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Old 05-31-2008, 12:20 AM #1
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Default Need help with driver for laser/led combo...

Hi, I'm building a laser/led combo powered by two 14500 rechargeables (8.4v peak in series).

There's one central board (2.25" x .75&quot that has a switch and both drive circuits on it.

My plan was to use DDL drivers for both the laser and the led.

The laser takes 25 - 70mA at 1.8 - 2.5v, so I chose an 18ohm resistor & a 50-ohm pot for that DDL.

The LED takes 600mA (at ~1.7Vf). I can use a 2ohm resistor and a 50ohm pot for that DDL.

BUT!

If P = (Vs-Vf)*I,
the power dissipation of the LED circuit would be as high as (8.4 - 1.7)*.6 = 4 watts!!! Can that be right?!?

I really like the DDL because there's so much good info on how to use it, and I already have the parts I need to build them (still waiting on breadboard and protoboards).

But now I'm thinking that I need to find a more efficient option for driving my LED.

I'm open to advice, recommendations, or corrections. Thanks!


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Old 06-01-2008, 12:34 PM #2
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Default Re: Need help with driver for laser/led combo...

Whoa are you sure that LED can take 600mA? That almost sounds like instant *POP*, but anyways your calculation doesn't seem right to me.

Power = Current * Voltage
Power = Current˛ * Resistance
Power = Voltage˛ / Resistance

600mA * 1.7V = 1.02Watts

Use a DMM and find out exactly how much that LED is pulling, just sounds way too high.
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Old 06-01-2008, 04:41 PM #3
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Default Re: Need help with driver for laser/led combo...

Xylicon - Thanks for the feedback, and for the handy pic! (now saved to desktop)

The operating specs for this LED are: Vf 2.4, If = 1 amp. I've operated it at 520mA for 30 seconds with no probs.

It's an unusual LED: really high current at low voltage.

My calculation in my first post was my attempt at a rudimentary power assessment.
8.4v * 600mA = 5 watt dissipated across the whole system
1.7v * 600mA = 1 watt dissipated at the LED (in the form of photons)
I'm assuming that this means that the other 4 watts must be dissipated across the LM317 and the sense resistors (as heat).

That's correct isn't it? (I'm pretty noobish, so if I'm doing that wrong, somebody school me )

After reading around some more, I think a better design would be to put the 2 14500's in parallel, and drive both the laser and the LED from a 4.2v system. My laser will operate at 2v and my LED will operate at 1.7v, so a lower supply voltage suits my needs better. (right?)

That would mean no DDL, which is a bummer because I really believe what Daedal says about the benefits of regulating current to diodes.

But maybe CCT is still possible!

Is there any such critter as a CCT driver that starts with 3.5 to 4.2 supply voltage, and can supply a steady 45mA at 2v, or a steady 600mA at 1.7v?

Bottom line - I'm open to suggestions! Thanks!
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Old 06-01-2008, 07:56 PM #4
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Default Re: Need help with driver for laser/led combo...

Quote:
Is there any such critter as a CCT driver that starts with 3.5 to 4.2 supply voltage, and can supply a steady 45mA at 2v, or a steady 600mA at 1.7v?
I don't know I just recently got into lasers/modification so as far as drivers and power supplies go I really don't have a clue

My electronics knowledge hits a wall when it comes to lasers at the moment.
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