Wall Wart Mystery component

Discussion in 'General Electronics Chat' started by Marko, Feb 10, 2008.

  1. Marko

    Thread Starter Member

    Feb 10, 2008
    Greetings from yet another new guy,

    I have a curiosity question regarding some wall wart transformers. Here's hoping the following narrative won't put any would-be readers to sleep.

    I have in my possession an AC to DC adaptor wherein two of the four full wave bridge rectifying diodes on the secondary side are fried, and the transformer has no longer has power going through it. It seems there was a short in the secondary plug end that goes into the device to be powered that caused the diodes to burn up and the transformer to fail. I discovered the secondary was not open, but the primary was indeed open. So I began to peel back some of the tape covering the primary windings in an effort to expose said windings in hopes of discovering a burn through without too much digging. However, I didn't get very far before I discovered a tiny electronic component. At that point, I tested the continuity of the primary windings downstream of this component, and found them closed (yay!).

    Of course, this now reveals the question: what IS this mystery component? I presume it is some sort of fuse, but wondered if it were something else, like a filter.

    The size of this rectilinear component is 7/32" x 9/32" x 3/32", and the markings are:
    TF 125ºC
    There are two leads which come from one side (a 3/32" edge), which threw me, making me think it might be a film cap.

    Particulars for the wall wart are:
    Input: 120V AC 60Hz 14W
    Output: 9V DC 1000mA MAX.
    Plug: S5.5x2.1mm (center bore is positive, outer shell is negative)

    Other info:
    Diodes are IN4001's
    1000 uF, 25V electrolytic Cap on the secondary side

    I had come across this sort of thing before on another adaptor, and bypassed the apparently nonfunctioning component to see if the adaptor would work. It did. However, I still replaced the entire adaptor because I didn't know what part the mystery component played in the grand scheme of things. Now that I've come across this mysterious component again, I figured it would be good to know what it is. If it is a fuse, how do I go about finding a replacement? O.K., so that was more than one question.

    Thanks in advance,

  2. scubasteve_911

    AAC Fanatic!

    Dec 27, 2007
    Just a guess, maybe a thermal fuse?

  3. SgtWookie


    Jul 17, 2007
    Good guess, scubasteve ;)

    A typical thermal fuse looks similar to this:
    They put thermal fuses in those things to keep them from starting a fire if the output is overloaded for a period of time. The one that failed had a link that was designed to melt at 125°C. Otherwise, the wall wart would've gotten so hot the plastic and insulation would've melted, and very likely started a fire.

    I haven't seen one of the type you're describing. The M30 marking initially made me think it was a MOV, until I read the TF125°. They're usually available in 121°C or 128°C. Go for the lower rating, if you choose to repair it.

    I suggest tossing the wall wart and getting a new one, as they're not really worth repairing and are relatively cheap to replace. Besides, the insulation inside the transformer might still be damaged.

    If you DO decide to try to repair it, you MUST use a thermal fuse of the same or lower rating.
  4. pfofit

    AAC Fanatic!

    Nov 29, 2006
    Also called a thermal cutoff

    The one wookie showed is most likely rated at maximum 10 -15 amps is probably physically large for placement inside your dinky transformer. Current is not so important as long as it is greater than normal operation . Temperature is what it is there for.

    a description can be found here http://www.thermtrol.com/7amp.htm
    with lots of types and ratings.

    The smaller rectangular ones that you have are harder to find, in fact the lower amp ones are harder to find as well.

    One must ensure that whatever you do, the replacement must be in contact with the windings like where and how you found it. In your case it is meant to open when the windings get hot, not the inside of your wall wart.

  5. Marko

    Thread Starter Member

    Feb 10, 2008
    Thank you all for the info. Truly, finding the little square axial leaded ones is difficult unless one wants to buy a lifetime plus supply of them. I may be able to fit the axial lead variety against the windings. Yes, the transformer is small, but I think there is room - just. So I will probably go that route.