Questions on Modifying Tri-Colour LED Driver

[Byork]

I've got this tri-colour LED ceiling light for free. It switches between white, warm white, and both colours at every flick of the light switch. When running on any single colour, it is not bright enough for my needs, and it's a pain the flicking the switch 3 times to have both colours on every time. Thus, I'm trying to modify the driver to have both colours on all the time.

The chips(SJ-680) at the top and bottom(both same) appears to be the constant current ICs, each controlling the current on each colour of the light, and the one in the center appears to be the chip(JXC0905B) controlling which CC IC gets activated. I've tried searching for both chips online, but fail to find any info on them. Nevertheless, I've managed to trace out most of the pins.

After measuring with a multimeter, I've figured that by applying 5V to Pin X, the CC IC gets activated. So I'm assuming it is VCC. What I did was, I connected my own 240v to 5V transformer, to Pin X on both ICs, and it worked perfectly. The light switches on both colours all the time.

Here's my issue, without using my own transformer, I couldn't figure out where could I tap the 5V from on this board to activate both CC ICs simultaneously. I've tried shorting Pin X on one CC chip, to Pin X on the other CC chip, but it didn't work, the light did not turn on at all. The bottom side of the board is also empty, without any components.

The JXC0905B chip appears to be receiving it's power directly via AC, without passing through the 4 diode bridge rectifier, and through a 2M resistor, I'm not very clear on that as well.

I know I could easily just get another CC driver, or just use the transformer method mentioned, but I think it would be interesting to try to understand/figure this out. Any advice would be greatly appreciated.

[anon35827816]

I think it's a good attempt at analyzing the schematic, but not entirely successful. Take the datasheet of the led driver chip and see how the application example fits into this device; it will help for sure.

Anyway, there's a good chance the thing will work by simply removing U1 altogether, or cut its traces to both led drivers. The reason is that the led drivers often have a pin labeled "enable" or "dim" which is low to mute the light (it can be pwm driven). An internal pullup resistor pulls up the en/dim pin to the Vcc of the internal voltage regulator of the led driver chip, so leaving the en/dim pin floating usually leaves the light on permanently.

[Byork]

Thanks koraks, you are right! I was initially thinking Pin X is the enable pin too, but I couldn't understand how/where it is drawing its actual power from, so I started to guess Pin X was VCC instead. But you are right on the money, removing the middle chip did the trick. I do wish these Chinese chips have their datasheet easily available. I really appreciate your explanation, for I have learnt something new from it. Many thanks again.

[anon35827816]

Great, nice to hear it works now!

Yeah, datasheets are sometimes hard to find. For most led drivers like this one I've been able to find the datasheet - albeit often in Chinese, but the essential information can still be gleaned from it. However, for this particular driver chip, it seems to be difficult or even impossible to find it. Fortunately most of these chips work in very similar ways!