Laser Power Supply (Wiring/circuit design question)

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01 Mar 2012, 7:11 pm

So I'm building a 250mW 404nm infared Laser. The diode itself is rated at 200mW, but the intention is to run it at 250mW. Anyway, I don't have the diode itself yet, but I have connected some wiring to a 12v rail on an old computer power supply. It checks out at 12v, and it is well over necessary 20mA without a resistor. The strange part is when I put the 1/4W resistor on. The meter reads as about 12mA with that. At the 12 volts, this only comes to ~150mW. What am I doing wrong?

Fnord
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01 Mar 2012, 7:32 pm

largosan wrote:

It depends on what you are trying to do. Your math checks out...

12v / 12mA = 1K-Ohms

12v x 12mA = 144mW

...

As for the diode, over-driving it is a sure way to ruin it. If you need to run it at 250mW, then get a diode that's rated at 250mW. Better yet, to make sure that the diode will handle 250mW, then use one rated for 500mW instead. Trust me, the extra expense for the higher-rated diode will be less than the expense of buying the lower-rated diode twice.

Oodain
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01 Mar 2012, 7:51 pm

Fnord wrote:

As for the diode, over-driving it is a sure way to ruin it. If you need to run it at 250mW, then get a diode that's rated at 250mW. Better yet, to make sure that the diode will handle 250mW, then use one rated for 500mW instead. Trust me, the extra expense for the higher-rated diode will be less than the expense of buying the lower-rated diode twice.

this,

incdiently blue ray lasers come from 100mW to about a watt easily, they can be had for cheap on ebay as modules, a little desoldering and you have a perfectly good laser diode,
they run at 405nm as far as i remember, dunno if that will do as i dont know what the laser is meant for.

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RazorEddie
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02 Mar 2012, 5:23 am

Your figures assume you are running the diode at 12v and it is 100% efficient. In practice they are around 20 - 30% efficient and run on a much lower voltage.

Before you do anything you need to know the rated voltage and current for the laser diode. I just did a quick search and found the specs for a roughly similar 300mW laser: 3V at 400mA.

Your resistor needs to drop 12 - 3 = 9V at 400mA (0.4A).
V/I = R
9 / 0.4 = 22.5 ohms. The nearest standard resistor is 22R which would give a current of 9 / 22 = 0.409A.

Now you know the resistor value you need to know how much power you will be dissipating in the resistor.
V * I = P
9 * 0.409 = 3.68W. The nearest standard resistor will probably be 4W. Watch out as it will run quite hot.

Obviously you will have to substitute your own numbers from the diode data sheet.

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largosan
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02 Mar 2012, 7:14 am

Oodain wrote:

Fnord wrote:

I've already ordered the diode. I thought about using a blu-ray laser diode, but I managed to crush the low powered diode from a cd drive while I was practicing with that. Also, it was ultimately cheaper to get the 200mW diode($6, free shipping). In all reality the purpose of the laser is something to build, for the sake of building.

largosan
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02 Mar 2012, 7:17 am

RazorEddie wrote:

That explains a bit. Thanks.

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02 Mar 2012, 12:54 pm

And do be careful where you wave it: people only have one pair of eyes and that level of output is enough to scorch retinas.

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kVArc
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05 Mar 2012, 8:31 am

I'm not really familar with laser diodes; but I know a couple of things about them:

- Laser diodes are extremely sensitive to electrostatic discharge. Always ground yourself (by touching a radiator, a water tap or another grounded metal object) before handling them.
- They absolutely don't like overload - 20% too much current can kill the diode instantly.
- Powerful laser diodes need proper cooling; otherwise they will overheat.

The best way to drive the diode is a constant current supply. It's very simple to use a LM317 voltage regulator as a constant current source.

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largosan
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05 Mar 2012, 9:06 pm

kVArc wrote:

Thanks for the warnings. I was trying to use as many things that I already had, and it seemed like a PC PSU would have a fairly constant current, because computer parts are often also sensitive. In the multimeter tests, the current seems pretty constant, and since I'm under-amperage anyway, I won't be going over 200mw. As for grounding myself, I already assumed that from having a computer background. And cooling. I'm using a piece of aluminum heatsink from an old dell, with a hole drilled for wiring and a larger hole for the diode. I have non-conductive thermal paste(Not the silver based stuff) to put in there, and if it gets too hot I can run a fan on it.

As for Cornflake's comment, yes, I will be careful where I shine it.

RazorEddie
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06 Mar 2012, 1:40 am

PC power supplies are constant voltage, not constant current. However as you are intending to use 12V with a resistor dropping it to around 3V, the resistor will regulate the current well enough.

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