# LED Lamp Conversion

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http://www.greenledlamp.info This project shows you how to convert a halogen desk lamp into a high efficiency LED lamp. The full project is on greenledlamp.info .The converted lamp can be powered using a AC/DC wall adapter or directly from a 12 volt solar source or other alternative power source. It is easy to build but does require some basic electronic skills.

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## Step 1: Circuit Design

This step shows you how to build the lamp circuit. For more details and circuit theory goto http://www.greenledlamp.info. I used a 12 volt 500mA DC adapter, 3.2volt 20mA/100mA white discrete LEDs, 100 ohm 1/4 watt resistors (8) and 33 ohm 2 watt resistors (2), female DC jack.

The LED array connects 24 LEDs into 8 parallel rows. Each row contains three LEDs and a 100 ohm resistor. The two 33 ohm resitors are connected paralle resulting in 16.5 ohms in series with the LED array.

## Step 2: LED Array

Building the LED array requires good soldering skills. The lamp has a 55mm opening to accommodate the LED light. We will need a piece of rigid plastic that can be easily cut and drilled to set inside the opening. I used ABS plastic central vacume pipe as it can be easily flattened by gently heating it in an oven at 225degF for five minutes then pressed flat until cooled. The result is a very flat piece of 2mm thick plastic that is very easy to work with and its is usually white making it a good reflector.

## Step 3: LED Array Reflector Plate

Draw out a template to fit the 24 LEDs using the pattern in the above picture. Trace out the hole in the lamp head onto the ABS plastic. Apply the template and mark each hole using a nail or punch. Remove the template and use a small drill bit to drill out pilot holes at each of the marks. Next drill test holes in a scrap piece of plastic to determine the correct size drill bit as some LEDs are made slightly larger or smaller than others depending on the manufacturer. Drill out all the pilot holes with your selected drill bit.

The holes should provide a snug fit that the individual LEDs can be pressed into. Insert all the LEDs as shown in the pictures paying close attention to the polarity of each LED. The result will be 6 sets of three LEDs around the outside and 2 sets of 3 LEDs in the center.

## Step 4: LED Array Soldering

Bend the leads of each set of three LEDs as shown in the picture and solder giving you eight sets of three LEDs. Next solder the positive (anode) lead wire to the anode of each LED set as shown then solder a resistor to each of the eight cathode leads of each set as shown. Bend the resistors into place and solder the negative lead wire to all of the cathode resistors.

## Step 5: Test LED Array

After the LED array is assembled it should be tested. Connect the two parallel compensating resistors to the negative lead of the array and solder on the female half of the DC jack. Connect it to the DC adapter and measure the voltages across the DC jack, the compensating resistors, the LED series resistor and one LED. Calculate the current for each LED section by dividing the voltage across the resistor by 100 ohms. Then calculate the current through the compensating resistor by dividing its voltage by 16.5 ohms. You should get close to 20mA and 160mA. The voltage across one LED should be close to 3.2 volts.

## Step 6: Lamp Assembly

Disassemble the lamp paying close attention on how it goes back together. Remove the original halogen bulb and socket, the AC wiring (electrical cord) and the transformer. We will reuse the low voltage wires, the switch and the lamp frame.

Remove the compansating resistors from the LED array and fit the array into the lamp head. Secure it with silicon and let dry. Solder the LED array leads to the lamps low voltage wires making sure you know the negative and positive leads at the lamp base. Protect the connections in the lamp head with shrink tube.

Fit the female DC plug into the back of the base by cutting or filing the hole left by the AC cord and secure with glue. Solder the compensating resistors to the negative lead of the LED array and to the negative side of the DC jack (the picture above shows the 3.9 ohm resistor not the two 33 ohm resistors we are using). Connect the LED array positive lead to one side of the lamp switch. Connect the other side of the lamp switch to the positive side of the DC jack. Make sure all connections are secure and protected by shrink tube.

We are going to reuse the transformer as a weight in the bottom of the lamp. Simply cut the transformers 4 lead wires and secure them with electrical tape and reinstall the transformer in the base. Reassemble the base and test the lamp to ensure there are no loose connections or short circuits.

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## 14 Discussions

You seem to have copied the .info website here... What's the story? Also, the website claim a consumption of 2.5W but 24 LEDs, using your numbers of (3.2v @ .02amp) accounts for only 1.5W. So the circuit is wasting 40% of its power heating up the adapter and resistors?

6 replies

yes I did copie the website as it is my website ... the circuit is wasting some power but its is entriely made from recycled components ... couldnt find 120 ohm resistors...

True, 120 might be hard to find, but if you could get 100 Ohm and find a 20 ohm to 50 resistor, it would put you where you need to be. OR if you had a Load (and I do mean like 100+) 100 ohm resistors, you can put two in parallel to make a 50 ohm resistor, add that in series with a 100 ohm resistor, and you'll have a total of 150 Ohms. I

We used to 'shave' our resistors - let's say I need a 120 and had only a 100, I'd take an xacto knife and nick the 100 to just under the enamel. With a little bit of practice, we can get anything up to 200ohms from the 100's.

there is about 1.5watts consumed by the LEDs, another half a watt consummed by the eight 100 ohm resitors and another half watt by the compensating resistor... but that was as good as I could get it with the available components as I did not want to buy anything... another option is to simply remove the two 33 ohm resistors and use a lower voltage dc adapter.

Have you tried replacing the 100ohm with another LED? If you have 12.8v from your adapter, which is very likely, then you'd get 25% more light and save yourself the hassle of destroying circuit boards for their resistors! I'd start and measure the voltage across the wall-wart, then the LEDs. Chances are you can just use another LED to drop the voltage you are currently burning up in the resistor and have all 4 LEDs running with about the same brightness. Now THAT is being green!

I do mean to criticize here, but you can get rid of the resistor thats in series with the whole circuit. Since your want only 20mA to get to the LEDs, you can calculate the resistor using ohms law. Since the resistor is going to have a voltage drop of 2.4v, you would need a 120 Ohm resistor (2.4v/20mA). Closest standard value is probably 140, Which would still be OK! Less current would get to the LED, but in my experience, unless your reducing the current by something drastic Like 15mA) a 5mA difference in current doesnt make a difference in brightness. Less current also means less power being wasted in the resistor. Also, power adapters are rated for that load. If your adapter is 12V @500mA, That means it will be 12V when you connect a load of 500mA on it. I have a feeling the 2.5W is a typo. Theres no WAY a transformer can waste that much heat. Their 95% to 99% efficient.

2 replies

It'll be close: About 0.5W is burned up just in the resistors, and the efficiency of a 500mA transformer running at 160mA cannot be that great.

try to find 120ohm reisitors on scrap circuit boards ... not very easy ... 100ohm is much mor common on power supply boards ... I might have miscalculated the watts i will check .. thanks for the comment