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Is this a THERMAL CUT-OUT?

Discussion in 'General Electronics Discussion' started by Martaine2005, Sep 5, 2019.

  1. Martaine2005

    Martaine2005

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    May 12, 2015
    Hi guys, I have two identical 1:1 240v transformers which I believe are from UK bathroom light with shaver socket.
    In series with the LIVE (hot) to the transformers are little round devices about 8mm x 3mm thick. I have Checked them and they are both around 40 Ohms. Putting pressure both sides with warm fingers, the value drops to low 30 Ohms. The wires are soldered directly to them (no legs). They resemble tiny round magnets. I have not seen these before. I hope my picture uploads from this phone.
    I also think that these lights with shaver sockets are at least 25 years old. These 1:1 transformers work but I am just curious.
    Could they be NTC’s from the past?.

    Thanks.
    EDIT: photo wont upload. I’ll have to wait until the weekend when I am in front of my laptop.

    Martin
     
  2. techforce

    techforce

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    0
    Dec 26, 2008
    They sound like NTC thermistors. Thermal cut-offs (TCO's) are found in things like microwave ovens, where they are fastened to the housings of the magnetron and the chassis to monitor safe temperature conditions. Usually they are a NC switch that opens when
    the temperature of whatever they are attached to hits their rated activation range.
     
  3. Martaine2005

    Martaine2005

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    May 12, 2015
    Thanks for a quick reply. These are definitely floating in the wind. I did suspect an old NTC by it’s change with temperature. It’s just that I have never seen one like this before.

    Martin
     
  4. Martaine2005

    Martaine2005

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    May 12, 2015
    img_0008 (3).jpg

    Here is the little device. It took several attempts to re-size the photo.
    There are no numbers or codes on it. I just noticed it's not quite focused, But that is the best I could do with my phone.

    Martin
     
  5. davenn

    davenn Moderator

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    1,749
    Sep 5, 2009
    Looks like a poly-fuse for over current protection

    If it was a true thermal fuse(cutout), it would be up against the winding under several layers of the yellow tape
    and that would normally be under with the primary winding


    Dave
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    As the resistance drops with rising temperature, that's indicative of an NTC as stated before. A Polyfuse would show a PTC characteristic and I doubt that it would have been present 25 years ago.
    My theory is that these NTCs were/are used to limit inrush current to the load while presenting a (comparatively) low loss load in operation (when they had gotten warm from the power dissipation).
     
    73's de Edd likes this.
  7. Martaine2005

    Martaine2005

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    630
    May 12, 2015
    Thank you Dave and Harold. I will stick with my gut instinct that they are NTCs.
    I was just curious as I have never seen one like this before.

    Martin
     
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