This is an “Ledberg” LED downlight from IKEA:

I really like it – the LED is a nice, warm white, it has a very high output (up to 3W) and the diffuser lens makes a nice soft spot. It’s low-profile, easy to mount, doesn’t require heatsinking, and is a complete solution including a 24VDC power supply, inline switch and driver circuit. And it’s only about $8. You can buy a three-pack (single power supply) for about $16.

I bought it with a little project in mind which will be the topic of a future post.

I was very curious about how the LED is being driven so I cracked this thing open while I was still on the bus home and spent the rest of the bus ride itching to get home to get it apart properly and satisfy my curiosity. Inside the puck enclosure is a tiny PCB with just 5 discrete surface mount components driving the single LED chip:

The back is a single copper layer for heat dissipation, with some curious non-plated through-holes, and the polarised socket.

The PCB is coated in a white layer which makes the PCB tracks barely visible, but I was able to make what I think is an accurate and complete circuit of the driver (I’m travelling at the moment so don’t have a multimeter, magnifying glass or soldering iron).

Here is the parts list:

R1	18R		0805 package
R2	10K		0805 package
R3	10R		0805 package
Q1	S8050 (NPN) 	SOT-23 package
D1	Possibly WSD-3535A120W110-01

This seems to be a standard, cheap-and-cheerful two-transistor current limiter design. In this circuit, R1 is used to control the current through the LED by setting the voltage across the base-emitter junction of Q2. R1 senses the current and when the voltage across it reaches the bias voltage, Q2 conducts and diverts the base drive from Q1, cutting its output current. R2 is used to set a biasing current between Q1’s base and Q2’s collector and lower value should drive more current through the LED. Something like that.

I’m not sure what the function of R3 is.