I recently bought a few of these:- **broken link removed**
The power-on indicator consists of four white 5 mm LEDs around the on-off switch. They are in two strings, each with a resistor and a diode in series and run from 230 V 50 Hz mains.
I find lighting that flashes at 100 Hz bad enough, but these run at 50 Hz. Also, I can't see why the LEDs are in two strings, as the LED voltage is around 12 V, so far below the mains voltage. There is only about 0.5 mA through the LEDs.
Anyhow I've changed the circuit to put all four LEDs in series, and removed one of the diodes and resistors, and added an electrolytic to get rid of the flicker, and they seemed to work.
After a month or two, some of the LEDs have gone out, and some flicker or are intermittent. The flicker isn't the 50 Hz modulation, it's more like a loose connection or the flicker that an unstable neon light has, so the light turns on and off sort of randomly, once or twice a second.
I've swapped out some of the LEDs, and tested the ones that I've taken out. I've found that it takes quite a lot more than 0.5 mA to light some of the LEDs, and others seem to "loose" around 0.5 mA intermittently, so that at 0.5 mA, they turn on and off. At larger currents the brightness varies quite a lot. Once the current is up to 10 mA or so, there's no visible variation.
I'd not seen before LEDs that took that much current to light, nor LEDs where the brightness varied with a constant current.
Are these particularly poor LEDs? How common is it to have variations in brightness, both between LEDs and with time? Is it this behaviour what has made designers vary brightness with PWM instead of reducing current?
Finally, has anyone any idea why the LEDs weren't originally in one string of 4 LEDs?