How often happened that you or someone in your family, by mistake, to overthrow the desk lamp? If you are like me, maybe this has happened many times...
If when I had installed in the desk lamp an incandescent bulb which costs 30 cents, the event was just unpleasant, now when I have a CFL bulb which costs
over 6 USD, the event is "expensive". For this reason, the last time my offspring slammed my desk lamp with an innocent "UUPS!", I exclaimed
I decided that I have to replace the classic bulb or CFL from the desk lamp, with something more shock resistant...
But that could withstand the treatment of a 10 years child and at the same time to make enough light to work at desk in good condition, to be reliable
in operation and decently priced? If only a few years ago, there was no a solution to this question, the answer is now clear: a power LED. As I already had
some Cree MX6 Q5 LEDs with 3W power and maximum light output of 278 lumens used in LED Lighting - first version
I decided to use them for this project too.
If the LED selection was the simplest part, powering him will be much more complicated ... or maybe not :)
For simplicity, I decided to use a switching power supply of a mobile phone, perhaps together with an electronic adapter that would give me the necessary
voltage and current to power the LED.
For this purpose I have chosen the power supply of a defective mobile phone Siemens A52 having, according to the manufacturer, an output voltage of 5V and a
current of 420mA.
According to manufacturer specifications, MX6 Cree Q5 LED can be powered at maximum current of 1A at a voltage of 4.1V, and I expected I'll need a 1 ohm
resistor to lower the voltage by about 1V from the 5V (given by the power supply) to the 4.1V accepted by the LED, and that only if the power supply would
handle the maximum current of 1A.
To check the maximum current supported by the power supply I connected to his terminals various resistors, measuring in each case the voltage and then
calculating the current. Here are the obtained values:
To my surprise, the power supply seemed to have by construction the current limited to about 0.6A, value he looks like it stand without problems. Testing
in the same way other mobile phones switching power supplies, I found that absolutely all of them are limited by construction to a current from 20% to 50%
higher than the one specified by the manufacturer, which now I find it makes perfect sense: any manufacturer will design the power supply so that it will
not overheat even if the powered device would be damaged, even in short circuit ... and the easiest way to do this is by... current limiting!
I had therefore, a constant current generator limited to 0.6A, very effective (a power adapter cell phone heats up only slightly during operation),
powered directly from 230V AC, ready-made by the factory, with very small size. And this is simply GREAT.
Most of the power supplies of mobile phones, especially the latest, are small enough to fit in the body of a CFL energy saving bulb, especially if
the latter is more "fat", as in this case.
For the beginning I made "the autopsy" of the power supply in order to extract the "organs" to be inserted in the body of the new bulb. As most power
supplies are assembled by soldering, extracting consists of cutting with a saw blade... So pay attention to bruises for those less skilful ...
In order to fit inside the body of the lamp, it was necessary to mechanically adjust it as in the image below:
To fix the board inside the bulb, I used sanitary silicone, taking advantage of its resistance at high temperatures. Before closing the bulb, I attached
to the cover (I used a screw) the heatsink on which is located the power LED.
And here's the bulb assembled. Note that despite those written in the image below, the power consumption is just under 2.5 W and the luminous flux is
about 190 lumens, perfect for a economical, durable and resistant desk lamp. And all this for up to one hour of work, except of course the time for drying
the silicone and heat-conductive adhesive used to fasten LED on the heatsink.
No restless offspring have suffered during this project, on the contrary, she was the first and happy beneficiary of LED lamp :).
As you can see, desk is perfectly lit and because of the angle of 120 o of LED lighting, during the night those sleepy will not be disturbed.
I was so excited about the success and ease of implementation of this project that after a few hours I had finished another LED bulb.
Note that instead of the cell phone power supply used in this project, you can use a stronger one having a 5V voltage and a rated current of 2A, which could
power a Cree XM-L T6 9W power LED having the luminous flux of 910 lumen, as used in LED Lighting - third version.
This is what I'll do, I'll replace the LED power supply from LED Lighting - third version and from
LED Lighting - first version with a much simpler and more efficient mobile phone power supply.