gEDA-user: Laser diode operation?

John Griessen john at
Thu Sep 4 12:21:04 EDT 2008

Rob Butts wrote:
> Referrer to the following datasheet 
[please and thank you works well here]
for the laser diode I'm using:
> If I use the following circuit at the link below to power the laser diodes
> what should I calculate Ipd to? 

You have to measure the current resulting from actual operation as wired 
-- see below.  You start small and change the pot. until you get the 
desired current.  Desired current will come from the above spec. (No, I 
have not read it).

  From what I can see that would be 0.2 mA.
> Also, should/could I string 10 of these circuits and parallel?

This Sam's laser faq circuit has a volt bias and neg feedback via Vbe of 
the PNP transistor such that if the current in the laser diode increases,
the base current increases, so the base voltage goes up, cutting the 
transistor off.   So the transistor Vbe will be around its turn on 
voltage.  It's a current regulator depending on the gain of the PNP 
transistor near cutoff.

John Griessen

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