I have a Phillips 5.5W 240V LED bulb that has stopped working (ie. no light), and I want to fix it (I realise this is probably not an economical use of my time but it's fun).
I have attempted to reverse engineer the circuit and I think I've got it mostly, but something looks odd. Schematics and images attached. I have tested the chain of 6 LEDs and they turn on at around 30V. The 2.2uF capacitor was my first guess, but it seems fine and I am getting 320V DC across it, but I am ordering a replacement anyway so I can replace it. In the meantime, I want to understand the operation of the circuit better. I don't really want to scope the signal because I don't want to damage my scope, but I am able to use a multimeter as I set up the probes before turning it on.
I don't understand what the transformer is for... I would have guessed it was for isolation, but it seems like the LEDs are not isolated. Are they just using it for some kind of feedback? Also surely Z2 needs some resistor to pull it up?
Ignoring the transformer my guess is that R8 and R9 create some voltage feedback to regulate the current through Q1. Q3 is some kind of protection that turns off Q1 - perhaps R10 (unmarked) is a thermistor and turns on Q3 when it gets hot? I'm a bit lost. I would have thought that Z2 would need something pulling it high.
I am measuring about 400mV across Z1 and 0V across Z2.
Any suggestions for operation would be appreciated.
Matt