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Old 05-29-2008, 05:48 PM #1
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Default Questions about DDL:  PD, and distance to LD

Hi.

I have two noobish questions about the DDL circuit as a driver for a LD.

1 - As far as I can tell, the DDL only has leads connected to the laser anode and cathode, and the photodiode is not used. Is this because the current regulation capability of the DDL enables precise LD output power setting, which then obviates the need for the photodiode feedback? Thanks for helping me understand this.

2 - My project has a main circuit board located 3 inches from the LD. I'm thinking of locating the entire DDL circuit on a new main board and using 3 inches of 22awg wire to carry the DDL output power to the LD. If this is completely inadvisable, maybe someone can say so and explain why?

Power supply = 2 x 14500 rechargeable protected lithium cells, each is 3.6v nominal, or 4.1v peak when fresh off the charger.

LD is 10mw 904nm, 25 to 70mA op-current range, 1.8 to 2.5 op-voltage range.

The objective is to operate the LD at the lower end of its output power range, ie probably drawing only 30mA or so.

Thanks!


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Old 05-29-2008, 06:03 PM #2
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Default Re: Questions about DDL: *PD, and distance to LD

1 photodiode isnt needed b/c the driver does the regulating.

2 length of wire shouldnt matter as long as its not like a hundred feet of it.

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Old 05-29-2008, 07:54 PM #3
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Default Re: Questions about DDL:  PD, and distance to

Quote:
Originally Posted by Jess
1 - As far as I can tell, the DDL only has leads connected to the laser anode and cathode, and the photodiode is not used. Is this because the current regulation capability of the DDL enables precise LD output power setting, which then obviates the need for the photodiode feedback? Thanks for helping me understand this.
That's pretty simple, it's because the photodiode isn't connected to anything.

Quote:
Originally Posted by Jess
2 - My project has a main circuit board located 3 inches from the LD. I'm thinking of locating the entire DDL circuit on a new main board and using 3 inches of 22awg wire to carry the DDL output power to the LD. If this is completely inadvisable, maybe someone can say so and explain why?

Power supply = 2 x 14500 rechargeable protected lithium cells, each is 3.6v nominal, or 4.1v peak when fresh off the charger.

LD is 10mw 904nm, 25 to 70mA op-current range, 1.8 to 2.5 op-voltage range.

The objective is to operate the LD at the lower end of its output power range, ie probably drawing only 30mA or so.

Thanks!
The only problem I could think off is that the wire might be too strong, when you solder leads to the LD's pins, make sure you use a wire which can be easily bent, so that you don't stress the LD's pins. As far as the power source, 2 14500s should be wonderful, and btw, you don't have to worry about the voltage going to the LD, it will get sorted out by itself, all you have to worry about is the current.
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Old 05-30-2008, 05:04 PM #4
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Default Re: Questions about DDL: *PD, and distance to LD

Thanks!
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Old 05-30-2008, 07:57 PM #5
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Default Re: Questions about DDL:  PD, and distance to

#1 - the DDL supply is constant current, not APC (automatic power control). For a pointer this usually doesn't matter, however it does mean the power output of the LD will vary with temperature, and as the LD heats up. If you need the optical power of the LD to remain constant, that's when you need an APC supply. Half of the LD's output hits the monitor photodiode - an APC supply adjusts the current to the diode to keep its actual output stable, regardless on the temp. It does this by monitoring the output from the third pin (assuming it's hooked up).

#2 - should not be a problem.
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