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Re: gEDA-user: Laser diode operation?
On Friday 29 August 2008 11:15:03 pm Robert Butts wrote:
> I'm using ten of these and parallel. I WAS going to just use a 1 amp 5 vdc
> power supply with a 2.8 V zener diode to adjust the voltage to 2.2 V. I
> take it this is too simple.
This hideously outdated page gives a good introduction to the subject:
http://www.repairfaq.org/sam/laserdps.htm
Does not have to be complicated, but you never want
to parallel LEDs/LASER Diodes as they are a current
driven device. Each of your ten diodes would need
a current limiting resistor so that one of the diodes
does not steal all of the current from the other nine,
because they will not be perfectly matched.
Here is a real world example of why paralleling
LEDs is a bad idea. The boss man assigned me the task
of updating a product from the 1980's that used a bunch
of LEDs in parallel, without resistors. They did this to save money and board
size. Amazingly it worked *almost* always. The bench tech. in repair
said that he did have to replace a lot of LEDs due to miss matched brightness
(Always make a point of talking to the people in the trenches when working on
designs).
I updated the design, put in the proper resistors. In the design review
meeting the boss said "I didn't need those in my design, take them out".
Ok, your the boss (Resume anyone?). :-( The boards came in and usually
had a single LED of the four in parallel that was super bright, which boss
immediately started blaming on bad LEDs, the three dim ones. Yeah, right.
Anyway after wasting several thousand dollars on LED rework, he let me respin
the boards with the proper resistors in series with each LED (I had left the
space for them when I did the layout, as I knew this was coming).
Bottom line is that maybe you could get away with paralleling LEDs with 1980
vintage LEDs, but you can't do it with LED technology today. Skimping by
trying saving pennies (resistors) will always cost you many dollars (rework
costs) in the end.
--
http://www.wearablesmartsensors.com/
http://www.softwaresafety.net/ http://www.designer-iii.com/
http://www.unusualresearch.com/
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