I'm trying to convert an E14 RGB LED bulb to be able to power it from a lithium battery. Inside the bulb are 2 PCBs - a transformer and a LED driver - which are connected by 3 wires. Black and red are DC ground and positive, white is for something else.

When I apply DC voltage to the driver board it works only while I'm touching the white wire with a finger. I tried to connect this wire to positive or to ground directly and via a capacitor - nothing seems to work.

On the transformer PCB this white wire is connected to one of the mains terminals via a 1 μF capacitor. On the driver board it is connected to ground via a 6.8 nF capacitor.

Maybe somebody here knows what this third wire is for, and what circuitry could possibly replace my finger?

rgb bulb

transformer pcb

led driver pcb

  • \$\begingroup\$ I don't know much of the circuit there. I wish you can get the diagram of the circuit it will be better. But what I want you to try is, connect a resistor on the white with and connect the other terminal of the resistor to the positive wire and see if it's going to work. If not, then remove it from the positive and connect it to ground. Show us the results. Resistor should be between 1k upwards. Test it and show the results. \$\endgroup\$ Sep 2, 2022 at 17:59
  • \$\begingroup\$ Can you show us schematics? Without some kind of schematics, any answer would be little more than a guess. \$\endgroup\$
    – Hearth
    Sep 2, 2022 at 19:12
  • \$\begingroup\$ Sorry, I don't have schematics. \$\endgroup\$
    – Artur
    Sep 2, 2022 at 20:15
  • \$\begingroup\$ @Artur Then start working on reverse engineering and drawing some schematics, because I don't think this is answerable without a better understanding of what the circuit does. \$\endgroup\$
    – Hearth
    Sep 2, 2022 at 20:40
  • 1
    \$\begingroup\$ It would be interesting if that white wire was intended to pick up the 60 Hz of the powerline and your finger gave it enough AC hum to make it happen \$\endgroup\$
    – infixed
    Sep 2, 2022 at 22:17


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