Four Channel LED Driver

Submitted by Ed_B on Mon, 11/22/2010 - 00:00

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Here's a purely practical project I've done for student use in my department at SAIC. It's a breakout board for the Recom RCD-24-X series of constant current LED drivers. An LED driver's output current must match exactly the requirements of a given LED. Recom makes quite a variety of these modules.

 

In the Recom part number, X is replaced by the reguated current value. The board shown has one 1-amp and three 350mA modules.  I've noticed that 350mA is a common value for power LED supply current. The retail price for the Recom moules is about $12 to $15, depending on the output. (Click READ MORE to see the whole post.)

 

 My assignment was pretty open-ended: An easy to build kit for making a four channel dimmable LED driver board with current regulation up to 1 amp.  On the "make it easy to build" side of things, an LM317-based constant-current regulator circuit can't be beat. These regulators are really inefficient, but I checked out the applcation anyway, just to cover my bases.

 I wanted to be very sure there weren't any missed opportunities in the analog approach. The worst case load is the 1 amp LED's, so my testing focused on those. The test circuit shows larger than necessary power resistors and an LM317T in the standard current regulator configuration (the circuit diagram is all over the web). The target power supply voltage was 12 volts, so to try to get a better match between the LED and the supply, I used rectifier diodes in series with the load (rectifier diodes give a drop of around 0.6V with no additional thermal burden).  Using four diodes got the circuit to run passibly warm. The last part of the test involved adding a control input. The LM317 feedback voltage is developed at the load side of the power resistor. I disonncted the feedback pin and inserted a 4.7K resistor at that point. I also added the collector of a small NPN transistor to the feedback pin. The emitter was grounded. This arrangement allowed a small signal at the base of the transistor to have fair range of control over the LED. WIth a 350mA LED, the componants were smaller, the waste heat was less. If efficiency isn't a huge concern, the 350 mA version of the circuit still seems promising. It's cheap and easy to solder. With a 1 amp LED the circuit was bulky and ran hot, so the analog approach ended here. The design would require  some form of switch-mode current regulator.

The LED lighting industry can not be surveyed in a casual manner. All segments of the LED lighting industry are hot and growing fast. A comprehensive view of the technology can only come by spending a lot of time paying attention to trends and players (big and small). The question of "buy it or build it" isn't an easy one to answer. Most of or students who would build this circuit kit as a learn-to-solder experience in a lighting class wouldn't do well with surface mount parts. Also, the parts count per regulator doesn't seem to much below eight in most reference circuits published by the chip manufacturers. This seemed like a problem set that could make use of a modular solution. 

Digi-Key, one of our regular suppliers that does retail sales, distributes for Recom corp. I decided to use Recom RCD-24-X modules on a custom board as the basis of a design. I did the board design in Kicad and had the proof set run at PCBfabexpress.com.

With a 12 volt power supply, each module can run around three LED lamps.

 The KiCAD design files are here.