These Days power leds are available very cheap.I had seen a lot of stuff on the net about the “1 watt led” and i fell instantly in love with these beauties!!These babies have a very high lumen per watt output as compared to either incandescent or the flourescent technologies.For the layman this means very high efficiency-you get more bang for your buck.But driving these things may be a bit difficult .
I did some reasearch and found that the humble resistor we all connect to the leds while driving the usual 5mm leds does not work very efficiently for the power leds and hence complicated driver circuits are required.to understand how the driver works we need to first understand the basic principle on which the leds work. LEDs are current driven devices i.e one can increase the brightness of the LEDs by increasing the current through them. Sounds simple right!!.But it is much complicated than that the LEDs require a particular forward voltage known asforward breakover voltage. This is the voltage required to ‘turn on’ the LED.For white LEDs this is usually 3.2-3.4 v. Red and green LEDs usually have a much lower forward breakover voltage.Once the forward breakover is reached the leds literally act as a short circuit hence the current limiting resistor.These resistors cannot be used because of aforementioned reasons.Hence we use a constant current source. SMPS(switched mode power supply)s are usually used as a source for power LEDs. They usually switch on and off at a very high frequency.And that too at a very high efficiency of about 85-95%. The only drawback of such a power supply is the prohibitive cost,especially when the supply current is in a couple of amperes
All said and done i decided to make my own 1watt led lamp. I’m using a 1w LED with nothing but a 6N printed on it as you can see(i think this might be some cheap chinese made replica of the philips lumileds luxeon I). An lm317 adjustable volatage regulator and a 3.9 ohm 1 watt resistor. A heatsink i salvaged from an old TV receiver box.I’m using a nokia mobile charger model ACP-12E has a rated output of 5.7v@800ma.But my multiimeter gave me an output of 7v!! maybe because it was the open circuit voltage.
I chose the lm317 because it has a constant current mode (see the application notes) and also its damn cheap.The lm317 can handle currents upto 1.5A .I suggest attaching a heatsink to the Lm317 as well as theLED. Altough the LM317 has thermal protection buit in the led may heat up and an overheated LED is as good as dead .The LM317 regulator gives out a constant voltage of 1,25 volts between ADJ and Vout, so by adding a resistor between these two outputs, you’ll get a constant current.to find the resistance value use ohms law V=IR.Here the V=1.25v and required I is .350A hence we get R as 3.5714ohms. the nearest standard value is 3.3ohm .But we will use the 3.9ohm as a safe measure. The constant current source has a voltage drop of 3 v. And since the voltage drop across the LED is 3.2 v we would require around 6v as vcc.Avoid using higher volatge power supplies than required since excess voltage is wasted as heat. I suggest using a supply of 6 to 9 volt.Also the wattage of the resistors may be high enough to handle the heat generated.
The finished product is shown below.This might not be the most efficient way to power these 1w LEDs but it’s the simplest and the most cost effective. The efficiency may be increased by using the right supply voltage to reduce the power wastage.