the blog of uwezi

Ok, let us continue the reverse engineering of the E27 LED bulb LEDARE from IKEA. After describing the actual light emitting diode assembly in the previous post, I will today look deeper under the hood into the power supply.

The power supply itself is located in a plastic cylinder which is screwed to the cast metal housing of the lamp. It is the most elaborate design I have seen so far in any LED or CFL-light. The circuit contains three inductors in the line filter and PFC (power factor correction), two bridge rectifiers, two transformers and only one electrolytic capacitor. Since electrolytic capacitors are the most vulnerable components in these circuits this promises some longevity for the lamp.

The heart of the design can be seen on the bottom side of the circuit board. It is an TPS92070 High-Efficiency Integrated Dimming LED Lighting Driver Controller from Texas Instruments. Even though the LED light bulb itself is marked as not-dimmable, the controller chip itself supports dimming.

At least the best one I have seen so far. After giving a lecture on LEDs yesterday I finally unpacked and tested my latest buy – the clear E27 LED bulb LEDARE from IKEA.

The specs of this LED bulb are not overly impressive, with an efficacy of a mere 50 lm/W it is about as efficient as a compact fluorescent lamp and with its 400 lm it corresponds to a 30 W to 40 W light bulb. Running at 8.1 W this means that it will consume only 20-25% of the electricity of a comparable incandescent bulb.

But what really amazed me was the high quality of the light. It is a nice warm white, stated as 2700 K on the package, much comparable to the best compact fluorescents I have seen and close to an incandescent – if this is what you are after. Not only is the visual impression of the light very good, also the color rendering is very good.

So, how does IKEA accomplish the pleasant spectral composition of the light bulb? I taught my students that the best way to do this is by using the correct blend of phosphors, like in compact fluorescents. But then there was a distinct red appearance when I looked closely at the LED bulb. The LEDs themselves are hidden underneath a light-spreading plastics cone inside the clear glass dome of the bulb – interestingly this cone is screwed to the bulb, not glued as you might expect.

However, the glass dome itself is glued to the solid metal lamp socket which serves as heat sink for the internal converter and the LEDs themselves. I was not able to remove it without breaking, but the rest of the bulb remained undamaged. So how does it look under the hood?

The solid cast metal socket carries a COB (chip-on-board) unit with 6 LEDs:

• four white LEDs, being standard blue LEDs with a phosphor-containing blob of resin on top
• and two big red-emitting LEDs in between.

The LED chips are connected in a string with a voltage drop of 16.5 V. One interesting fact about the red LEDs is that there are two bond wires from the board to the front side of the LED chip.

A short inquiry on the internet revealed that the COB module is made by the company Tridonic. More information can be found on their homepage. (more…)