LED Desk Lamps





Introduction: LED Desk Lamps

About: I am in my third year of my electrical apprenticeship. This is my second career; formerly I was in the firefighting / EMS field. I enjoy my work and am always curious about new hardware (devices, wire, con...

What to do with discarded LED PCBs? Use them as desk lamps! Duh! A while ago I recieved these 12" strip PCBs with 30 high intensity LEDs on each. I took 4 and attached them to a board to see how much light they would emit together. Turns out that they put off what appears to be about the same as a typical 18 inch flourescent undercabinet fixture. I now have two that illuminate my computer desk.

Step 1: Fabricate Mounting Board

Because these strips do not weigh much and also do not creat much heat at all I chose to secure them with cable ties. Also, I had these laying arround, so why not? I then took the scrap 3/4" thick board and measured to length and width and cut with a handsaw. I think the dimensions ended up being about 12" long by 6" wide. Lastly the mount holes for the cable ties had to be drilled. I drilled these a little narrower than the strips. One pair at each end.

Step 2: Spacer Fabrication

The strips still needed to be spaced off of the board to allow the wires to have room. I took the nylon casing / tubing that the strips were originally installed and cut them to about an inch long each and cut the top of the tubing off lengthwise. I then inserted the strips into these at each end and cabletied everything together.

Step 3: Installing the Fixture

Each fixture was mounted underneath my book shelfs using 4 1" drywall screws. The power cable is a 2 wire 14 AWG cable, which was ran through the cabinet in the corner and through the top where the transformer sits.

Step 4: Electrical Connections

This was the easiest step of all. These strips have miniature molded plugs that allowed them to be daisy chained together. I just pluged each one together and then attached the two leads from a 120 volt primary / 24 volt 40 volt-amp secondary transformer.

Step 5: Conclusion

The previous information was in error due to a crappy multimeter.

After consultation with an engineer friend. I found out that the initial resistor " R1 " value is at 180 Ohms and that the remaining eight resistors "R2 thru R9" each have a value at 460 Ohms.

He also advised to measure voltage across each LED and voltage across all the LEDs in each branch circuit. With that information, the total resistance for each branch circuit was calculated. For simplicity, each branch will be called, Branch 1, Branch2, etc.

Branch 1 had a total resistance of 960 Ohms. Branches 2 thru 9 each had a total resistance of 860 Ohms. The total resistance for Branches 2 thru 9 is 860/8 = 107.5 Ohms. The total resistance for the entire board is
(107.5 x 960) / (107.5 + 960) = 96.67 Ohms.

The total Voltage for the board is 20.7 Volts. To obtain the total power usage I use this formula.
Power = Voltage Squared / Total equivalent Resistance


20.7 x 20.7 / 96.67 =
428.49 / 96.67=
4.4 Watts per strip

Each fixture has four strips @ 4.4 Watts each. This equals 17.6 Watts per fixture. This is just less than the original fluorescent ones that these replaced.

However, the LED fixtures have some key advantages. First, they require no real parts that need to be replaced periodically, i.e. tubes, ballast, etc. Second, they do not make noise, the fluorescent ones hummed a bit. Third, these can be dimmed, sort of. Dimming is accomplished in a stepped mode by switching off each strip individually to obtain a required light level (this is a feature to be added in the future).



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    I have a better solution for the application.http://www.fuzhouled.com

    I've been searching for a good solution to the flourescent bulbs I currently have, and this looks like a great solution.

    Do you have any idea on the brand name the strips are (the part number appears readily available in your photos)

    What about the part number on the power supply?

    these would never burn out right? how much dose it cost to make these? cool idea im always looking for a way to use reduce, reuse, and recycle.

    1 reply

    They will eventually "burn" out, but the lifetime of them is superior to that of incandescent and flourescent. I paid nothing for these, the cable or the small transformer. They all were being discarded.

    Oh... I didn't mention it before, but I think this is a pretty good Instructable. I'd appreciate if you could add a wiring diagram for the setup.

    Im not one to cry code, but that pic in step 4 is pretty bad

    2 replies

    Other than the missing coverplate for the 110V receptical, what is out of code?

    I'd cover the 20v bus bar as well. Just for safety's sake.

    You're calculations have a bit of a problem. You see, this is not entirely a resistive circuit since each 4 LED module has it's own regulator. I suggest that instead of calculating based on the total resistance, that you measure the total current while the unit is connected. Disconnect one leg of the secondary and connect that to the Multimeter's Amps connector. Then connect the common to the point the secondary wire was connected. IOW; The amp-meter is connected in series with the device. From this you can determine the Wattage. I expect it will still be very low, but not in the microamps range.

    2 replies

    So, would you say that Chavez is correct with his estimate? My DMM is a cheap crappy one and does not measure AC amps. However I will be getting a loaned one in a few days to measure the AC amps.

    I expect with 120 LEDs, you're probably pulling 4 to 6 amps @ 120V. Still significantly less than incandecent or even florecient. Note that your Lumins are probably rather low, but LEDs supply directional light so none is wasted directed the wrong way.

    To get an idea about LED lighting and the currents used, check out C Cranes LED Light Bulbs.

    120 high intensity leds, a transformer, and four full bridge rectifers and resistors and you calculate 1 milliwatt!!!, without a schematic, I can not give you a good estimate but roughly you have 30 Watts or 30000 mW so you don't think it's a typo. would you agree with me LasVegas!! . clamp an AC current meter on one of the wires from the AC wall outlet and find out the rms voltage and let us know what you get.

    Great idea. Where would one go about finding LED PCBs if they don't have them to begin with? I was thinking about mounting one behind/below my LCD monitor to create a desk light right onto the keyboard/desk.

    1 reply