LM317 for driving a laser diode

Hi, I would like to run a laser diode in continuous operation. I found circuits using LM317 like this



There seems to be no restrictions on input voltage and load forward voltage? current is simply 1.25/R ??

I read somewhere else that input voltage has to be 2.5V more than load diode's forward voltage. Is that sufficient or am I missing something?

Wiring a regulator like that makes it produce a constant current output regardless of what you put in to it as long as you meet the minimum overhead voltage requirement. It works quite simply by trying to regulate the voltage across the resistor instead of across the LED, in fact as far as the regulator is concerned, the resistor IS the load. Without additional resistors to divide the 'ADJ' voltage it works as a 1.25V fixed voltage regulator, maintaining 1.25V between it's OUT and ADJ pins. As the voltage across the resistor is constant and the resistor value is constant, by implication the current through it must be constant as well.

All you have to do is ensure there is sufficient input voltage which will be the Vf of the laser diode + 1.25V across the resistor + about 2.5V across the LM317. The maximum voltage you can apply is 40V + the laser diode Vf. Note that higher voltages will cause increased heat dissipation. W=V*I for both the regulator and the resistor.

As the laser is current operated, you should choose a resistor value to suit the laser current. For example if it needs 50mA you would choose (I=V/R) =1.25/0.05 = 25 Ohms. The resistor will dissipate (W=V*I) 1.25*0.05 = 0.06W and the regulator will dissipate (Vin - 1.25 - Vlaser)/0.05 Watts.

Brian.

Thanks, that cleared things up.

If Vf is 6.4V, then 6.4+1.25+2.5 = 10.25V So I guess 12V input should be fine!

Hi

6.4+1.25+2.5 = 10.25V

Click to expand...

Only if 1% of tolerance is allowed...

Klaus

Huh, I don't understand.

From above:

All you have to do is ensure there is sufficient input voltage which will be the Vf of the laser diode + 1.25V across the resistor + about 2.5V across the LM317

Click to expand...

12V is comfortably over the sufficient voltage input, I don't see a problem of 1% tolerance..

I think 12V should be fine, I think Klaus is mistaking the calculated voltage from the 12V you say you can supply.

Brian.

Hi,

6.4+1.25+2.5 = 10.25V

Click to expand...

Your result is about 1% off. It should be 10.15V

Klaus

Oops! Missed that one, you are quite right Klaus.

12V should still be more than adequate though.

Brian.

betwixt said:

.................
All you have to do is ensure there is sufficient input voltage which will be the Vf of the laser diode + 1.25V across the resistor + about 2.5V across the LM317. The maximum voltage you can apply is 40V + the laser diode Vf. Note that higher voltages will cause increased heat dissipation. W=V*I for both the regulator and the resistor.
...............

Click to expand...

The input voltage won't affect the resistor dissipation as that's determined by the current, which is a constant.