I got a Chinese torch with a charger supposedly needed to charge the (removable) Li-Ion cell in it. The connection is direct to the unprotected cell, but the charger shows a spec of 4.2V +/- 0.5V.
Before tossing it away, I wanted to check how was made, to see how much cheap it was and possibly learn something out of it. So I tried to devise its schematic:
If it is correct, seems to be an RCC power supply with indirect regulation. What I am not sure is, among the correctness of the schematic, how the GREEN and RED LEDs are working. From a user perspective, the GREEN is "always-on" and RED turns on proportionally to the output power. I also found very useful this video to get the basics, but the feedback on that one seems different - but the rest of the product, including the torch, was very much the same.
My guess is that during the on-cycle (BJT on), the winding 2 turns-on the GREEN LED and closes the 91Ω resistor. Then, if there is need to provide output power, more energy is stored in the core, hence the off-cycle (BJT fully off) lasts more as the winding 3 has to provide more energy on the output keeping the reverse polarity for longer - energy which I presume was provided during the on-cycle for a longer time as well. So during the off-cycle, the winding 2 will make a current flow through the 91Ω resistor in the other direction, hence dropping enough voltage which is used to turn on the RED LED.
Additionally, I would like to understand, if these assumptions are correct, how that Zener is used to regulate the output voltage (as was used here, for example) - but how in this very schematic? EDIT0: less relevant, as with the new schematic now makes a bit more sense.
So far seems that the 25uF capacitor has no usage here. Also, if the LED mechanism is right, during the off-cycle the return current used as a drop on the 91Ω to lit the RED LED, seems to be blocked by the GREEN LED. So something does not add up, essentially I don't find some returning current path - I also triple checked the schematic, but I am still not 100% sure I reversed engineer it correctly.
I had no chance to probe the circuit, as I am not comfortable to work with high voltages. Feel free to suggest any improvement to the question.
EDIT0: the zener was not correct. Now the schematics is updated and Zener relavant question updated too.