LED power supply help

Forum 2005-2010 (read only) General Frequently-Asked Questions
1

Hey guys, I got a small problem.

I want to power 5 Cree XR-E LED's and I'm pretty new to power management. The voltage on the LED's are 3.7v and I want to push them at around 700mA.

Now for the like of me, I can't seem to find any type of power supply to that would suffice.

If I'm wiring these in serial does that mean I'll need about:... 18v @ 3.5A? Or am I suicidal...

2

In series the voltage adds up but the current doesn't.

You need some sort of constant current supply to give you the current. High power LEDs are difficult to use, no quite a beginners project. There has been a lot of posts about this recently so do a search.

3

Yes, you need to either built from a proven design or purchase a constant current LED driver. There are several manufactures for these modules. Here is just one example of a driver that can handle four series connected 700MA LEDs:

http://cgi.ebay.com/3W-LED-Driver-for-Luxeon-White-Green-Blue-Lights-New_W0QQitemZ350226589565QQcmdZViewItemQQptZUK_BOI_Electrical_Components_Supplies_ET?hash=item518b21a77d&_trksid=p4634.c0.m14.l1262&_trkparms=|293%3A1|294%3A30

Lefty

4

LM317/LM117 can be used as constant current source, up to 1.5A, but I did not see it applied to a LED driver. Any explanation why? Is it because the required resistor would need to be an odd value and also high(er) wattage?
Thanks.

5

Any explanation why? Is it because the required resistor would need to be an odd value and also high(er) wattage?

Well it would certainly work and therefore qualify as a constant current LED driver, however it would not be very efficient and most modern purpose built LED drivers utilize switching regulators.

Lefty

6

OK, thanks. It is basically the difference between linear and switching regulator, where the latter is more efficient (>90%) because of much less dissipated heat.

7

where the latter is more efficient (>90%) because of much less dissipated heat.

Wrong way round, it dissipates less heat BECAUSE it is more efficient.

8

National Semiconductor has a good online design utility called Web Bench that you can use to design a constant-current switching supply with their simple switcher line. The big difference between that and a constant voltage supply is that you arrange the feedback so that it's proportional to the current rather than the voltage.