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constant current source for LED's

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jrz126

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I just ordered up a handful of those 1W leds for my dad's aquarium. I'd like to make some sort of constant current source to power them. I could just use some power resistors as current limiters but I was hoping there was an easier solution.
I'm not sure of the exact specs on the leds, but can probably figure they have a forward voltage of ~3.2V and a current of 500-700 mA
 
My boss has just done something like this as a "homer"
He basically used a chopper cct. and a big inductor.
Since he was planning on running from 12V (6V LED drop) he was able to use a MOSFET at switch in the order of 100kHz (was sourceing 1A)

With the LED in series with the Inductor it made a perfect current-source chopper.

He was toying with the idea of working open-loop (ie fixed duty PWM) rather then add a sense resistor+diff-amp, dunno what he ended up with
 
Resistors ARE the easyest solution.You just need a regulated power suoce.A linear regulator wod get prety hot.Meaby if you have an old switch mode power suply laying around.
 
If you take a look at unipolar stepper motor driver ICs from www.allegromicro.com , they have a number of devices with built in current control up to 3A. All of their chips are very easy to use and are inexpensive.
 
You're not the first person to run into this, the power LED's combination of current and low voltage is tough. Linear regs and resistors are not really practical.

The key is a simple buck converter in constant current mode. This not only means no heat is generated but you get more current out than drawn from the supply. In your case a 12v 200mA supply would do fine.

The easiest (and cheapest) thing is a Supertex LED driver. You need one of these, a MOSFET switch, a small but properly chosen inductor, a low ohm shunt resistor, a Schottky diode, and a few typical resistors/caps.
**broken link removed**

You mention 1W devices but talk about driving 2.24W to them. Is this for 2 or 3 or what?

What device do you have? The thing to be careful of is these devices can generate a good deal of heat. A bare emitter will destroy itself almost immediately if not heatsinked. Even a 1W Luxeon Star needs to have consideration given to its mounting to make sure it doesn't overheat.
 
Well here is the Simulink sim info to show what I was talking abt
 

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This is such a common problem that there are lots of ICs especially designed for this purpose. Google "LED driver".
 
Thanks for the advice guys, turns out the leds are 3.42v and 350mA.

I havent had time to work on this project lately, (I think I have about 5-7 projects that Im working on :? )

oh and it looks like they are having a sale on the leds or something. when I ordered my Luxeon I's a couple weeks ago, they were 6.25 each, now they're around $3.00, even the luxeon III's are down to about 3 bucks.
Looks like I need to order more :D
http://www.lumiledsfuture.com/products/family.cfm?familyId=8
 
Hi Jeff,
I can't get your link to work but isn't it for investing in LEDs?
1) You pay a low price up front.
2) If the price of the LEDs drops, they ship them to you and keep the profit.
3) If the cost of the LEDs rises, they don't ship any to you and wait for the cost to drop lower than you paid. :lol:
 
I second the supertex buck converter - its great stuff ... just make sure your switch is rated for your supply voltage!

my hv9910 is bucking 24v down to 14.6v for a series string of 2 watt nichia jupiters ... by my measurements (comparing switch power to output power (v*i out versus v*i on the switch), it is 90% efficient

I was originally using an lm317t configured for constant current, but the darn thing wasted so much energy, not to mention required an unresonably large heatsink

with the hv9910 circuit, nothing gets warm, except the leds, and they're designed to handle it (woohoo ceramic leds!)
 
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