8W LED Bulb - From 220VAC to 120VAC

[lynspyre]

Hello everybody,

A friend brought me from Spain (220VAC) a LED Bulb (8W / ~40mA). As an electronics enthusiast, I decided to give it a try and see how it was powered on and if I could make it work on 120VAC.

Here are some pictures of it and the schematic I drawed.
Thanks in advance.,

[Seekonk]

Seems to have a lot of resistance for 8W.  Does it currently work at all at 120V? If it does, you could just lower the resistance.  Measuring the voltage on the LED will help determine what it needs if it does illuminate somewhat. Might be just easier to jumper the resistor and run a capacitive to vottage doubler in the base of the lamp it is used in.

[lynspyre]

It runs at 220VAC, as stated in the package. It also says it's dimmable, I haven't tried to run it at 120VAC yet, i'm looking for an isolating transformer. I openned because some LED bulbs come with transformer baser circuit and if I run it at 120VAC I would have toasted it.

I measured the LEDs, as the have partial "wire" outside the silicone protection, I managed to power each one with the DMM diode mode, but got no reading on voltage drop. Don't know why, DMM is good, i've recently repaired an 3D Printer SMPS with it, and readed voltage drops of its diodes.

[SeanB]

You would not get a forward voltage drop on the LED itself because the forward voltage drop is around 3V, and your meter likely only shows up to 1.999V in diode test, though it might put out up to 4V open circuit in that mode. However those LED strings have a typical forward voltage drop of around 100V, if they are all LED's in series, or a forward voltage of 3V if they are all in parallel. You can just plug it straight into 120V and see if it lights, and if so just use it that way, otherwise you will have to fit into the inner space a voltage doubler to run it off 120VAC.

As you already have one capacitor and a bridge rectifier all you need is another 15uF 400V capacitor, and unsolder the negative lead of the existing capacitor, add the new one positive to it, connect the new one negative to the hole you unsoldered from, and then remove the power lead not soldered to the resistor, and solder it to the junction of the 2 capacitors, and you have the voltage doubler that will light it on 120VAC.

[lynspyre]

It should be something like this, right?

[SeanB]

Pretty much right.