An AC-powered LED lamp
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.
It seems the circuit is an example of a capacitive power supply. Here are some links:
- a good write-up on a Capacitive Power Supply Circuit
- another pretty good description
- another example example (but seems with a few errors)
- one of a few questions on EE.SE on the subject.
The basic stages in the circuit:
- AC power supply
- Capacitive current limiter C1 with residual voltage protection R1
- full wave rectifier
- DC low-pass filter (R2/C2) with RC time constant of 1.8s
- LEDs in series with R3
Capacitive reactance = 1/ωC = 4680 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.
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.
| 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 |
- 38 LED Lamp DIY Kit - as purchased on AliExpress
- Inside some of my earliest DIY LED lamps - excellent video by bigclivedotcom, discussing circuit design and construction of LED lamps like this