missed
    opportunity, but until you tell us how you partition the feedback
    between the Peltier and the local heater you are essentially saying
    that you have a magically better way of managing your system which you
    aren't going to tell us about in enough detail to let us copy it.
    The control theory books that I've looked at distinguish between
    controlling an inertial mass and and controlling an inertial mass with
    a time delay in the feedback loop. The math for these two conditions
    is - obviously - different, but the texts did point out that time
    delay was always present in any real system.
    That I can see. but I am still interested in how you partition the
    feedback signal between the Peltier junction and the local heater.

    True, but you are being a little mysterious about the details of the
    control loop that you use to get around it.
    Or - in control theory language - the presence of a fixed delay around
    the feedback loop. Because the delay through the heater is less than
    the delay through the Peltier junction, you've got two different
    feedback loops in the one system, and you haven't told us how you
    split the error-correction signal to exploit these two separate
    actuators with their two different sets of dynamic properties.

    While your inner heater doesn't have as large a pure delay as the
    Peltier junction, it is still in series with the temperature sensor,
    whose pure delay forms part of the pure delay around both of the
    control loops. Feedforward does have the advantage that it by-passes
    the delay in the sensor (but not the actuator), but it is a purely
    open-loop correction.
    I'd beg to differ. You haven't been at all clear on how you contrive
    to exploit the different properties of the two different actuators -
    and my recent posts make my incomprehension perfectly obvious.
    Spamming your book? Have sales been slow recently?
    And does that chapter spell out how to partition the feedback signal
    between the two actuators?

    If it does I'll probably buy the book (but not until I'm safely in
    Australia and can stick it on the shelf next to "The Art of
    Electroncis" and Williams and Taylor on filter design, that are out
    there already.
    As I said, I came across it in a paper on precision calorimetry, which
    is - of course - all about measuring the heat that gets dissipated on
    the controlled volume aka calorimeter.
     
  19. Bill Sloman

    Bill Sloman Guest

    If not explained in any kind of useful detail.
    I've not yet got a text on control theory. I've dipped into quite a
    few of them, and got what I've needed, but the ones that I saw
    confirmed Sturgeon's Law, that 90% of everything is rubbish.
    If Phelan's book is part of the 10%, it's quite likely that I'll buy
    it.
    Used it back in 1993, so it's referred to the 1996 paper. It's
    electrically conductive, which can be inconvenient with power
    transistors, but it's great for power resistors aka heaters and
    Peltier junctions. Unlike Thermopads, you did seem to get the thermal
    conductivity that the manufacturer predicted.
     
  20. Bill Sloman

    Bill Sloman Guest

    What's magic about a quarter? I imagine you mean that the close-in
    resistive heater is dissipating 25% of the maximum power you can
    afford to have it dissipating. I'd have though that that running it at
    half the maximum dissipation you think that you can get away with
    would maximise your headroom, but obviously there may be subtleties in
    there that I don't know about.
    They may not have known enough about the gritty details of putting a
    temperature controller together to have actually noticed that you were
    being vague and elliptical and skating over important detail, or -
    more likely - weren't interested enough to care. Obviously, there's no
    dishonour in being vague and elliptical - it saves a lot of typing,
    and if you been immersed in a particular technology for a while one
    does tend to slide over the practical details which have been drummed
    into your head by repeated exposure.
    That would be Chapter 19 which didn't actually make it into your book
    - which could be just as well. The phrase "carbon thermistors" came up
    twice, and thermistors aren't any kind of carbon resistors but a
    sintered mix of metal oxides or sulphides. Faraday apparently made the
    first one out of silver sulphide.

    http://www.technologystudent.com/elec1/therm1.htm

    I couldn't find an answer in it to the question of how you partition
    your control signal between the fast local heater and the slower
    Peltier junction - which is not to say that it isn't a brilliant and
    totally admirable bit of work. The "carbon thermistor" pratfall
    doesn't detract from that at all - you are perfectly entitled to the
    occasional human error, and - if pressed - you can always claim that
    your publisher insisted that you included a couple of deliberate - and
    obvious - errors as anti-piracy devices.
    Who is quarrelling? I'm trying to get you to be more explicit about
    what you actually do, and I'm getting a bit grumpy about the time that
    it's taking to define exactly what it is that you are actually doing,
    but I certainly don't hold you in lower esteem now now than I did when
    this thread first got going.
    I'm about to turn 70. My shiny new aortic valve is working fine, but
    the other heart valves are a bit leaky - nowhere near enough to create
    any kind of problem, but I may see more of my Australian cardiologist
    than I like (and he's a very pleasant character, if totally brilliant
    in his field - one of my friends from undergraduate days is a
    professor of oncology with a dodgy heart, and he got his cardiologist
    to take me on too).

    I'll do my best to follow your instruction, but I can't promise to
    succeed.

    <snip>
     
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