Intro: LED Desk Lamps
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).