# Current Limited Laser Driver

Discussion in 'The Projects Forum' started by coolnewt, May 9, 2015.

1. ### coolnewt Thread Starter New Member

May 8, 2015
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0
I'm trying to build a circuit to drive a laser diode that has an operating voltage of 3V and operating current of 180mA-250-mA. I have built the below circuit referencing Figure 14 in Section 9.3.3 in http://www.mouser.com/ds/2/405/lm317-556636.pdf. I know I still need to adjust the voltage, but I am puzzled as to why my ammeter is indicating such low current. It will not even light an LED. The 12V DC source is from a wall wart and can source 3A. Any insight on what I'm doing wrong?

2. ### crutschow Expert

Mar 14, 2008
13,521
3,388
The LM317T requires at least 3V across it for proper operation. Your circuit only has 1-2V.
Just connect the LM317T directly to the 12V source.
Of course the LM317T needs to be on an appropriate heat sink since it will be dissipating up to 2.25W at 250mA and a 3V laser diode load.
Note that with a 15Ω resistor the constant current will be about 1.25V / 15Ω = 83mA .

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3. ### coolnewt Thread Starter New Member

May 8, 2015
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0
Thank you crutschow! I thought it was 3V for Vin minimum, but now I see it is Vin - Vout as you stated must be at least 3V. I'll give this a try.

4. ### coolnewt Thread Starter New Member

May 8, 2015
4
0
That worked, measured 82mA . Is the best way to achieve 3V, to hook up a 3V regulator to this output?

5. ### crutschow Expert

Mar 14, 2008
13,521
3,388
You can either regulate voltage or current, take your pick. You can't regulate both at the same time.
Normally you regulate the current to a laser or LED and let the voltage be whatever the diode takes at that current.

Last edited: May 10, 2015
6. ### coolnewt Thread Starter New Member

May 8, 2015
4
0
Ok, thank you for the info.