In this Instructable, i am going to show you how to convert one of those power-hungry halogen desk lamps in to a 10x more efficent LED lamp with about the same brightness. I apologize for any grammar or spelling mistakes wich might appear, Im a German, so dont expect perfect english :p.
Step 1: Take It Apart!
See a skrew, remove a skrew! These are the parts you will find in this Lamp. If there are some special skrews on the bottom of the Lamp, carefully break the lid free from the skrews. Be patient, because you will need it later on, as well as the other parts, except the halogen bulb and GU 4 holder.
Step 2: Get Your Parts!
You will need for this Project:
- an afternoon
- some soldering skill
- white LEDs ( I used 20x 3,1 V; 20mA)
- a resistor ( See how to calculate it in step 9)
- a piece of perfboard
- a dc power adaptor with a Voltage of 12,5 to 17 V (mine is 16,5 V dc)
- a female dc jack
- an 1000 uf electrolytic capacitor with a voltage rating of at least 2-3 V higher than the power source (this component is not really necessary, it is used to smoothe out the voltage)
Step 3: Modifying the Reflector
Cut out the area marked red.This can easily be done with small scissors and a metal file, since the material is very soft. Of course you can also use a rotary tool. This is where the Led module is going to go later on, so measure the dimensions of the hole. Also do not forget to stick a piece of electrical tape under the hole to prevent any short circuits.
Step 4: Creating a PCB- Layout
Next, draw a PCB-Layout according to the dimensions of the hole in the reflector. Since you want to power the LEDs with 12,5V or more, you will need 5 columns of each 4 LEDs in series in parallel. Its important to keep an eye on the polarity of the LEDs, since they are light-emmiting-DIODEs. In the pictures above you can see my PCB layout, wich you can copy or use as a starting point for variations according to your needs.
Step 5: Cutting Out the PCB
Cut your PCB out using a rotary tool with a cutting wheel or a small hacksaw ( I found out using a nearly dull blade works best). Here is another tip: Leave about one to two rows space on every side, so if your PCB-layout needs 17x8 copper dots, cut it to 19x9 or 19x10 copper dots, so if you mess up cutting, its not a big deal.
Step 6: Soldering the Leds
This is where it gets a little tricky for those who dont have much soldering skill. I recommend to not solder every led first and then soldering the traces, because this will get messy really quickly. I would rather begin with the leds of the first column and then soldering the traces a.s.o...
If you have finished, check your connections with the contenuity function of your multimeter to prevent any short circuits, before you solder the positive and negative breakout wires of the columns together.
Step 7: Finishing the Upper Part
Now stick the Led-module in the hole and secure it with hot glue. You might need to enlarge the hole a littlebit to fit the module inside, wich, again, can easily be done with a metal file or a rotary tool. Last but not least connect plus and minus to the cut off cables, wich run to the bottom part of the lamp and screw the glass and the yellow thing back into place. The upper part of the lamp is finished.
Step 8: Modifying the Lower Part of the Lamp
First, enlarge the groove, where the original powercord left the casing, so your dc jack will sit flush with the edge. Then secure it in place with hot glue. Make sure to solder wires to the jack, since this will make connecting a lot easier later on.
Step 9: Calculating the Resistance of the Current Limiting Resistor for the LEDs
Now you need to find out two things:
1. How high has the resitance to be?
2. How much power is it going to disappate?
In the picture above you, can see my calculations. Keep in mind that your LEDs and power source might be different, so you have to do your own calculations with your numbers. Another important thing to know is, that while LEDs get warm, their forward voltage changes. In my case the forward voltage dropped from 3.1 to 3.0 V per LED. So now you can calculate the resistance with ohms law (R=U/I) and find out the power rating with this formula: P= R*I^2
Since I did not have any power resistors laying around at the time, I used a whole arrangement of 1/8 W resistors to disappate the generated heat properly. And yes, I know that using a resistor is not the most efficent way to power the LEDs, but I wanted to keep this Instructable simple, and even with the resistor the lamp is over 80% more efficent than before.
Step 10: Finishing the Electronics
Connect everything as shown in the schematic in step 2. If your resistor has to dissapate a significant amount of heat, say >0,3 W, I recommend mounting it near the ventilation holes to keep it cool. If your casing does not have these holes, you can easily drill some with a power drill and a 2-3 mm universal drill bit. Also keep an eye on the polarity of the electrolytic capacitor and the LEDs, since reverse polarity can cause the capacitor to explode and the leds to stay dark.
Step 11: Adding Weight and Closing Everything Back Off
First, remove those pesky skrews using a pair of needle nose pliers.
You might need to cut another groove in the lid, so the dc jack does not hinder you from putting the lid back on.
To add weight to the base, so the lamp will not tip over, you can mount the transformer back on the lid with skrewing it and the plastic clamp back on. Lastly glue the lid back on or use fitting skrews, if you have some and stick the foam feet back on, again with using hot glue. Now just give it power and enjoy your new efficent LED desk lamp! Please leave me some feedback in the comment section, since this, again, is my first instructable. If you liked it, you can support me by voting for me in the contest or follow me. Thank you very much for reading this instructable.