Would a string of these work for each laser? The Battery would be 5 vdc.
Battery(+) o----------------------+--------+
| |
_|_ __|__
PD /_\ _\_/_ LD
| |
Ipd| | Ild| |
v | v |
| |
R1 | |/ E
+--/\/\--+---------| Q1 PNP
| 1K |\ C
/ |
VR1 \<-+ |
S1 10K / | |
Power \ | |
_|_ | | |
Battery(-) o----o o-------+--+--------------+
In this case, the power output is determined by the equation:
Vbatt - (Ipd * (R1 + VR1)) = Vld + Vbe1
Or:
Vbatt - Vld - Vbe1
Ipd = --------------------
R1 + VR1
Where:
- Vbatt = battery voltage under load.
- Ipd = total photodiode current.
- Vld = voltage across the laser diode.
- Vbe1 = Base-emitter drop (.7 V) of Q1.
Since Ipd is proportional to optical power output, like LP-LD1 and LP-LD2
(above), brightness is dependent on battery voltage. In this case, it is a
much more non-linear relationship as Vld and Vbe1 set a threshold of about 2
to 2.5 V below which there will be nothing and then output will increase
based on Vbatt/(R1 + VR1). The circuit operates on 3 V but 4.5 V seems like
the minimum to get any decent output.
On Sat, Aug 30, 2008 at 11:23 AM, Kai-Martin Knaak <[EMAIL PROTECTED]>wrote:
> On Fri, 29 Aug 2008 23:15:03 -0400, 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.
>
> It is. You need to protect the diodes from over current rather than over
> voltage. Laser diodes will die in micro seconds, when exposed to over
> current. A proven design is to use an adjustable voltage regulators as a
> current limiter. See the bottom of page 18 of the data sheet by national
> for this rather simple circuit:
> http://cache.national.com/ds/LM/LM117.pdf
> Each laser diode needs its own protection.
>
> ---<(kaimartin)>---
> --
> Kai-Martin Knaak
> http://lilalaser.de/blog
>
>
>
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