Bedside Reading Light

As the reading light had a 50 W halogen lamp it was waaaay too bright. So I decided to replace the incandescent lamp with LED lamps. From Hong Kong I ordered 100 pieces 3 mm LEDs in white, and set out. I felt that using the blind current of a capacitor was a good idea to convert the line voltage of 230 VAC to about 10 V, which would nicely drive the LEDs with a comfortable current.

The LEDs were arranged in a series/parallel matrix, with 12 LEDs parallel and that three times in series. The LEDs need about 10 mA each, so total current was set to 0.13 A. This was set by the total capacitance of 2 µF, which was made using several high-voltage foil capacitors. Some protection was needed, as well as a rectifier since the LEDs need DC.

In the end the project was not successful. First the zener diode died. Apparently it had to work too hard to keep the voltage in check. Or something. This caused a kind of short across the LEDs which only very fainted glowed. I removed it as I didn't expect real big power surges. Then the current limiting resistor fried, causing the electrolytic to explode (well, actually it just popped and sizzled...). Instead of just beefing it up I figured that the LEDs would limit any current surges themselves. How wrong I was! These devices are very brittle, and only the slightest glitch will cause failure. So all LEDs burned out. ALL OF THEM!!! Interestingly enough, the failure always results in a shorted crystal. Now that's good to know.

In hindsight the LED failure was silly. Had I given it more thought (better zener, beefier current limiting resistor) it would not have happened. When these LEDs are protected they will not fail.









© Zappy TV 2004