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Old 02-28-2011, 01:16 AM #1
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Default Question about typical overvoltage allowance

Question to you guys that like to push their diodes hard... I ordered a diode whose seller says it's 2.2V nominal... (808nm / 300mW eBay special, my fingers are crossed that it works at all) Do you guys think I'm safe using a typical 3V supply (eg. LM1117 base), or do I have to design a lower voltage one? Just increase my load-series resistor a bit?


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Old 02-28-2011, 01:18 AM #2
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Default Re: Question about typical overvoltage allowance

If you are using a LM1117 voltage will be regulated by the diode. You need a current regulating power supply, then you don't need to worry about voltage at all.
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Old 02-28-2011, 01:51 AM #3
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Default Re: Question about typical overvoltage allowance

As long as I keep my current where I want it, I can really (sort of) ignore suggested voltages? Can I in theory run a diode off 5V (at a lower current? P=IV??) without it imploding and ending the universe?

Sorry 'bout the nooby questions, the teaching is much appreciated : )
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Old 02-28-2011, 05:14 AM #4
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Default Re: Question about typical overvoltage allowance

If you were connecting the diode to a set voltage supply, say a 5V battery, there might be problems, because the diode only drops 2.2V, while the battery is supplying five. The excess voltage would be applied across the diode and wires, etc.

On your linear regulator, the LM1117, its only job is to produce 1.25V between its output and the ADJ pin (assuming you got the adjustable one). When you put that resistor between those two points, the resistance determines how much current is produced. But what about the voltage? Well the diode has a specific drop--2.2V in your case. The linear regulator will develop a charge that will increase to the forward voltage level of the diode. When finally that voltage level is enough, current will flow through the diode. So the linear regulator will essentially give you the voltage you need so long as you have enough input voltage. It almost seems like magic, but the linear regulator is not a passive device like a resistor.

Short answer: the linear regulator will take care of the voltage for you as long as the input voltage is greater than its Vdropout + Voutput. Vdropout is 1.25 or so on your LM1117 I think.
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Old 02-28-2011, 05:33 AM #5
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Default Re: Question about typical overvoltage allowance

Lumin:

If you use a lithium ion (around 3.7V), and an LM1117 that you wire in a circuit following the "DDL" driver schematic, and if you choose a resistor value to setup the driver for the correct CURRENT, you will be A-OK with that diode.
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Old 02-28-2011, 05:51 AM #6
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Default Re: Question about typical overvoltage allowance

Oh my goodness! Thanks for the replies (BB twice for the in-depth explanation!), 2.2V _DROP_! That's where I got confused, I just thought of it as a component that required a 2.2V potential to lase! I totally didn't think about them in their proper diode context ("laser module" in my head) Now the whole "only the current matters" response makes more sense!
Much appreciated : D
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Old 02-28-2011, 05:55 AM #7
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Default Re: Question about typical overvoltage allowance

Quote:
Originally Posted by Bionic-Badger View Post
It almost seems like magic, but the linear regulator is not a passive device like a resistor.
Disregard everything Bionic-Badger said.... I don't care how much rep power he has, I simply REFUSE to stop believing that it's magic
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Old 03-01-2011, 12:07 AM #8
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Default Re: Question about typical overvoltage allowance

LOL technology = magic explained, no? : ) And there's definitely something comforting in believing that my doide's regulated by spike-proofing magic!
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