How to Design Your Own LED Lamp

We probably wonder just how this little bead of acrylic smaller than a pea manages to radiate a visible beam of very intense light in many different colors of the spectrum. We must first categorize its placement in our lighting world. Each Led uses a solid state technology in order to perform its feat. The color is dependent on the materials used. Some use GaAlAs (GaliumAluminumArsenide as junctures of material. Some use a InGaAlP(IndiumGaliumAluminumPhosphor) combination. These are generally much more efficient than the former, though the specifics are not important for the moment.

1. Step 1
   Figure 1 shows the basics, but Led configurations are wide and varied. All that is required to power any given Led is a means to limit the current flow through that given lamp. This is accomplished using a simple resistor of proper value and power rating for any given design. In Fig.1 near bottom is schematic of LED circuit showing R1 connected to Anode of typical LED. For the sake of simplicity, if 10 volts is applied at point B and the LED shown drops a voltage of 2.0 volts @20 ma., then the value of R1 will be 400 Ohms. Of course, a common and standard value of 390 Ohms would be used. The power rating of the resistor R1 would be .160 Watts. 1/4 Watt would be used(Next higher standard value). At these current values, the life expectancy of the LED given any led-lighting design would be over 10 yrs. (at least 30+ times that of the widely used
   incandescent light bulb).
   
2. Step 2
   Figure 2 shows how more than one Led can be used in the original single Led circuit of Figure 1 by just placing them in series and changing the value of resistor R1. Since we know that the current flowing through R1 will be the same, i.e. 20 ma., because LED1 and LED2 are in series, we can calculate the value of R1. Applying 10 volts at point B again, resistor Ri calculates out to be 300 Ohms. Here we can use 330 Ohms, another more readily available standard value. The power rating for R1 in this case again would be a 1/4 Watt value, since it would consume only .120 Watts. We are using 5mm LEDs in the examples above, but Led diameters can vary from 2mm...or less to more than 10mm. The operating designs currents are usually 20 ma. in general, though many of the more recent Led designs are set at 70 ma. and higher. These higher nominal
   operating currents are due mainly from newer Led junction design. The Led itself would consume only .04 Watts, i.e., 40 milli-watts, a very small power consumption for a light bulb. Some of the more robust LEDs on the market today are available in 1 Watt, 2 Watt, 3 Watt, 5 Watt and even higher. Of course, the package design is forever changing to accommodate this increase in power. The Led efficiency, the amount of lumens in light output per Watt consumed is increasing at a much greater pace than other lighting technologies to where it is hoped to reach 150 lumens/Watt by the year 2012. The incandescent is about 15 lumens/Watt and does not see any increase in the near future.
   
3. Step 3
   Figure 3 shows the 2 Leds of figure 2 in a parallel wiring method. This approach uses 2 times the current which again will flow through R1. By wiring LEDs in this manner lessens the need for a higher Voltage to supply several Leds. We still have 2.0 volts dropped across the Leds. Note the resistor R1 now becomes 1/2 the value used in Figure 2, now
   calculated to be 200 Ohms. We will use a 220 Ohm(standard value). Since twice the current will now flow through R1, the wattage will be 320 mW(.32 Watt). We will use the next nearest standard value of .500 Watt. The brightness of the 2 Leds in this wiring configuration will be exactly that shown in Figure 2. The two simple methods shown above
   of wiring together different combinations of Leds are used throughout the Led lighting industry in one form or another.
   
4. Step 4
   Figure 4 shows a combination of a parallel and series design. This design is only to outline the variation where the designer wants to keep the same 10 volts supply. One can visualize the inability to place several Leds in series would require a greater than 10 volts supply. Or, by simply placing several Leds in parallel would require too large
   a resistor to handle such a large current. R1 again, as in Figure 3 would be 220 Ohms and we would use a 1/2 Watt resistor. If we placed all Leds(LEDs 1-4) in parallel, the wattage required by R1 would be 2 Watts( the standard value), even though the resistor would consume 1.280 Watts. The current flow through R1 would now be 80 ma.,i.e., the
   reason for the higher wattage that would be needed for R1. This high heat dissipation would not be a desirable design because of the wasted power through R1. Referencing Fig.4 Led configuration, a solution to this dilemma can be found on page 82 of 03-02-06 issue of ELECTRONIC DESIGN.
   
   Source: http://www.ledyjm.com/led-blog/How-to-Design-Your-Own-LED-Lamp.html