We all use LED Bulbs to save energy. It is available in 3W, 5W,7W,9W,15,18W etc . What this Watt indicates? Is its Power output similar to CFL, Tube light etc or light intensity similar to CFL or Tube light? If you consider the specified watts as the real output, then it is a mistake because the Capacitor power supply used in LED bulbs cannot give the specified output. For example, a 9 Watt LED bulb shows that it is only 3.9 W both in theoretical calculation and in power measurements. Let us hack one LED Bulb to see how it is behaving.
The images given below are that of a 9 Watt LED Bulb. It has a PCB with two sides. The facing side has 14 SMD LEDs, an SMD bridge rectifier , two 470 SMD resistors and two 474 SMD resistors. On the back side, there is a 145 J 400 V X rated capacitor and a 10 uF 100V electrolytic capacitor.
Now let us see the current output from the 145 J capacitor and the power out through calculations.
145 J Capacitor is 1.4uF . As a general rule, 1.3 uF gives 0.06 Amps or 60mA current. Therefore 1.4uF capacitor gives around 0.08 Amps or 80 mA. There are 14 LEDs and the forward voltage of each LED will be between 3V and 3.6V. We can consider it as 3.5V.
Now see the Power P in Watts
P = IV
P= Current from the Capacitor x V ( LED voltage x No of LEDs)
P =0.08 A x 3.5V x 14 = 3.9 Watts.
So if 145 J ( 1.4uF ) is used, it can give maximum 4 watts in such an arrangement.
See the images below.These showing the real condition in Power meter. The 9W LED bulb is using 88 mA current and its Power is 2.81 W. That means, the Watts shown in the bulb is not its power output but something different related to intensity of light.
We do not know the exact forward voltage of the LED used. So let us calculate from lower limit 3V to upper limit 3.6V to get the power. Use the same formula changing only the LED voltage.
3 V – 3.3W
3.1V – 3.4W
3.2V – 3.5W
3.3V – 3.6W
3.4V – 3.8W
3.5V – 3.9W
3.6V – 4W
So to get the 9 Watts power similar to its specification, the current should be 185mA, which needs three or four 145 J capacitor in parallel. But the LEB bulb using only one 145J capacitor which is giving only 80mA.
P= 0.185 x 3.5 x 14 = 9 Watts.