Background
A number of years ago i installed lights along the top of kitchen cabinets to provide some 'atmospheric' additional lighting. They are of a simple design. A plug mounted AC step down transformer provides power to the light unit via an inbuilt switch. The bulbs are 'pygmy' two pin types, rated at 12V 20W. Somewhat wasteful but effective. Recently one unit failed and i assumed the bulb had failed. Changing the bulb did not solve the problem. I assumed that it must be a problem with the transformer so i used a digital multimeter to check the continuity of the transformer primary via the AC pins. This indicated an open circuit. A known good transformer from another of these lights provided a reading. I decided to see if the unit could be repaired.
This is the type of light being repaired. The light unit is fixed to the top of a bookcase or kitchen cabinet and connected to the transformer via a two pin plug. The transformer can be hidden on top of the bookcase or cabinet.
Surprisingly for such a low cost unit the case was held together with Torx screws. So often this type of unit made in the Far East is glued together.
I removed the torx screws and carefully pried the two halves of the case open. I was then able to remove the transformer from the case. I could see that under the outer layer of yellow tape a component was held in place. I guessed this would be some type of thermal fuse to protect the transformer windings from over heating. I carefully removed the outer tape and exposed the component. I manged to read the printing on the component and was able to confirm that it was indeed a thermal fuse. The thermal fuse was wired in series the primary winding of the transformer. I was able to use a multimeter to confirm that the transformer primary winding was still continuous and had not been damaged.
I removed the torx screws and carefully pried the two halves of the case open. I was then able to remove the transformer from the case. I could see that under the outer layer of yellow tape a component was held in place. I guessed this would be some type of thermal fuse to protect the transformer windings from over heating. I carefully removed the outer tape and exposed the component. I manged to read the printing on the component and was able to confirm that it was indeed a thermal fuse. The thermal fuse was wired in series the primary winding of the transformer. I was able to use a multimeter to confirm that the transformer primary winding was still continuous and had not been damaged.
I got the component catalogues out and very quickly located a compatible replacement. The parts were ordered and a couple of days later they were delivered.
I was a little worried that the thermal fuse had blown, reassuringly there was no sign of excess heat or burning inside or on the outside of the case. I did a bit of research and found that it is quite common for thermal fuses to age and fail although no over temperature condition has occurred. As the transformer unit is almost 8 years old i was happy to try to repair it without worrying about there being an underlying problem.
Carefully de-soldered the old thermal fuse and prepared the transformer wire ends for re-soldering. I then carefully wrapped the very thin enameled copper wire around the leads of the thermal fuse. I soldered the wires in place using as little solder and heat as possible. Unfortunately the thermal fuse did it's job, the heat from the soldering iron caused it to blow! I started again and this time i attached a 'bulldog' type paper clip to the body of the fuse and while soldering the leads blew strongly on the clip and thermal fuse body. I soldered one lead at a time and checked the continuity after each step. At the second attempt the repair was successful. The fuse was then mounted on the primary winding and taped into place. The transformer was then re-installed into the case and the screws tightened. The light was tested and worked as expected.
For a cost of approximately £0.30 and a couple of hours work including diagnosing the problem, ordering parts and performing the repair i once again had a working set of lights. I ordered a few spares in case any of the other lights of the same age should fail. The lights are still in production but the body colour has been changed from white to silver and to purchase a replacement i would have to travel all the way across town.
For a cost of approximately £0.30 and a couple of hours work including diagnosing the problem, ordering parts and performing the repair i once again had a working set of lights. I ordered a few spares in case any of the other lights of the same age should fail. The lights are still in production but the body colour has been changed from white to silver and to purchase a replacement i would have to travel all the way across town.