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Help with thermal fuse specs?

Discussion in 'Electronic Repair' started by DaveC, Dec 15, 2003.

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  1. DaveC

    DaveC Guest

    Found a white, square, 2-lead device located in a small transformer, under
    the insulation tape, soldered in series with the primary winding of a
    120v/12v small transformer in a wall-wart power supply. It is open (no
    infinite cont.)

    It has these markings:

    Tf (that's T sub-f) 115 deg C

    It is a little smaller than a TO-220 package (no tab heat sink, 'tho).

    What is this? I presume it is thermal cutout or fuse of some kind. What specs
    do I need to know to replace it with kind unit?

    Can anyone cross this number to a current product?

  2. DaveC

    DaveC Guest

    Of course, this should say "infinite continuity."
  3. Alan McClure

    Alan McClure Guest

    Of course, it should REALLY say "infinite resistance"
    or "no continuity".

    ARM ;-)
  4. DaveC

    DaveC Guest

    Blast and damn those early-morn posts!
  5. Jim Adney

    Jim Adney Guest

    It certainly sounds like a 115C thermal fuse, but all the ones I've
    ever seen were cylindrical, about the size of a 3A diode, but with a
    metal shell and axial leads. One lead insulated.

    I used to be able to buy replacements at Radio Shack, but I believe
    NTE sells a wider assortment.

  6. DaveC

    DaveC Guest

    Yeah, I got an NTE replacement. It is larger, and cylindrical, as you say.
    But there was room, and after ruining one by soldering it in without using a
    heat sink (Doh!!), I successfully installed one. Works fine, now.

  7. Asimov

    Asimov Guest

    "Jim Adney" bravely wrote to "All" (15 Dec 03 22:14:50)
    --- on the heady topic of "Re: Help with thermal fuse specs?"

    JA> From: Jim Adney <>
    JA> It certainly sounds like a 115C thermal fuse, but all the ones I've
    JA> ever seen were cylindrical, about the size of a 3A diode, but with a
    JA> metal shell and axial leads. One lead insulated.

    Yes, I've seen square thermal fuses...

    .... You may be a tech if you're entertained by a 6-pack and sparking HV.
  8. Jim Adney

    Jim Adney Guest


    Isn't it amazing how quickly they respond to temperature? ;-)

  9. GPG

    GPG Guest

    Short the "thermal" fuse and wire in a conventional one.
    These fail because of repeated transformer currents on switch on.
    Use an appopriate value fuse.
  10. Sofie

    Sofie Guest


    Not really a good plan......
    ........a conventional fuse will not provide over-temperature protection and
    fire hazard prevention.
    The thermal cut-out fails not because of "repeated transformer currents on
    switch on" but rather because of over-temperature due to the windings in the
    transformer over-heating.
    Usually the thermal cut-outs, in addition to the temperature rating, have a
    CURRENT rating well in excess of the transformer application..... 2 amps
    for the small thermal cut-outs and 10 amps for the "common" ones available
    from Radio Shack, NTE, ECG, etc.
  11. Ben Miller

    Ben Miller Guest

    Exactly how will this protect the winding against over-temperature
    conditions? What is an "appropriate value"? Have you run the UL temperature
    tests to confirm the effectiveness of your recommendation? Modifications
    such as this are not a good idea.

    Ben Miller
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