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Old 07-02-2009, 10:54 PM #1
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Default Limit of red long open can diode

I'm wondering what's the limit in current of a long open can red diode.
I made an experiment today, which was an epic error. I made a labby style Red diode feeds at 505mA , and massively heatsinked.
I also have my ancient mxdl built which contain the same diode at exactly 460 mA.
When i switched on my labby, the output of the red was weaker than my mxdl at 460 mA. I didn't know why (coz it was at 500mA), and let the labby lasing for 2 minute (it's very long 2 minute) to test the efficiency of the heatsink;
In fact, the output of this one was weaker compared to the mxdl, because the current was nearly above 1 A ! (1.08), there was an error in my circuit.
The diode didn't died thanks to the heatsink i think.
I made an other driver which work perfectly at 500 ma, and hooked the diode to see if there was some degaradation of the diode (2 minute at 1A).
The output was brighter than my mxdl (460mA) so there was no degradation. I saved my diode.

But i have a question. I never fried a red diode with a too high current, i would like to know if there is some report of a maximal admissible current for these diode? i guess that we can go to 550 mA to have better output, but it start decreasing after "the knee". Where is the knee for long red diode ? i dont have one lpm so i can't investigate more; i don't remenber some precise Power, Intensity curve thread about red diode ( they stop at 500mA)


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Old 07-03-2009, 12:20 AM #2
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Default Re: Limit of red long open can diode

the lpc-815 for instance gets it's first "kink" at about 430mA. So a common current setting for them is ~420mA if you want max power with max lifespan. The LPC for instance will give about 250mw through an aixis acrylic lens. Though the Meredith Red lens will allow about 300mw to pass through with a 420mA current setting! I'm surprised your diode didn't die at an amp of current... it may come back to haunt you though..

hope this helps!
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Old 07-03-2009, 12:37 PM #3
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Default Re: Limit of red long open can diode

The only way to find out would be to do a large-scale experiment with dozens of diodes. The practice of setting the current to just below the first "kink" or "knee" is a ROT that seems to work, but there's no scientific backing. I too have been running an LPC-815 at 680mA for a couple of minutes by accident and it's still fine, so I guess there's headroom (but it did get quite hot rather quickly).

With semiconductors, generally, the higher the temp, the faster they age. Beyond some temperature, the aging process gets so fast that they more or less die instantly (electric fields also contribute).

So the question is not "what is the highest current I can run them with", but "what is the highest current I can run them with, IF I WANT THEM TO LAST xx HOURS (on average)?", and the answer will also depend on duty cycles and heatsinking - many variables. Most people probably don't care whether their laser lasts 10.000 hours or just 1.000, but I guess most would rather use a setting where it lasts 50 hours and not just 5.
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Old 03-16-2011, 03:17 PM #4
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Default Re: Limit of red long open can diode

This is a question that probably belongs in a new thread, but it's vaguely related: when you guys talk about mW you are referring to optical power. How do your measurements of that compare to the actual measurement of mW electrically speaking? For example, 300mw from 420mA isnotwhat I would expect to see if I calculated based on the voltage unless it was really low voltage. It seems the optical power registers much lower than the electrical power. Am I way off?
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Old 03-16-2011, 04:42 PM #5
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Default Re: Limit of red long open can diode

Optical power, not electrical power, and also, it's matter of efficency.

About the differences, you have to take in consideration the part of the electrical power that is turned in heat from the device in its normal working cycle, plus the optical power, for compare them with the electrical power (current and voltage) that you feed it ..... if, as example, someone say that you can get 300mW of optical power from 420mA of current (assuming they are speaking about a red LD), you need to consider this: you are feeding the laser diode with 420mA of current at 2.8V (usual threshold voltage for a red diode, for BR and 445nm ones it change), so you are feeding the diode with 2.8*0.42=1.176W of electrical power ..... getting 300mW of optical power, this mean that your diode is turning in heat 0.876W, with a total efficence of approximatively 25.5%.
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Old 03-16-2011, 05:41 PM #6
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Default Re: Limit of red long open can diode

mauswiesel;

I did a lot of testing with normal and flat package Red diodes.
Flat Red Laser Diode Testing - 2nd Phase

The LOC reds will deteriorate quickly at over 300mw.

There are some new Sharp flat package 4-pin Red diodes that can go well above that level consistantly (450-500mw).

They are hard to mount and use with our lenses.

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Old 03-16-2011, 07:13 PM #7
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Default Re: Limit of red long open can diode

I've killed 2 LPC-815 in my driver testing. LPC-822 has been reported to be slightly more robust in that if you overdrive it, output drops, but if you bring it back to the optimal drive current, it makes a full recovery.

Full details on lpc822 here: NEW LOC Laser diode review.
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Old 03-17-2011, 01:08 AM #8
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Default Re: Limit of red long open can diode

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Originally Posted by lulighttec View Post
It seems the optical power registers much lower than the electrical power. Am I way off?
You're spot on really. With lasers, we usually state te optical output power. Things like lightbulbs and led's are often quoted with their electrical power input instead of their optical output.

Compensating for efficiency, you'd probably end up with a 2 watt-electrical led yielding the same optical output as a 200 mW-optical laser.

It's also good to notice that laser diodes are in fact extremely efficient at turning electrical power into light. The efficiency of a laser diode can be as good as 25%, much better than leds, cfl lights and all... but at a price tag.
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Old 03-17-2011, 03:11 AM #9
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Default Re: Limit of red long open can diode

Yeah, it's too bad. Ion lasers would sound more impressive if they were rated on input power.
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Old 03-17-2011, 05:32 AM #10
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Default Re: Limit of red long open can diode

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Originally Posted by LarryDFW View Post
There are some new Sharp flat package 4-pin Red diodes that can go well above that level consistantly (450-500mw).
I'd love to experiment with some of these. Is there a reasonable source for them somewhere?
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Old 03-17-2011, 01:12 PM #11
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Default Re: Limit of red long open can diode

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Originally Posted by Cyparagon View Post
Yeah, it's too bad. Ion lasers would sound more impressive if they were rated on input power.
Please don't give manufacturers ideas here

There seems to be a bit of a movement the other way, where lightbulbs and leds get rated by optical power (in lumen) rather than electrical input. I think it is quite logical, since the output is what you need.

Stating the input power is like stating the gallons of fuel per mile for a car and marketing that as a good thing.
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