I was tearing apart a transformer that was not working (from halogen lamps) and I found this little thing inside:

A component The transformer

Eventually I found out it was a bad connection that made the transformer faulty and am trying to reassemble it, but I am not sure how this component was wired. What is this component, what purpose does it serve and how do I properly wire it back?

Thank you!

  • \$\begingroup\$ Paint? Varnish? \$\endgroup\$
    – winny
    Feb 14, 2018 at 20:23
  • \$\begingroup\$ Looks like copper contacts painted... \$\endgroup\$
    – Eugene Sh.
    Feb 14, 2018 at 20:25
  • \$\begingroup\$ Everything was covered with some sort of white paint yes. It made the disassembly even more destructive at the end. But now the plastic is broken and I must find another way to wire it back. It might have gone bad because heat from transformer made paint liquify and move over contacts, but I'm not sure. The stratches seen on contacts were made by me to test the transformer. Very weird doing on manufacturer's part. \$\endgroup\$ Feb 14, 2018 at 20:25
  • \$\begingroup\$ THermal switch I am guessing. \$\endgroup\$
    – Trevor_G
    Feb 14, 2018 at 20:37

1 Answer 1


That "little white thing" is a thermal cutoff. It is a sort of fuse that reacts to external heat.

They are used to prevent things from burning up. The one you have is rated for 130 degrees celsius.

Measure it with an ohmmeter. It should read as a complete short circuit (very nearly zero ohms.)

If it reads as an open circuit (high resistance) then it was exposed to too much heat and opened the circuit. This probably saved you from having a fire.

Replace it with a part rated for the same temperature.

Under no circumstances should you leave it out or replace it with a wire. If it has opened up, then it is needed.

Try not to place as heavy a load on the transformer, or maybe not run it as long. Also make sure not to cover it up - it needs air circulation to keep cool.

  • \$\begingroup\$ I'm a part-time landlord, so I used to have quite a lot of 12V halogen downlights in my flats. I phase them out now. "105W" chinese transformers will burn with 100W of halogens. The plastic chars and melts, it's a mess. Without the mandatory thermal cutoff it would have resulted in a fire, several times by now. These days if I must use a 12V system, which I try to avoid, I will use the same 105W transformer... and put 20W of LEDs on it. \$\endgroup\$
    – bobflux
    Feb 14, 2018 at 20:45
  • \$\begingroup\$ Thank you. Well according to my multi-meter it is still functional (not zero ohms). My multi-meter is analog and it says infinity across contacts. But the theory that the fuse blew wouldn't be far fetched because the enclosure in which the thing was sitting is practically falling apart from heat and has a darkened indent in plastic where transformer was sitting. So possibly indeed the tiny tiny wire from transformer has burned and severed the connection. If I understand correctly and this fuse makes a short main fuse for apartment should also have blown (but it didn't)? \$\endgroup\$ Feb 14, 2018 at 20:47
  • 2
    \$\begingroup\$ It SHOULD be zero ohms. If it is high resistance then it opened the circuit to protect your house. High resistance= replace it. \$\endgroup\$
    – JRE
    Feb 14, 2018 at 20:55
  • 1
    \$\begingroup\$ Your description "falling apart from heat" sounds like most burned transformers I had. I installed these not in airtight plenums under rockwoool insulation, but in well-ventilated places. These transformers are built on the cheap. If it overheated, even if you fix it, it will fail again under the same conditions unless the halogen lightbulbs are replaced with LEDs which use much less power. Considering such transformer is very cheap, put it in recycling. Don't waste time repairing it. Don't use these cheap transformers at their rated powers. Always derate by a lot. \$\endgroup\$
    – bobflux
    Feb 14, 2018 at 20:57
  • 1
    \$\begingroup\$ @peufeu Right! Moreover, there might be also legal issues. IANAL, but if the OP fixes it and then fails again, the OP might be liable for all the damage it may cause. Here in Italy any modification or repair made to part of an electrical system should only be done by licensed electricians, otherwise the system loses regulatory compliance. Of course, this is not a big problem if you know what you are doing and do a quality fix (one can beat mediocre electricians ;-), but fixing that crap is risky business! \$\endgroup\$ Feb 14, 2018 at 21:13

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