#067 AC LED Bulb

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

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:

• 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 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

Parts

Credits and References

• 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