HOW to Recycle the LED Strips of a Spiral LED Bulb (a “reverse Engineering” Study of a Commercial LED Bulb)

For some years "spiral" LED bulbs - among several other LED lamps - have been available as a direct replacement of conventional incandescent light bulbs. LED bulbs come with different ratings in term of energy consumption and Kelvin color temperature, and can be directly (without external transformer) screwed to the original socket. With reference to the socket diameter the main standard attachments here in Europe are the so called E27 or E14, mains voltage is 230V/50Hz.

Fig. 1 shows a spiral LED bulb rated for 230V/50Hz / 20W / 155mA (I explain below the meaning of this value) / 3000K.

After ca. 5 years of life the lamp has stopped working, although no external damage (burn or crack of the LEDs) was visible, so I decided to investigate and possibly to save and reuse the LED strips the spiral is made of.

After carefully breaking the spiral glasses (two, interleaved) and severing the lamp’s basement I could separate the internal components:

- 2 strips of 36 LEDs each, connected in series (fig. 2)

- 1 AC/DC converter (fig. 3 and 4)

Fig. 3 shows that the cause of the failure is a burned 1Ohm resistor on the AC input, probably intended to act as a fuse, as eventually did. Rather than trying to repair the module (the bulb was in any case destroyed), I was more interested in knowing the DC output voltage, in order to check and possibly reuse the LED strips. Looking at the SMD face of the module one can make out the marking of the main IC, a CS7320S switching regulator. Unfortunately, no datasheet is available for this component, unless you make an order for mass production. On the other hand, the electrolytic capacitor on the output is rated 200V, which suggests a quite high DC output voltage. Therefore I tried an alternative way to solve the problem.

First of all, I thought that the 155mA printed on the bulb can’t be the primary current, otherwise the power would result: P = 155mA x 230V = 36,65W, instead of 20W. So, I assumed this is the secondary (DC) current flowing through the LED chain. Next step, I carefully checked the traces on the strips, and I realized that each strip consists of a series of 6 groups, where each group in turn consists of the parallel of 6 power LEDs. Then, I decided to split one strip in two parts (18 LEDs each), and I connected one part (3 groups in series) to an adjustable power supply. Starting from 0V I slowly increased the voltage: until 25V no current was flowing and all LEDs were dark, then beyond this value the current suddenly raises and all the LEDs light up. The current reaches 155mA exactly at 27V.

Now, 155mA x 27V gives 4,185W, considering that this half-strip with 18 LEDs is a quarter of the total (72 LEDs) the total power absorbed by the strips results 4,185 x 4 = 16,74. Finally, considering an efficiency of the AC/DC regulator around 90%, the input power from the mains results 18,6 W, which is not far from the nominal value of 20W.

Other interesting notice: the electrolytic capacitor on the output of the converter is rated 200V, this is consistent with a total value of 27V x 4 (remember, the 2 strips are connected in series) = 108V DC as output voltage of the supply unit.

Finally, 27V : 3 (remember, my test strip consist of 3 groups in series) = 9V, this is the nominal supply voltage of each LED (that can be in fact measured directly on the chips)

What is all this stuff worth? Well, for example, if we don’t need the full brightness of a 20W LED bulb (around 1500-1800 lumens) but we can get by with let’s say 400lm, for limited illumination periods (e.g, the internal compartment of a closet or a kitchen wall unit) and no mains outlet is available nearby, we can easily recycle the LED strips for building a battery operated LED lamp. But this is the subject of my next Instructable…

Coming soon...