identification of heating element leads ?

Discussion in 'General Electronics Chat' started by prometei, May 9, 2015.

  1. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
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    0
    greets,

    I've got a heating element for a soldering iron and I'm a bit confused about the leads.

    The two green leads give a voltage in the range of zero to about 4 mV if I heat the element with a lighter.
    The blue and red leads stay at zero when the element is heated. So I assumed that the two green leads are the thermocouple and the blue and red pair is the resistive/heating coil. But the soldering station I have (ZD-937) has the triac tied to the green wire pair, i.e. to the thermocouple (if I'm not mistaken).

    Am I wrong by assuming that the green wire pair is the thermocouple, i.e. it is the heating coil because it is tied to the triac?

    thanks
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    given your testing info, i would say the greens are the thermocouple..post pictures of the whole thing, also if you have a dmm measure the resistance of the heater and thermocouple.
     
  3. Reloadron

    Active Member

    Jan 15, 2015
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    Most soldering irons do not use a thermocouple but use a thermistor of sorts. Rather than looking for voltage I would look at resistances. You should find two with a fairly constant fixed resistance and a pair with a resistance that changes if you externally heat the element. I am not saying a thermocouple is never used but that most soldering pencils (temperature controlled) I have used and taken apart used a resistive type thermal sensor.

    Ron
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    The heating element in my desoldering tool has a temp sensor that is polarity sensitive, so I would assume it is a thermocouple. The heavier gauge wires are the heater. When replacing the element, I make temporary connections to the temp sensor to insure proper control before final reassembly.
     
  5. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
    56
    0
    hey guys,

    turns out the thermocouple is not placed at the tip of the heating rod as I assumed. If I heat up the other end of the ceramic rod it works as it is supposed to, i.e. I get a reading from the blue/red leads pair. As you can see on the picture the smaller part is the thermocouple. I'm not sure if this a good design. I was expecting the thermocouple to be embedded inside the heating coil or placed at the tip of the heating element, i.e. under the soldering tip.

    Heating Element heater and thermocouple leads 04.jpg
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    Apparently some soldering irons use the temperature coefficient of the element itself.

    A micro in the base stand samples its resistance between the controlled pulses.
     
  7. Reloadron

    Active Member

    Jan 15, 2015
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    While I can't recall the details that very subject came up sometime back. Likely this or the other forum I frequent and I am only active in two. I have a few Weller pencils laying around and I am sure they use a resistive element but they are years old. I remember however, reading about exactly as you mention.

    As to this thread? I would still try measuring the resistance of the red wires and heating the thing.

    Ron
     
  8. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
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    cold resistance ca. 2.5 Ohm, heated up with a lighter for ca. 20 seconds resistance goes up to ca. 8.5 Ohms.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    The first mention of coeficient type temperature sensing I can remember, was in a magazine project - it seems that in the decades that followed, a few soldering iron manufacturers at least toyed with the idea.

    Pretty much most soldering iron elements are resistive - otherwise they'd go bang when you plugged them in.

    AFAIK: the basic non-controlled elements are wound with PTC wire, at a certain temperature the resistance becomes so high that it only draws enough current to maintain that equilibrium. When you use the iron and take heat out of it, the PTC element cools and draws more current - it at least makes some sort of effort toward regulation, but not up to the standard of thermostatic control.
     
  10. Reloadron

    Active Member

    Jan 15, 2015
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    Interesting as I would have expected higher numbers for a RTD type sensor. Have a little type K thermocouple laying here. Just measuring the thermocouple I get about 3.2 Ohms. This is with the ohmmeter + lead on the TC + and ohmmeter - lead on the TC -. Heating the TC with a Bic lighter flame I get to about 22.0 Ohms. Reversing the ohmmeter polarity of the test leads and when heating the TC the ohmmeter drives negative which is to be expected. If the resistance behaves the same in your case with the ohmmeter leads reversed it is a resistive sensor, if the Ohmmeter drives negative then you have a TC.

    @ ian, pretty much how I remember reading about it. Yes, the heating elements are just a resistive element and on most I have seen and taken apart the heat sensors are just resistive sensors. :)

    Ron
     
  11. prometei

    Thread Starter Well-Known Member

    Apr 13, 2008
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    0
    yep, it goes negative so it's a TC.

    Thanks for the tip!
     
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