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maintaining constant temperature?

Discussion in 'Electronic Basics' started by [email protected], Nov 13, 2005.

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  1. Guest

    Say I have a metal plate the size of an eraser head. I want it to stay
    at 140 F using 4 AA batteries. It seems like I could simply use a
    thermister, but I really don't know much about the electronics world.
     

  2. Yup, you have said it all !!
     
  3. Rich Webb

    Rich Webb Guest

    What's your target run time? What kind of thermal isolation is there
    between the metal plate and any other heat sinks? What is the ambient
    temperature? What are the consequences of failing to maintain 140 F?
    What other loads are presently being, or are being considered for, the
    4 AA cells?
     
  4. Guest

    I was just going to put the thermistor next to it. There wont be other
    loads, and it will be in room temp.

    But I don't know what kind of thermistor I need or anything. I'd like
    it to stay at 140F +/- 2F.
     
  5. Rich Webb

    Rich Webb Guest

    As a swag, a CMOS 555 hooked up as a variable duty cycle astable driving
    a saturated transistor switch might work. Try a 10K thermistor from Vcc
    to DISCH, a 22K between DISCH and THR/TRIG paralleled with a switching
    diode, and a 0.1 uF cap at TRIG. This won't ever turn completely off,
    which might be a good thing depending on ambient losses.

    Alternately, use the thermistor in a voltage divider compared to a
    setpoint pot, both input to an LM339 comparator as a bang-bang
    controller.

    Haven't run the numbers on what kind of heating to expect with only
    4 AA cells to work with, nor the expected run time.
     
  6. Guest

    I see...



    No, wait-I'm confused. DUMBFOUNDED as it were...
     
  7. The simplest power controller is an infinite gain (on-off) type that
    turns the power full on if the temperature is low and turns it off it
    the temperature is too high. A comparator is a device that switches
    off or on depending on which of two signals is more positive.

    Based on the power of the heater, the heat stored in the heater
    element (that keeps coming out for a while after the power is turned
    off, and takes a while to come out when the heat is turned on),the
    thermal mass of the object being kept hot, and the time delay between
    that mass and the temperature sensor, the temperature will ramp back
    and forth through the desired temperature. Whether or not it stays
    within the required bounds under this method of control depends on the
    the factors I mentioned earlier.

    The only way to keep the temperature in much narrower bounds is to
    pulse the heat faster than the time delay between the heater and the
    temperature pickup, and vary the duty cycle of those pulses with a
    more complicated scheme than the simple one I first described. One
    such more complicated scheme is the PID algorithm (a combination of
    Proportional gain times the temperature error, another gain times the
    integral of the error, and a third gain times the derivative of the
    error.
     
  8. Guest

    Ok, nevermind all the IC, I can just tweak until a simple thermistor
    and source work properly. Say for a constant input of 5V and up to 1A,
    happen to know what sort of thermister would I want to get at radio
    shack to be able to reach 140F?
     
  9. Radio Shack isn't going to have much that is helpful. But Digikey
    sells a wide range of thermistors , including some that switch from
    conducting to insulating over a very narrow temperature range, as
    temperature rises. This type is called a PTC thermistor for Positive
    Temperature Coefficient. I use a few of these soldered between a pair
    of brass ribbons mounted in the front of my motorcycle helmet to act
    as a self regulating defroster bar. You may be able to use one or two
    of these connected in parallel and mounted around your heated mass to
    act as both heater and control system. But it may take more than 5
    volts to get it up to its switching temperature, since they have about
    50 ohms resistance when cold.

    The Digikey part number for a 50 ohm PTC thermistor that switches off
    at 60 C ( 140 F) is 235-1123.
     
  10. I think that you may misunderstand what a thermistor is, or does.

    A thermistor is a component that varies its resistance depending on
    temperature. Thermistors are normally used as temperature sensors.
    They may be used in a circuit that controls a heater, but they don't
    do the heating themselves.
     
  11. Ken Moffett

    Ken Moffett Guest

    The devce you are using is a "Switching PTC Thermistor", a name given to
    it by the manufacturer: Thermometrics.

    http://www.thermometrics.com/assets/images/ptcnotes.pdf

    Most people in the field would not generally refer to this as a "PTO
    thermistor". The "silister" in the above URL is what most people refer
    to as a thermistor. The Switching PTO Thermistor curves look like those
    of "PTC resettable fuses". The are probably the same technology under a
    different name.
     
  12. Concert pipe? You need 155C to evaporate the THC. ;-)

    Good Luck!
    Rich
     
  13. Guest

    lol. no comment.

    Anyway I was intending to just use the thermistor as a heat source, I'm
    not melting an iceberg. Oh well, I'll figure it out later.
     
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