Repairing A Lamp – The Reverse Engineering Way

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This article is about one occasion where I had to use the concept of reverse engineering which is the method of breaking things down to understanding its design and working. Although reverse engineering is a skill that an engineer should posses, I was always so focused on my own designs that reverse engineering never crossed my mind. That was until one day when my LED lamp suddenly blew. Naturally, I was curious to know why. So I removed the mounting screws and dismantled it to pieces. The main  circuit board actually had all the LEDs and the side on which the wires come from the power supply was intensely charred.

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Most of the board except this was almost untouched. So, I wiped out the soot and examined the board’s PCB traces to understand how the LED’s and other components were connected.

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In the first glance I saw that the LEDs were in series. So, I knew that it had to be a constant current circuit. A quick search using the IC (on the left) number – CYT1000A which is a Linear Current Driver showed me that I was right. So the circuit was very simple – an array of LEDs connected in series to a constant current driver and then all that would be required is a DC supply. It is only logical that the charred IC on the right had to be a rectifier of sorts so as to convert the AC supply into the DC version required by the circuitry. When I desoldered and cleaned the IC to took a very close look at it, there was the number MB6S etched on it. MB6S is a 600V Reverse voltage tolerant bridge rectifier from Fairchild semiconductors. Thus, I had reverse engineered the circuit. It turned out that it was a circuit given as a typical application in the current driver IC’s datasheet.

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But I still didn’t have the answer to what I was actually curious about – what went wrong? I used my multimeter in continuity mode to check the connections between the wires and the components. The AC (input) terminals of the MB6S were short-circuited. However, there was no electrical connection between the wires attached to the board and the MB6S input terminals. If the lamp was to work, there had to be a connection so that the power can be supplied. So patiently from the input terminals I kept check continuity to see where the break in connection was. And not surprisingly the break in connection was exactly above and below the circular hole (the hole is there to provide space for the mounting screws). Like a detective, I had enough evidence now to judge what went wrong. This is what I think happened – An abnormal transient in the AC supply made the voltage higher than what the diode is capable of blocking. Therefore the diode breaks down creating a short-circuit between the supply lines. This draws a lot of current through the wires to the IC input terminals through the PCB trace around the circular hole. The huge current produces most heat where the PCB trace is narrow (above and below the circular hole) since the resistivity is comparatively higher there.  The huge current burns the plastic solder mask resulting in the charring and the narrow metal film ‘melts or vaporizes out of contact’.

If my deduction was right, simply replacing the bridge rectifier would make the lamp work again. So, I used the commonly available W10 bridge rectifier which is more bulky but has far greater tolerance (1000V) than the MB6S (600V).

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I soldered the supply wires to AC terminals of the W10 and then soldered the DC terminals of the W10 to the correct DC terminals on the lamp. After all the soldering, came the moment of truth. I connected the AC supply to the mains and switched it on.

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It worked and I was right! Then it was a matter of finishing things up with some hot melt glue to insulate all the metal terminals and prevent the wires from excessive movement (which will eventually cause any soldering to wear away and break contact). Then I tucked the wires and neatly assembled it back into the recognizable lamp.

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Finally, the mounting screws went in place and the lamp was as functional as it used to be. As simple as the lamp might be, it was still interesting to reverse engineer it. Only when I understood the circuit, did I find what could have gone wrong and only when I understood what went wrong, did I find a way to undo it. The end result is that, I basically salvaged a lamp but this is one of those cases where the process is much more rewarding that the result itself.

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