I'm new to these forums but I'm a Make subscriber (since issue 1) in the UK. I've got some high brightness white Cree LEDs that I'd like to drive with some kind of constant current driver. I've searched the forums for ideas but not found anything particularly so I'm asking here and hoping someone knows what I need.
What I'm after is a chip to take an unregulated DC wall wart output and give me around 350mA. Here's my list of wants (hopefully there's something that ticks most of the boxes): * Fairly high efficiency (I don't just want a heater!) * Easy to hand solder (most of the new Maxim and Linear ICs I've found are in tiny packages) * An input (PWM or analog or something) to adjust the current to dim the LED * Simple to hook up (if I can avoid weird diodes and inductors that would be ace!) * Preferably available as a sample
Has anyone got any ideas or found anything that works for this?
The LTC and Maxim ICs that are in the SOT23 packages are not hard to solder but you should have a PCB.
I believe that the CREE LEDs have a Vf of 4V. If you got a 5V regulated wall wart you could use a simple current sink to get 350mA. The current sink would consist of an opamp, FET and power resistor. Dropping 1V across the current sink gives you 80% efficiency and is very easy to build with TH parts.
I have a schematic of a current sink (AKA load-cell) at
The buck puck LED drivers come in a variety of flavors, including a variety with an analog potentiometer input so you can adjust the brightness. I haven't used them personally, but they seem to be a good fit for driving the type of LED you are using.
The original thread for reference which you might also find useful:
If you use a 5V wall wart, you won't be that bad off, in terms of heat, with a linear regulator. The LED forward voltage will be about 3.6V, so (5V-3.6V)0.35A = 0.48W wasted. That's not horrible. If you get a lower voltage wall wart (some cell phones had 4.5) then the losses will be even lower. Of course, a switcher will be cooler, but a linear circuit definitely avoids "weird diodes and inductors". Also keep in mind the real heat problem you want to be careful about is the LED itself, and you'll have that with either a linear or switching current regulator. At 3.6 x 0.35A, that's 1.26W, most of which is still heat and not light.
One other comment is don't fear surface mount parts. Things like SO-8s are pretty easy to work with. For simple circuits like this, you can hand-cut copper-clad board with a hobby knife. Don't try to cut skinny traces like on real PCBs, just make islands and remove as little copper as necessary. Hand draw the layout first to figure out the minimum amount of cutting.
Mark: Thanks for the link to the previous thread. The Buck Puck drivers look the business, unfortunately they're about three times what I paid for each LED and I'm cheap!
jcl and lens: thanks for the pointers and advice on surface mount, great stuff.
I've already found out just how hot these things run at full whack, luckily I plan to make a lamp out of a heatsink because it looks nice, which has the happy side benefit of being a heatsink!
I think I'm going to go down the Maxim or Linear route and try and find a device that's in a big enough package that I can solder and then find some inductors and schottky diodes. I'll see how it goes, if it works out I'll blog about it.
For a 1W LED the lower efficiency converters will have a large thermal pad underneath. This needs to be soldered to a ground plane. You will have to use either a toaster oven or a hot plate to solder it.
Hopefully you will be able to find a part with a high enough efficiency.