Introduction: Converting a 230V AC Bulb to USB Power!
I came across these neat flame-effect bulbs on eBay, which flicker and have a subtle animation built in. They're usually powered by 85-265V AC mains input, but for portable applications like a fake flaming torch or lantern this is not ideal.
I modified the bulb so that instead of the original power supply, these bulbs can be powered by any 5V supply, straight from a single li-ion battery, or even from 2-3 AA batteries.
Step 1: Disassembly of the Bulb
The top diffusion housing is just clipped on, with a bit of prying it popped off to reveal the AC-DC driver, and on the other side of the board, a flexible PCB is soldered onto it.
The flexible PCB has the microcontroller and LED array soldered onto it before it was then rolled up and soldered in place. Taking a closer look at this PCB, there are only two electrical connections to the driver board from the DC output side. If a voltage is applied to these connections which is the same as the AC-DC driver output voltage, then the bulb should work fine.
The metal end cap of the bulb can be pried off, revealing the AC Live connection is just pinched in place against the plastic.
Step 2: Testing the Driver Output Voltage
To test the output voltage safely, I soldered two wires onto the DC output and wrapped them around my DMM leads as pictured. Then I powered the bulb and checked the DMM to see that the voltage was around 6.3V.
I was hoping it would be 5V, but the slightly higher voltage makes sense as pairs of LEDs can be driven in series with ~6V. This does make converting the bulb a bit trickier however as I did not have an adjustable boost converter on hand that would fit in the base of the bulb.
Step 3: Modifying a Boost Converter - Theory
I had this boost converter module laying around and after looking at the datasheet of the IC, I realised I could modify it for my needs.
This boost converter provides a fixed 5V output from any voltage in the 2.5V to 4.5V range. Since I need ~6.3V on the output and not 5V this module wouldn't work as-is.
In the above image of the circuit you can see that the IC regulates the output voltage by a direct feedback path from the output (thick line). If a voltage divider was placed between ground and the output voltage, and the node of the voltage divider was connected to the "VOUT" pin of the IC, then we should be able to trick the IC in regulating above its set point.
For large changes to the output voltage, other components like the inductor and capacitors might need to be changed, but as I am increasing the voltage only slightly, there is no need to change anything else.
Step 4: Modifying a Boost Converter - Practical
After removing the USB jack, I desoldered the IC to take a closer look at the layout of the boost PCB.
The middle pin "VOUT" is connected to the tab on the IC, so I cut the copper separating this connection from the rest of the board. I calculated resistor values and chose the closest resistors I had on hand; 220kOhm and 50kOhm to form the voltage divider.
These resistors were then soldered in series across the output of the boost converter, and the middle node was soldered to the VOUT tab on the IC as shown.
I applied 5V to the board from a power supply and measured the output voltage of 6.56V. This reading is slightly higher than what I wanted, but as there's a zener regulator for the microcontroller this is an acceptable voltage level.
Step 5: Reassembly of the Bulb
With the metal end cap removed, a wire can be passed through the small hole in the base. In this case I show a short USB cable being used, but you could also use any other type of cable for connecting directly to a battery.
I tied a knot in the USB cable for strain relief, a cable tie would work also. The ends of the USB cable are soldered onto the modified boost converter which is then connected directly to the bulb's DC side.
Note that I left the AC-DC circuitry in the bulb as it holds the flexible PCB together, it serves no other purpose and could be entirely removed in this setup.
Squeezing everything back in place, you are left with an odd looking bulb with a cable dangling out the end. I also made a version with a 2 pin JST connector which can be connected to a battery of your choice - in this case I went with a protected 18650 cell that has the matching JST connector.
Step 6: Done!
So here it is; a Mains powered bulb modified to a low voltage battery operated bulb which can be used in a wide variety of displays or other visual effects. Of course if you have a mini adjustable boost converter you don't need to worry about modifying any circuit boards.. but where's the fun in that? :)
Thank you very much for looking if you've made it this far and I hope you found it interesting!
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