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Topic: How to drive high power LEDs (Read 2 times) previous topic - next topic

random_vamp

I'm working on a project in which I want to control a few high-power LEDs via an Arduino, all running on a 12V DC system.  A friend of mine suggested these LEDs as they are being used in the ridiculous flashlights given to some Google employees: http://www.digikey.com/product-search/en?WT.z_header=search_go&lang=en&site=us&keywords=SST-90-W45S-F11-GN400&x=-1451&y=-51

The specs claim that these should be run at 3.25V with a power draw up to 9A (but I'll probably want to run them somewhere below that).  The question I have is how best to actually use these monsters.  Using a straight up voltage-resistor-LED calculator comes out at using resistors in the range of ~1.5 Ohm 50Watt resistor so I ordered a couple of options in the 1.5-2 Ohm range, however this does not sound like a very efficient option as I suspect there will be a lot of heat generated on the resistor.

Does anyone have suggestions for alternative circuits to use to drive these?  Checking ebay I found a few 120VAC options, but nothing for 12V DC.  Optimally I'd like to be able to dim these, either by having a variable current source or via PWM.

Magicj

Have you considered using a constant current driver ? Not sure on your size constraints but a Meanwell LDD driver is pretty small and relatively cheap and it has an input pin to allow dimming via PWM straight from an Arduino.

random_vamp


Have you considered using a constant current driver ? Not sure on your size constraints but a Meanwell LDD driver is pretty small and relatively cheap and it has an input pin to allow dimming via PWM straight from an Arduino.


Thats actually why I asked here.  Doing a search for these I only see ones that go up to 1A, I'd like something that can go near the top of these LED's range which the datasheet says caps out at 9A, so say at least 6A.  If you know of a constant current driver with that sort of range I'd really appreciate it.

Magicj

Sorry, I didn't have a close look at the specs for the LED.

Not sure on the size of your flashlight but these are not going to be easy to keep cool, even at 6A.

random_vamp

I'm not actually putting them in a flashlight, these are intended for headlamps/spotlights on an art car for Burningman.  I'll be able to strap them down in thermal contact with the vehicle frame and run a fan over them if needed.

If there's a basic formula for an easy circuit to drive LEDs I suspect we can scale it up to drive these, but I'd rather buy something than spend the time doing that if possible.

fungus


I'm not actually putting them in a flashlight, these are intended for headlamps/spotlights on an art car for Burningman.


Headlamps - plural. That means you can wire them in series which makes life a bit easier.

The simplest way is to go for the 50W resistor, it will look something like this: http://www.ebay.com/sch/i.html?_nkw=2ohm+50w+resistor

Cooling won't be easy.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

rickbruce

this will be help you
http://www.stagelasers.com/led-moving-head-light/171-factory-price-led-eight-jaw-fish-bg-led-8-50

Headroom

You need to control these with a switched constant current power supply.
The very low resistor value you calculated is based on a fixed forward voltage. As the LED heats up that forward voltage is going to change and you would need quite a different resistor. Other wise due to the changed forward voltage a higher forward current would flow.
As you can see in the spec sheet at higher forward currents, higher forward voltages result. And so on....

As fungus has already indicated, you will need a very substantial heat sink. There are online calculators that will help you with calculations and define what heatsink will be required.

Perhaps provide more details as to what you are trying to achieve. These LEDs you linked to are monsters and I am wondering if that lumen output is really needed.
http://trippylighting.com

http://ledshield.wordpress.com/

cjdelphi

The SST90 is way over priced for the amount of lumens it puts out, it's really only good for flashlights due to it's low forward voltage.


If you had the time, or still have time...

http://www.ebay.com.au/itm/271206815330?var=570158292403&ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649


Buy a few of them, they run directly from about 5 - 6v (supply 12vdc direct these go pop as i found by accident.... twice)

cjdelphi

And as to how to power these things....

I use a TIP31 power transistor with a 68ohm resistor going to the base, then I use analogWrite and use PWM to give varying brightness (i cap off at around 200 mark) it's trial and error start off with a low
pwm value say 10, use the multimeter and keep and eye on how many amps it's consuming, then gradually increase the value and don't let it exceed what you have in mind ....

A mosfet would be a better job though, might not even need a heatsink at all using this method... (supplying 12v to the anode of the LED directly, and sinking with the mosfet)

random_vamp

Would TIP120's suffice?  I'm not familiar with the difference between the various TIP series. 

This seems like an interesting means to limit current, I'll try it out on some less pricey LEDs first.

I do also have a bunch of mosfet's that I just ordered to try replacing the TIP120's I'm using elsewhere in this project.  It would be incredibly convenient if I could use the same chips to drive these high-power LEDs as I'm using to drive lower powered 12V LED strips.  I'm putting together a module that I'll be making a dozen or so of, if they're fully interchangeable that would be perfect.

fungus


This seems like an interesting means to limit current, I'll try it out on some less pricey LEDs first.


It doesn't limit the current, it limits the average current.

Read the datasheet very carefully before doing it on an expensive LED.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

Tom Carpenter

#12
Jul 11, 2013, 09:52 pm Last Edit: Jul 11, 2013, 09:55 pm by Tom Carpenter Reason: 1
For that sort of power, you are best going for a switching regulator type driver. Using a resistor or linear voltage regulator or transistor like one of those TIP120's is just plain silly, wasteful, and will result in enough heat to fry an egg.

Rather than dissipating 9V of your 12V supply, what these do is act like a transformer and essentially change voltage to current. You have a high voltage (12V) low current side, and a low voltage (e.g 3.5V) high current side.

A quick example would be: http://www.linear.com/product/LT3763

Never used them, but it should give you a starting point to what sort of IC you will need to drive it.
~Tom~

dc42

I looked for an LED driver chip that can supply 9A, bit all the ones I found need external mosfets.

One possibility that doesn't need external mosfets is to use a VNH3SP30 motor driver chip (or buy the breakout board from Pololu) along with a dual comparator, 2 inductors and 2 current sense resistors, to make a 2-channel switched-mode constant current regulator. You could drive 2 of those LEDs in series from each channel.
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Headroom

What exactly would be wrong with the external MOSFETs ?

I mean aside from the fact that I believe designing a proper board seems may be a skill the OP yet has to acquire. OTOH that is not soooo difficult. Many of the spec sheets for LED driver chips provide very good application examples and sometimes there are additional App notes with designs for demo boards.
http://trippylighting.com

http://ledshield.wordpress.com/

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