# A basic buck converter circuit to power 3W LEDs from PC power supply?

I'm just finishing up my first electronics course (edx.org), and I want to build some LED grow lights for starting seedlings in the winter. I have high power LEDs, like this one here, and I have a PC power supply, which I think will be easier than trying to build my own AC to DC converter, at least for now.

But now I need to take the 5V input and do something to feed 1A to these LEDs. I asked a previous question here, before I realized that a transistor as a current source is a waste of energy. One of the answers to that question suggested a "buck converter with feedback and a current sense resistor". I don't know what this means. I just read about the buck converter, and I understand the basic idea. I drew a circuit at circuitlab

Now, how do I use feedback to control SW1 so that a desired current is flowing through the LED? In class we have learned about op amp feedback, and I think I could create a square wave pulse with an op amp, or 555 timer, or arduino, but I'm still lost. The only "current sensor" I know is a simple resistor, but if I put one in the path of that LED, I'm wasting energy again.

I will edit the circuit drawing and update the question if I get suggestions.

## 1 Answer

For starters, don't use a 555, Arduino, or op amp circuit. There chips designed for this sort of thing, and they will work better and are easier to use. But your design is a little unique, which can be both good and bad.

You don't want a simple "voltage regulator with a buck topology". You want a CURRENT regulator with a buck topology. The reason for this is that you really want a specific current to flow through your LED, and the voltage is of minimal importance. (Voltage is important only in that you want to make sure you have enough of it.)

To make it even more interesting, you don't need a very good current regulator. LEDs can withstand quite a bit of noise before it becomes visible (or messes with other things). The average current needs to be spot on, but there can be quite a lot of ripple and the design will still work fine.

Fortunately for you there are chips specifically designed for this: Driving LEDs with a current regulator in a buck topology! A quick look on the TI web site found the TPS92510. TI has, literally, 65 different chips for driving high brightness LEDs. I am sure that other manufacturers, like Linear Tech, have some too.

• Thanks. I will mark one of these as an answer after I do some more work and reading. It seems like the next step for me is to learn how to search for chips like this, and how to read their datasheets so I can use them in projects. Also, looks like that chip and many others need surface mount soldering, which I have yet to do. Dec 19 '12 at 18:55
• Okay, so it seems those chips are the answer to my question, but to make an LED driver, they require more circuitry that I'm comfortable with at this point. For example the user guide on that TPS92510 shows an "application" with about 20 other circuit components. So I think for this project I will just buy an LED driver (constant current -- not sure why you'd want constant voltage) and attach a series string of my LEDs. I'll postpone "rolling my own" circuits to some other easier projects. Dec 21 '12 at 5:13