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Project Notes

#067 AC LED Bulb

Build and analyse a commercial LED lamp kit with capacitive power supply.

The Build

Notes

So I got this LED Lamp Kit, mainly because it was quite cheap (1.59USD) and I was interested to see what’s inside. It arrived fast (free shipping), assembled without problem (no dud parts) and even came with an extra 7 LEDs thrown in for good measure.

So is it a very good lamp? Not particularly, unless you are after a fairly dim warm white glow. Perhaps good enough for a night-light. It’s also not the safest. When assembled, there’s no exposed AC or even DC power, however the only protection is the “snap-in” LED board. If that falls or is knocked out, AC circuits and wires will be exposed.

Circuit Overview

The circuit is an example of a capacitive power supply. Here are some links:

The basic stages in the circuit:

  • AC power supply
  • Capacitive current limiter C1 with residual voltage protection R1 with time constant of 0.265s
  • full wave rectifier
  • DC bypass/smoothing capacitor C2
  • C2 residual voltage protection R2 with time constant of 1.8s
  • LEDs in series with R3

Capacitive reactance = 1/ωC = 4680Ω so at 230V it will deliver 49.1mA. Note there is no series resistor that is often recommended to handle possible startup voltage surge (“short”) into the capacitor.

OK, so the more I look at the circuit the more it looks a bit nasty, and it’s not clear the LEDs are being driven optimally.

Here’s an attempt to simulate it with CircuitLab.

LEDs

The LEDs are some variety of 5mm warm white, perhaps similar to those in this datasheet. I measured the actual forward voltage, and it ranges from about 2.69-3.02V. So the 38 LEDs will require a total voltage drop of around 114V; not surprising as I guess this is designed to support 110V AC as a minimum.

Construction

The Build

The Build

The Schematic

Internals Front

Internals Rear

Demo

Parts

Ref Part Notes
R1 390kΩ resistor AC residual voltage protection
R2 390kΩ resistor DC RC low-pass filter
R3 100Ω resistor LED excess voltage disipation
D1 1N4007 diode 1/4 full wave rectifier datasheet
D2 1N4007 diode 1/4 full wave rectifier datasheet
D3 1N4007 diode 1/4 full wave rectifier datasheet
D4 1N4007 diode 1/4 full wave rectifier datasheet
C1 684J 0.68μF polyester 630V datasheet
C2 4.7μF electrolytic capacitor DC RC low-pass filter
L1-38 5mm Warm White LED perhaps similar to this datasheet

Credits and References

About LEAP#67 PowerLED
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

The projects are usually inspired by things found wild on the net, or ideas from the sources such as:

Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.