Do You Know the Linear LED Driver?

Disassembling electronic devices, broken or not, can be very instructive. After recovering a dead 30 W LED floodlight in the dumpster and taking it apart a so-called (sequential) linear LED driver was discovered.

Disassembling electronic devices, broken or not, can be very instructive. After recovering a dead 30 W LED floodlight in the dumpster and taking it apart, an unusual circuit was uncovered. Instead of the expected basic string of LEDs in series with the main voltage and maybe a current-limiting resistor, the driver turned out to be a so-called (sequential) linear LED driver or direct AC LED driver. This clever design has a much better power factor than the classic design. Repairing the spot was surprisingly easy.

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Discussion (6 comments)

gert-ce 4 years ago

What a pity you did not replace the 3 LEDS . An analysis of the defect would have been interesting too.

ElektorLabs 4 years ago

Oh, but I did replace the dead LEDs (there were more than three dead), I even replaced all off them, but they burned out again immediately. One short, bright flash, that's all I got. The transistors seem OK, remains the driver IC, but since this IC seems to be unknown, I cannot replace it. Which is why I decided to simply replace the board.

Ok thanks. Anyway these replacement boards seem really cheap. WE might even use them for other lighting purposes!

2 Comment(s)

Treth 4 years ago

That method of driving the LEDs is completely new to me and interesting.
Would appear quite difficult to troubleshoot with both mains voltages and switching within the mains cycle, but very clever.

0 Comment(s)

n111ckk 4 years ago

I suspect that a similar technique is used to switch the multiple LEDs required on amber indicators. You wouldn't want up to 10 switchers for the individual LEDs, and resistive current limits would be very inefficient.

Instead, you put all the LEDs in one series chain with a constant current feed. Then you have a transistor connected to each LED pair node, shorting them to ground. So to just light one LED, you might have 9 transistors turned on, progressively turning them off to light more LEDs.

I used this technique with a quad open-collector comparator to provide a sliding LED indication to show the output power of an audio amplifier.