Renewable Energy Questions/Discussion > Renewable Energy Q&A

Using High Power LEDs to Make Lights

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rossw:
I've been doing a fair bit of late with some battery-powered LED applications, a bit like you Woof.
I've come to the conclusion that series linear regulation is a dead-end.
It does have the benefit of simplicity, I'll grant it that. But that's about its only benefit.
The wasted heat is power your battery is giving up that SHOULD have been used in making light.
So either you get less light output for you battery, or less run-time.

I've been making some indicators. These are 4 x 1W LEDs, I believe very similar to the ones you are using.
The PCB acts as a bit of a heatsink, but I'm also not driving them to their full power, to reduce heat and extend life.



The HUGE difference here is that with this circuit, I can run 4 or more white LEDs (or any other colour!) at high brightness, off a SINGLE 18650 CELL. (It'll work from about 1.8V to 5.5V). The boost converter uses a very small value resistor (I'm using 2.2 ohms) to set the current, so there is almost no heat or waste power in the resistor. Under a thermal camera, the only part of this board that gets detectably warm is the inductor, and even that is only about 4 degrees above ambient.

In the interests of power conservation, I would urge you (and anyone else dabbling with power LEDs) to skip the series resistor, and/or linear regulator solutions, and go straight to a switch-mode converter using a very low voltage sense (feedback) control. The one I'm using tries to maintain 0.25V across the feedback resistor.

Additional benefits they bring to the table is a "brightness" input pin that can be driven by PWM, or by an analog input, to dim the LED from basically nothing to 100%, with virtually NO LOST POWER. Nothing gets warm. 92% or 96% or something of your battery power gets to the LED, none of this "dropping lots of volts (at full LED current)" nonsense.

Just to demonstrate the point - in your above linear option, you're going to drop 1.25V across your resistor 100% of the time. So lets work at 100mA (where I have mine set). If you have 4 LEDs at 2.3Vf that's 9.2V across the LEDs. With your battery full and sitting at say, 12.8V, that's 3.6V you have to drop across the regulator. 1.25V across the resistor (125mW) and 235mW across the LM317, for a total of 0.36W wasted as heat from your total of 1.28W drawn from the battery.
As the voltage goes up, the worse this gets. At 13.8V, it's 4.6V to drop, 460mW wasted from 1.38W drawn from the battery.

The converter option with 0.25V sense would be about 0.99W drawn from the battery with 0.07W wasted as heat for exactly the same LED output!

MadScientist267:
I'm in full agreement. Buck converter is THE way to go for driving LEDs.

Linear (and by "extension", simple resistor), as Ross pointed out, is very wasteful at anything above trivial power levels.

"Plain" PWM is riddled with problems as well, and typically still requires the use of some sort of "absolute ballast" to limit current, so while it provides a means to control brightness, it doesn't do it very well (as mentioned, flicker can get really obnoxious) and I'm a firm believer that it's also harder on the LED dies because of the rapid thermal cycling... Even at higher frequencies (yes, law of averages helps there but it's still repetitive "heat and not heat").

Buck conversion (or boost as well, just not quite as common because of the voltages involved) gets around many of the problems associated with either and/or both.

Flicker and thermal cycling are reduced because conversion inherently requires the use of a cap at the output to catch the pulses, and converting rather than dissipating takes a lot of the losses out of the mix, yielding more efficiency. Current limiting is trivial to implement in most chips designed for SMPS (eg buck conversion) and what results is a stable consistent drive for a wide range of input with minimal waste.

For anything more than pretty much indicator LEDs, I personally will never use any other method ever again having seen the benefits of conversion up close for myself.

WooferHound:
I was trying to point out that the Buck Converters that I'm using don't work very well when set to currents less than 100ma and I'm using the LM317 regulator mainly to power ordinary 20ma LEDs. I'm about to build a small light for my computer keyboard that will require 60ma and I'll use an LM317 for that. There are two 20ma LEDs in series that light up my computer desk that need a LM317 to control them. Earlier I posted about making nightlights with 5v wallwarts and a 5 ohm resister. sure it's wasteful but it was still bright and only used 2 watts which is half the power used by the usual 4w incandescent bulb normally in a nightlight. I plan to get a large number of those 20ma CL2N3-G regulators for many future small projects.
For anything over 100ma the buck converter is the only way to go.

So after all that, I am making a Test Box using four LM317 regulators to give me 4 different current levels for testing LEDs and powering Prototype projects. There will be a terminal strip on top that will provide 4 individual outputs at the following current Levels
20 ma - regular LEDs
25 ma - laser diodes
310 ma - 1w LEDs
640 ma - 3w, 5w and 10w LEDs
When finished this will greatly speed up my project builds and testing new LED orders that I receive. It will be powerd from a Cigarette Lighter plug from my 12v solar power

WooferHound:
I finished my Test Box for experimenting and making LED lights. Has a Cigarette Lighter plug for the input, then then 4 handy Current Limited outputs for testing and prototyping new projects. I've already used it to sort out the color of some LEDs that got mixed together while working on my solar bicycle light project.



Got a buncha new stuff in the mail and testing 4 items out in the center of the picture. . .
-- 5 CPU cooler heatsink fans, $2 each. Got these specifically for using 10w LEDs and they do a great job keeping everything at almost room-temperature while staying quiet running full speed.
-- 3 Blue LEDs 10 watt $2.50 each. One of them is glued to the heatsink in the center. The colored 10w LEDs are brilliant as all the energy is directed into the one color.
-- 3 PWM motor speed controllers, $2 each. Being used as an LED Dimmer these work wonderfully down to about 5% brightness then off. The PWM frequency is 10k.
-- Got a lot of Reflector units in various sizes, some with a lens. These seem to double the brightness of the LEDs while directing the light.

WooferHound:

Building a 20 watt LED Lighting Fixture From Scratch

I've been building small lighting using LEDs and decided to make something that was more powerful and could be considered bright enough to work under.

It uses four 5 watt LEDs with two Buck Converters. A fan was added later because there was too much heat being produced. Not as bright as four florescent tubes 4 feet long, but plenty of light to work under. gets fairly warm but still cool enough for long life.

//www.youtube.com/watch?v=VXoJ6VCv3bY

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