Re: gEDA-user: Laser diode operation?

[John Griessen <john@xxxxxxxxxxxxxx>]

Mike Jarabek wrote:
> Robert Butts wrote:
>> Woe...
>>
>> 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.
>>
> I didn't mean to scare you.  You can probably get away with putting a 
> bunch of simple current limiter circuits on the diodes, like the ones in 
> the dollar store pointers.  The circuit looks pretty simple, but I never 
> traced it out.  The main advantage to a current control circuit is that 
> it will allow you to control the brightness as the diodes age, 
> especially if you use the PIN diode for power feedback.
> 
> I have even seen people drive these like an LED with a series ballast 
> resistor, but this does not protect the diodes from destruction as they 
> age, or protect them from over current.


Right.  When you care about replacement effort or price of the laser 
diodes, or even output, you will need to control the current or i**2*R 
power input according to temperature, and have it go down as temp rises. 
  The datasheet will define this relation.  To choose the current level 
to run at 25 deg C, you would measure an individual diode's light power 
output with a power detector, or use a "safe level" below the max to 
account for random variation.  The datasheet may or may not tell you 
about the variation in output power to current relation.   Volts will 
vary even more drastically with temperature rise, so if you have to work 
from volts, you will need to change it to mostly depending on current 
through a resistor and dropping power in those resistors.  A current 
controller will save you power, but add complexity and circuit cost.


John Griessen


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