Ideal constant current source?

Question

I've developing an LED fixture for my aquarium to grow plants. To do so, I've selected several high powered LED's. I'm currently considering driving them with LM317's because I know how to use those and they're simple. I'm not the most skilled with electronics. What is the ideal way to drive high current LEDs?

The LED's I've picked out are these specs:

2 (in parallel) 700mA @ 33V Luxeon K LED 12UP Warm White
3 (in series) 1.2A @ 8.55V Cree XTE 3UP Royal Blue, 25.65V total
2 (in series) 700mA @7.2V Luxeon Rebel 3UP Deep Red, 14.4V total

Answer 1

An LM317-based solution is probably the simpliest one if heatsink and extra power consumption is not a problem. It can also be turned into an advantage if you need to warm up water in your aquarium.

A better solution is to use a single-chip PWM controller, a power MOSFET and an inductor to drive the LEDs. A UC3843 and an IRF540 may suit you well.

Answer 2

Linear Technology do a series of very useful chips, and they do chips with three independant LED controls but these are not capable of supplying the current for the OP's needs. LT do a bunch of single LED-string chips though: -

• The LT3795 looks pretty useful and costs about £4. It can work from 4.5V up to over 100V.
• The LT3756 is a bit cheaper but would also do the job.
• The LT3956 is used mainly for white LEDs in car headlights but would do the job - it works across a good range of input voltages too. It also costs about £3.

The above are all ~90% efficient switching regulators that control current to several LEDs in series and are suitable for one string of LEDs only. You don't have to provide a big supply voltage to these parts - they can boost the supply up to whatever is needed to get the right current through the LEDs but, you do need to be able to provide power so, if you need 25.6 volts at 1.2A for the blue LEDs in series and your supply is only 12V, it will be called on to provide over 2.5A.

Answer 3

The trick in using an LM317 is to trick it into being a constant current supply, rather than constant voltage. And if you need LED strings in parallel, the 317 is cheap enough that you can use one per LED string; the major cost is the heatsink whether you use one LM317 or several.

Figure 43 of the datasheet shows how, though for 0.33A you would multiply the sense resistor by 3 (3.6R) rated at 2W.

This will waste some power, though (a) you asked for inexpensive and simple, and (b) as has been pointed out, an aquarium offers possibilities for water cooling!

Answer 4

LM317 is a linear regulator. LEDs are non-linear.

You need constant current, so either an analogue buck/boost converter circuit or the uC-controlled equivalent.

R is the load, your LED.

The BuckPuck is a common off-the-shelf solution; you could get one of these to run the lights whilst you design something bespoke.