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'PIC and laser diode'
2007\03\13@125732 by Peter Feucht

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Ladies and Gentlemen,

I have a new development here, which includes among other stuff a 2mW laser
diode. These diodes are as you might know relatively critical in case of
overcurrent. For the other software tasks are relatively slow and simple I
have plenty of time left to supervise this laser led. Has anybody done this
before? This is surely no very problematic application, I just don't want to
re-invent the wheel.

Any links and/or comments highly welcome.

Thanks in advance and best regards


Peter

2007\03\13@143130 by David VanHorn

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On 3/13/07, Peter Feucht <spam_OUTp.feuchtTakeThisOuTspampalmed-medizintechnik.de> wrote:

> Ladies and Gentlemen,
>
> I have a new development here, which includes among other stuff a 2mW
> laser
> diode. These diodes are as you might know relatively critical in case of
> overcurrent. For the other software tasks are relatively slow and simple I
> have plenty of time left to supervise this laser led. Has anybody done
> this
> before? This is surely no very problematic application, I just don't want
> to
> re-invent the wheel.


Are you thinking to regulate the current in software?

That's very likely to fail, unless you use a very slow ramp, and I would
still look to external hardware because you need to protect against glitches
on the supply rail.  Battery powered pointers have the advantage that the
supply is "noiseless", and there's nothing else on the supply to make noise.

What's generally done, is to use the photodiode output to control the
current in a feedback loop with an opamp or transistor amplifier. Then
adjust, using a power meter, till you get rated output.

The diode itself is extremely tiny, and slight overcurrent turns it into
toast very fast.



--
Jealousy is a disease, love is a healthy condition. The immature mind often
confuses one for the other, or assumes the greater the love, the greater the
jealousy. In fact they are almost incompatible; both at once produce
unbearable turmoil.
Jubal Harshaw, "Stranger in a Strange Land"

2007\03\14@144243 by David VanHorn

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On 3/14/07, Peter Feucht <.....p.feuchtKILLspamspam@spam@palmed-medizintechnik.de> wrote:
>
> David,
>
> Thanks for your comments.
>
> 1. Yes, I thought of regulating the current by any type of PWM.
> 2. The "very slow ramp" you speak of: What is "very slow" here? Seconds,
> milliseconds?


As slow as you can tolerate really. If you're going to modulate the current,
that could get very tricky in compensating your feedback so that you don't
overshoot.

3. You speak of "toast very fast". How fast is "very fast" here? ms? µs?


I haven't seen anyone give any hard figures, but it's in the single digit
microseconds, and it's similar to how a light bulb reacts to overvoltage, as
the current rises the output power increases dramatically past a certain
point.

http://www.sony.net/Products/SC-HP/laserdiodewld/application/index2_01_03.html

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