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Transformerless Power Supply Repaired

By Jestine Yong on November 26, 2013

My customer brought to me few units of different LED lights for repair. One of the LED lights was the ceiling round type light. The complaint was no power and as usual one has to open it up to check the power supply.

Below are the photos of the round type ceiling LED light power supply and circuit:

If you notice carefully it does not have the switch mode transformer. It has the bridge rectifier, 1uf 400 volt filter cap, power ic (LNK417EG), output filter capacitors, 4 coils and some other SMD components in the front and the back of the circuit board.

This is the first time I’m dealing with a transformerless power supply. I have checked from the internet to see if there is any transformerless power supply that uses LNK417EG but no information on this. Previously I did have blogged about transformerless power supply but the circuits were different from the one above. You can view the posts below:

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http://www.jestineyong.com/transformerless-switch-mode-power-supply-circuit/

and

http://www.jestineyong.com/transformerless–power-supply-circuit/

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I did have printed the LNK417EG datasheet out but it does not explain about the transformerlesscircuit design. It has the circuit that uses the common switch mode transformer design. Since I can’t locate the exact information and schematic for you thus I can’t explain in details how this complete circuit works.

Note: If you have the information perhaps you can share it with us in this blog-thanks.

Ok we have done the searches part and now let us move on to the repairing topic. The complaint was no power and obviously we have to check the main fuse first.

I found the fuse to be open circuited and the spec of the small rectangular fuse was a slow blow 400ma 250 volt. I proceeded to check on the e-caps and SMD bridge rectifier and both components were found to be good. There were no burnt marks in the circuit board.

My conclusion it could be only the fuse blown or the power IC have problem (shorted). I connected a bulb across the fuse two points and powered on. The bulb did not light up (which means a good sign) and there was output voltage (about 58Vdc) present which also means thepower IC was working fine.

I replaced the fuse and put everything back includes the metal cover of the power supply and powered on. All the LEDS were lighted up as seen from the photos below:

Conclusion-Even though the transformerless information is limited on this particular design, the troubleshooting method is still the same and I managed to repair it. Assuming if more than one component have problem, it still an easy fix because the power supply does not contain lots of components. In the future if you came across or already deal with such circuit perhaps you can share with us your experience in the comment section-thanks.