Modifying a 240V Compact Fluorescent Lamp (CFL) for 12V Operation
Experimental Project

The title of this page is a bit misleading since the actual fluorescent tube part of a CFL is neither 240V or 12V. It's the drive circuit that defines what voltage the lamp is going to run on, and whether it be mains or battery the tube will be driven the same. The reason for this project was that I had a 12V CFL that worked fine except it was a cool white type (6400K) which gave off the most hideous light and as such I would never have used it. So I thought what if I took the printed circuit board from the 12V CFL and fitted it into the body of one of my mains powered CFLs which are warmer and more pleasant (2700K), then I would have a warm 12V CFL. So I did, and it worked!

WARNING: Only attempt this if you are confident and know what you are doing as there is HIGH VOLTAGE involved

The disassembled mains CFL

I figured that it would be a good idea to use lamps of roughly the same power rating and physical size. Luckily the ones I had met this criteria. So the first thing I did was to break open the casings. The method I used was to cut around the join (which is normally bonded) with a junior hacksaw, being careful not to go too far in and damage the wiring to the tube. The three parts consisting of base, circuit board and tube could then be pulled apart, though the board was quite snug and needed prising out. The wires from the tube are usually (but not always) wrapped around pins to make a connection to the circuit board. These can be easily unwound and then rewound onto another board with no soldering required

WARNING: Be careful not to break the tube as it contains mercury which is hazardous to health

Make sure the 12V circuit board fits in the lower half of the 240V lamp body and that this meets with the top half (with the tubes) without any gaps. If you are happy, then it's time to reassemble the lamp with its new 12V electronics. Something to watch out for is that one or both of the leads that connect to the screw base may become disconnected if moved around too much. The one that goes to the tip (+) can be resoldered easily, but the one for the screw thread (-) is a bit more tricky. I got around this by drilling a small hole at the top of the thread where it meets the casing, passing the wire through and soldering it on the outside (see photo below)

Before joining the two halves of the body back together, I tested the lamp to make sure that it worked, remembering of course that it was now a 12V bulb and not to plug it into the mains! Epoxy resin was used to bond the assembly together, with any excess being wiped off before it dried. Tape was then wrapped around the joint to hold things in place while it set. I now had a finished 12V warm glow light bulb. Interestingly, mains CFLs seem to have smaller tubes than their 12V counterparts of similar power rating. This is a great advantage as many light fittings will only take smaller bulbs

A Philips Genie CFL modified to run on 12V

SAFETY NOTE: This is an experimental project and as such, results will vary. Care must be taken at all times to ensure that there is no risk of fire or electric shock. Remember that CFLs contain mercury and must be disposed of safely, usually by taking them to a recycling centre. Never throw them out with household waste. Do not attempt anything if you are unsure or lack experience

A modified 12V CFL illuminating a parking area (during some winter snow!)

For information only:

While playing around with 12V CFLs I dissected one of the circuits to find out what makes it tick. The schematic is shown below, but note that this is just the basic driver. Other components not shown here were for a delayed start circuit using a relay to give a 'pre-heat' function for the tube (to improve the life expectancy) and a reverse protection diode, fuse and thermal cut out in line with the supply leads. Even without these extra parts the circuit still worked, though I would always recommend keeping the protection components
. Also there were originally two 1K resistors from the 12V supply feeding each base, but I found that it kicked into life with just one. The tube connections were found by trial and error but you might get lucky!

The transformer was dismantled to find out how many turns it had and their direction, the results of which are shown below. Note that the windings are displayed separately for clarity

I don't know the details of the core used but while experimenting I replaced the original transformer with another one I made using a core salvaged from an old PC power supply. As I only had enough 34 SWG enamelled copper wire for 240 turns on the secondary, I decreased  the centre tapped primary to 6+6 turns. This was all guess work but it fired up OK (not literally), and the circuit drew about 900mA from a 12V power supply which is just under 11 Watts. Although this project was done out of curiosity, it's much less hassle to just buy a 12V warm CFL from eBay, but not half as much fun!