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Using LM56 to control heating element

Discussion in 'Electronic Basics' started by Hammy, Jul 28, 2007.

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  1. Hammy

    Hammy Guest

    I'm thinking of using the LM56:

    http://www.national.com/ds/LM/LM56.pdf

    My question is what is typically used as the heating element. I'm
    planning on using it to switch in a heater when the temp drops below a
    certain threshold and to switch it out when it hits another threshold.

    The temp here in the winter time can hit below -20C not including the
    wind chill. Is it possible (practical) to keep an enclosure a weather
    tight (PVC) (1.25" x 4") at a minimum of say 10C under such
    conditions? Or should I be looking at another device or method.

    What I'm thinking of doing is like fig 7 of the application note on
    the LM56.
     
  2. The heater depends entirely on what must be heated and how
    well it is insulated. This thermostat could control the
    heat to keep your entire house warm, if the output relay was
    large enough and you were willing to pay for the power.

    So lets start at the beginning. What are the dimensions of
    the enclosure?

    What is it insulated with? Can you come up with an R value?

    What waste (watts) heat is already being produced inside the
    enclosure?

    What is the warmest and coldest ambient temperatures the
    package will be exposed to?

    What are the temperature limits of the package?

    Until you have gone through that list, you do not even know
    if you have a heating or a cooling problem, or both.
     
  3. John Larkin

    John Larkin Guest

    That would work. I'd replace the 2N3904 with a mosfet and put the
    heating element, a power resistor maybe, instead of the relay coil.

    I think you could also do what you want without the IC... just a fet,
    a thermistor, and two resistors should do it.

    You might get a power resistor, stick it inside the actual box, apply
    some voltage, and calculate the temp rise factor for your box, in
    degrees C per watt, so you can figure out how much power you'll need
    to raise it from -20 to +10.

    What sort of power is available?

    John
     
  4. Hammy

    Hammy Guest

    Thanks for your input.

    I'll try to better explain what I'm doing. I've built a dual beam IR
    barrier similar to this.

    http://www.guardall.com/product.asp?PageID=245

    I'm putting the Tx in a 1 ½" x 5" weather tight (outdoor electrical
    junction box) which is made of PVC or hard plastic. The Detector will
    be going in an identical enclosure. Everything works fine (Thanks Ed).
    I'm just trying to figure out the most efficient way to regulate the
    temp for the brutal winters we get here (Canada). From a heat stand
    point everything should be fine unless the enclosure interior exceeds
    55C. There are no major sources of heat in the detector. The
    transmitter has a 7808 (TO-220) for the LEDS the driver and fet, the
    timer TLC556 (DUAL) gets a 78l05 (T0-92).The detectors have a
    78l05(T0-92) this supplies two PNA4602M and a dual comparator and
    single AND gate. This is all being supplied from a printer PSU 30Vdc @
    0.5A or 15W. I'm dropping this to 12Vdc with an lm317.

    The Tx draws 1.05W (the bulk of this is dissipated in the 7808 and LED
    current limiting Resistor) the RCV draws 0.150W.

    I was looking at the LM56 because I could use it to switch on a heater
    with one output and use the other output for thermal shutdown.

    I was thinking of using an old oven element I have in the garage? Just
    provide a two inch length with enough power to keep things warm. I
    just thought there may be something available that came with mounting
    hardware that someone may know of. Are large power resistor what is
    typically used?

    The temperatures in the winter can get to -25C and lower. Averages in
    the negative early teens (-12 to -15) to dam cold. Live here for a
    winter then tell me what you think of the Global warming theory (;.
     
  5. John Larkin

    John Larkin Guest

    It will probably work fine cold, without a heater, until all the snow
    blocks the beam.

    LEDs can get a lot brighter when they get cold, assuming the current
    stays fixed. Zap one with some freeze spray and see!

    John
     
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