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Platinum heater

Discussion in 'Electronic Components' started by Cornelius, Aug 27, 2003.

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  1. Cornelius

    Cornelius Guest

    Hello All

    I am a student attempting to manufacture thin film heaters by depositing
    platinum on a ceramic base. My heaters keep burning open when I apply
    even low voltages. Have any of you ever tried this. Can anyone give me
    any suggestions on how to make this work or how to predict the failure
    of such a heater or a resistor in general.

    Thank a bunch

    Cornelius
     
  2. Dana Raymond

    Dana Raymond Guest

    This is a stab in the dark... Do your heaters need to be heating something
    so as to not burn out?
    The thermal load will lower the thermal resistance, thereby keeping local
    temperatures below the melting point of platinum.

    Make sense?

    Dana Frank Raymond
     
  3. Theo

    Theo Guest

    Place the heater in some fluid, like water.
    The heater will not burn out and the water will warm up
    as a result.
    QED
     
  4. John Larkin

    John Larkin Guest

    What's the size? Film thickness? Typical resistance? Is there a
    serpentine pattern, or just an overall film layer? How are you
    terminating to it? How much voltage are you applying?

    John
     
  5. Cornelius,

    Your question is out of my expertise, but this story could be interesting to
    you.

    I once used steel wire to make a high wattage resistor as a temporary test
    load for a driver circuit. On the first attempt, I used a guitar string
    (0.009 in diameter) and long enough to give 5 ohm resistance. With 15 V
    applied, this quickly heated in the air (because of the small surface area
    and poor thermal conductance to air). In addition, the resistance of the
    wire dropped significantly as it heated, creating a type of thermal runaway
    condition. The wire glowed red hot and then burned out quickly. On the
    second attempt I used thicker stainless steel wire and immersed it in water,
    which worked well enough for temporary testing of my circuit. A week later,
    on telling the story to my neighbor, I found out he had a box of old 8 ohm
    100 watt resistors in his garage and two in parallel would have worked
    perfectly. (what are the odds?).

    Anyway, the point is to check thermal conductivity of the platinum to the
    ceramic base, and the temperature sensitivity of platinum resistance. If
    you find that platinum resistance drops as temperature increases, it might
    be better to use a current source rather than a voltage source. This would
    create a thermal feedback since P = I^2 * R and avoids thermal runaway from
    P= V^2/R.

    Steve B
     
  6. Phil Hobbs

    Phil Hobbs Guest

    If the ceramic is too rough, and the metal too thin, the film may not be continuous.
    This will lead to local hot spots. Make sure that the metal is several times thicker
    than the rms surface roughness, and work down from there. Also, you haven't told us
    where the heaters burn out--is it somewhere in the middle or right at the leads?
    What do the failed units look like? Are there hunks of ceramic missing, are there
    cracks, or just a toasted-looking region in the metal?

    Cheers,

    Phil Hobbs
     
  7. Cornelius

    Cornelius Guest

    Thanks to all who offered advice on my problem.
    In answer:
    I am using uncoated platinum metal. I am not sure it would be safe to immerse it
    in water.
    I measured the resistance of the films with a multi-meter. The are on the order
    of 2000-3000 Ohms, and yes they are in a serpentine pattern. The failure site
    varies on different runs but most occur at a transition pint in the film width;
    ie at a point were I from a width of several millimeters to a width of a
    fraction of a millimeter.
    I do suspect local overheating but I don't know how to confirm that suspicion or
    prevent it if that is the case. I will try and locate a current source as was
    suggested, but failing that I am still open to suggestions.

    Again thank you for all the positive feedback.

    Cornelius
     
  8. Cornelius

    Cornelius Guest

    Thanks to all who offered advice on my problem.
    In answer:
    I am using uncoated platinum metal. I am not sure it would be safe to immerse it
    in water.
    I measured the resistance of the films with a multi-meter. The are on the order
    of 2000-3000 Ohms, and yes they are in a serpentine pattern. The failure site
    varies on different runs but most occur at a transition pint in the film width;
    ie at a point were I from a width of several millimeters to a width of a
    fraction of a millimeter.
    I do suspect local overheating but I don't know how to confirm that suspicion or
    prevent it if that is the case. I will try and locate a current source as was
    suggested, but failing that I am still open to suggestions.

    Again thank you for all the positive feedback.

    Cornelius
     
  9. Cornelius

    Cornelius Guest

    Thanks to all who offered advice on my problem.
    In answer:
    I am using uncoated platinum metal. I am not sure it would be safe to immerse it
    in water.
    I measured the resistance of the films with a multi-meter. The are on the order
    of 2000-3000 Ohms, and yes they are in a serpentine pattern. The failure site
    varies on different runs but most occur at a transition pint in the film width;
    ie at a point were I from a width of several millimeters to a width of a
    fraction of a millimeter.
    I do suspect local overheating but I don't know how to confirm that suspicion or
    prevent it if that is the case. I will try and locate a current source as was
    suggested, but failing that I am still open to suggestions.

    Again thank you for all the positive feedback.

    Cornelius
     
  10. Cornelius

    Cornelius Guest

    Thanks to all who offered advice on my problem.
    In answer:
    I am using uncoated platinum metal. I am not sure it would be safe to immerse it
    in water.
    I measured the resistance of the films with a multi-meter. The are on the order
    of 2000-3000 Ohms, and yes they are in a serpentine pattern. The failure site
    varies on different runs but most occur at a transition pint in the film width;
    ie at a point were I from a width of several millimeters to a width of a
    fraction of a millimeter.
    I do suspect local overheating but I don't know how to confirm that suspicion or
    prevent it if that is the case. I will try and locate a current source as was
    suggested, but failing that I am still open to suggestions.

    Again thank you for all the positive feedback.

    Cornelius
     
  11. Cornelius

    Cornelius Guest

    Thanks to all who offered advice on my problem.
    In answer:
    I am using uncoated platinum metal. I am not sure it would be safe to immerse it
    in water.
    I measured the resistance of the films with a multi-meter. The are on the order
    of 2000-3000 Ohms, and yes they are in a serpentine pattern. The failure site
    varies on different runs but most occur at a transition pint in the film width;
    ie at a point were I from a width of several millimeters to a width of a
    fraction of a millimeter.
    I do suspect local overheating but I don't know how to confirm that suspicion or
    prevent it if that is the case. I will try and locate a current source as was
    suggested, but failing that I am still open to suggestions.

    Again thank you for all the positive feedback.

    Cornelius
     
  12. Cornelius

    Cornelius Guest

    Thanks to all who offered advice on my problem.
    In answer:
    I am using uncoated platinum metal. I am not sure it would be safe to immerse it
    in water.
    I measured the resistance of the films with a multi-meter. The are on the order
    of 2000-3000 Ohms, and yes they are in a serpentine pattern. The failure site
    varies on different runs but most occur at a transition pint in the film width;
    ie at a point were I from a width of several millimeters to a width of a
    fraction of a millimeter.
    I do suspect local overheating but I don't know how to confirm that suspicion or
    prevent it if that is the case. I will try and locate a current source as was
    suggested, but failing that I am still open to suggestions.

    Again thank you for all the positive feedback.

    Cornelius
     
  13. heating of any substance creates free electrons, which enhance conduction.
    The more you heat it the more free electrons, when a substance starts
    visibly glowing that is because you are tearing electrons from the inner
    shells and they are emitting photons as they fall back to the low energy
    states.
     
  14. Phil Hobbs

    Phil Hobbs Guest

    Spinach. The number of free electrons in a metal is a very, very weak function of
    temperature--every metal atom is already contributing at least one, so where are the
    others going to come from?

    Cheers,

    Phil Hobbs
     
  15. ddwyer

    ddwyer Guest

    My knowledge also limited but I suggest you do not have a uniform film.
    If evaporating initially deposit nichrome to give adhesion and then
    whilst under vacuum platinum.
    I once powered a transistor as a quartz crystal heater in a vacuum
    envelope.
    My high thermal resistance connections were through fine platinum wires
    which glowed red heater with 100mA.
    Interesting to note that platinum wire is a temperature sensor under
    other circumstances so its Tc is well known.
     
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