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A resistor at 150°C...how?

Discussion in 'Electronic Design' started by Francesco Piantedosi, Nov 24, 2004.

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  1. Hi all,
    I have to buil a circuit to use a resistor as a heater; what I want is
    to use this resistor to heat a surface at 150 °C.

    My first problem is to choose right resistor(I have ONLY 15 mm diameter)
    able to dissipate this heat power without crash!

    The second problem is design ctemperature control circuit...the
    sensor(LM35 or lm45) should be in contact with surface I have to heat
    and give feedback to power supply of my resistor...any ideas?

    Thanks
    Francesco
     
  2. Fred Bloggs

    Fred Bloggs Guest

    http://www.iprocessmart.com/sunrod/sunrod.htm
     
  3. John Fields

    John Fields Guest

     
  4. Rich Grise

    Rich Grise Guest

    I've been looking into something very similar to this, to warm a 5-10 l/m
    air stream to 155 C. The LM35 doesn't go that high. The thermistors I've
    seen aren't guaranteed at that temp. range. This leaves either a
    thermocouple, which is a PITA, or an RTD, which I don't even know how to
    search for - all I come up with is platinum and stuff, which cost more
    than I'm budgeted for the whole project.

    For the resistor, I found some thin nichrome wire on Ebay, which has a
    resistance of approx. 28 ohms per foot, which gives me a "blank check" as
    far as what I want the heating element to look like, since I'm making it
    from scratch. I also have a 7.5V 1A wall wart that somebody threw away -
    one of these days, I'm going to see how hot a 3" piece of this nichrome
    gets with the wall wart, but the bugaboo, of course, is sensing the temp.

    The controller shouldn't be too hard at all - there are a lot of suggested
    circuits that you could just copy. You could search on, say, "voltage
    controlled dimmer" and such.

    Good Luck!
    Rich
     
  5. John Larkin

    John Larkin Guest

    Several people make thinfilm platinum rtd's on little ceramic slabs,
    and they cost a few bucks each. google "thinfilm rtd"

    John
     
  6. Glass encased thermistors go quite a bit higher than 150C and have
    large signal outputs. Take a look at:
    http://dkc3.digikey.com/PDF/T043/1044.pdf

    Nichrome is great for heating a small air stream, because a small coil
    of it is self supporting in a ceramic tube and can go to red heat
    quite reliably.
     
  7. I take it this is a one-of research laboratory or hobby experiment.

    For a product design I would recommend differently.

    But, first, some questions need to be answered before an intelligent
    response can be given:

    1) How much hotter than 150C does the resistor have to be to
    hold the plate at 150 in the coldest expected environment?
    This is a function of plate insulation, plate size,
    airflow, dynamics: thermal capacity & resistance,
    transient thermal load, initial temperature (or, don't really care) ...

    2) How much steady state and transient _power_ needs to be fed to
    the resistor to accomplish #1? A function of thermal resistance.
    That means nothing until the amount of power is known
    Wrong move. Use a high temperature epoxy and attach a platinum
    RTD - you can also get iron ones, but they are best used around
    room temperature.

    Use a synthetic bridge (fancy name for a resistor of about the
    same R as the RTD in series with same) and measure the voltage
    across the resistor and the RTD. As usual, ratio and curve fit in software.

    If there is no software, use a four arm bridge with the RTD in one
    arm and a pot in another arm. Adjust pot till the plate is 150C.
     
  8. Rich Grise

    Rich Grise Guest

    ^^^^^^^^^^^

    Like I said - more than I'm budgeted for the whole project. ;-)

    Thanks!
    Rich

    <alter_ego>
    Yes, I know, I'm spending it all on booze, drugs, and harlots. They say
    "easy come, easy go" - well, I've had the "easy go" part down pat for
    quite some time now - somehow, that "easy come" part has been elusive for
    me. And for those of you who wish to blame booze, drugs, or harlots for my
    "sorry" "financial" "state", I say, "So what?" ;^j

    TA!
    R.
    </alter_ego>
     
  9. Just curious - would you have been this forthcoming with neat information
    like this if you'd already known that it's for a THC evaporator? %-}

    ;^j
    Rich
     
  10. Rich Grise

    Rich Grise Guest

    I must be missing something here. I know I've never really "understood"
    the theory of bridges, other than that they try to get balanced. But in
    the one you've just described here, where does the heating element fit in?
    Provided that the problem of the element design itself has already been
    solved, of course. :)

    Thanks,
    Rich
     
  11. The bridge is balanced when the ratio of the resistors on each arm is
    the same. Suppose you have a Pt100 sensor, we know that the resistance
    at 150°C will be 125.37 ohms.

    If you have a 5V supply you might like to use 4K99 precision resistor
    as the resistor in series with the RTD to allow about 1mA to flow so
    that the self-heating is not too bad, yet there's a bit of signal to
    work with.

    Then the other side of the bridge could be any pair of resistors
    provided the ratio is 39.8:1 provided that the bias current doesn't
    cause too much error. Offset voltage sensitivity is about 220uV/°C.


    Best regards,
    Spehro Pefhany
     
  12. John Fields

    John Fields Guest

    They don't _inherently_ try to balance, but surrounded by the proper
    circuitry, they can be forced into it.


    Like this:

    +V
    |
    +--------+--------+
    | | |
    | [R1]- - - --|
    | | |
    [POT]<---[METER+]---+ |
    | | ---> |
    | [HEATER]----> O THERMOMETER
    | | --->
    +--------+--------+
    |
    GND


    What happens is that +V is fixed and R1 is adjusted until the heater
    gets to the desired temperature as indicated by the thermometer, then
    the pot is adjusted to cause the zero-center meter wired across the
    bridge to indicate zero. Now, if the heater has a positive tempco,
    its resistance will increase if it gets hotter, causing the voltage
    dropped across it to increase, causing the meter to deflect upscale.
    when that happens, whoever's minding the store decreases
     
  13. On Wed, 24 Nov 2004 18:06:49 -0500, the Spehro Pefhany

    Grrr.. I used the °F values by accident. The numbers below are for DIN
    standard curve sensors.
    ^^^^^^^^
    370uV/°C


    Best regards,
    Spehro Pefhany
     
  14. John Fields

    John Fields Guest

    Aaaaarghhh!!!!

    Hit "send" by mistake...
     
  15. Why should I worry about evaporation?
    Accidents waiting to happen and weapons are what bring my help to a
    halt.
     
  16. John Larkin

    John Larkin Guest

    Good grief, are you Francesco Piantedosi too?

    John
     
  17. Rich Grise

    Rich Grise Guest

    Actually, no.

    But thanks for asking! :)

    Cheers!
    Rich
     
  18. [CUT]

    Absolutely NO! ;)
    BTW Thanks for the answers!

    Francesco
     
  19. Guy Macon

    Guy Macon Guest

    Given the limited budget, I wonder if it would be possible to
    derive the temperature from the resistance of the nichrome wire.
    It all depends on the tempco of the wire, of course. Just a thought.
     
  20. Robert Baer

    Robert Baer Guest

    Please be advised that the LM35D may not be speced beyond 125C (or
    whatever), but I have found that it is linear and as repeatable as a
    scientific thermometer or calibrated thermocouple at least to 185 C.
    One could use surface mount resistors for the heater, and surround the
    LM35; mounting everything on a sheet of copper for good thermal
    coupling.
    **
    How else do you think i characterize parts for high temp use?
    I have designed a number of electronic circuits, and guarantee
    parameters to 175 C and useable operation (at minimum) to 186 C.
     
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