CFL ballast design, and using dead lamps for repair

P E Schoen · 2013-01-21T09:04:24+00:00

I have a desk lamp, with a magnifier and a 12W T4 circline fluorescent bulb, that I use constantly for working on PCBs and electronics projects in general. I bought it several years ago and recently it started flickering and then died. I replaced the bulb, but still no joy, and after replacing the blown fuse and two damaged transistors, I found that the little transformer had an open winding. So, I thought, a 60W equivalent CFL is actually about 11-13 watts, and the little circuit in them should work. I had a couple of broken or dead bulbs ready for recycling, so I opened the bases, cut the leads, and extracted the PCBs. After a few unsuccessful tries, I was able to get it to work and now my lamp is once again operational. I found some schematics of the CFL driver boards here: Some of those circuits matched what I had almost exactly. It was a little difficult to follow the explanation of how they operate, but what was confusing is the four pins shown on the lamp itself, which is also how the bulbs are made. I assume they are the heaters that are usually activated with a starter, but I did not find any continuity on those pins. The desk lamp only had one wire to each of two pins on the circline bulb, but in the CFLs all four wires were connected to different points on the PCB. It would only work when I shorted the connections that would have gone to the heaters, and it seems to work very well. It starts to light at 50 VAC and reaches full brightness at 100-120V, at which it draws about 100mA. That's close enough to 12 watts for me! Here's a little clip of my repair project: This is good to know. Those little circuits in each CFL have a lot of good components, including a DIAC and high voltage transistors, and it's probably possible to use them to drive small fluorescent lamps for DIY projects or repair. I doubt they could be used for the 40W tubes, but a driver from a 100W CFL should work on a 20W tube such as are in many...

Jan 21, 2013 40 Replies

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Spehro Pefhany 13 years ago

An explosion like "pop rocks" or an explosion like Bikini atoll? Maybe your mains has more oomph, but I've not seen anything that would blow apart a proper housing from a residential power line.

Best regards, Spehro Pefhany

"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com

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CFL ballast design, and using dead lamps for repair

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