How to individually control multiple laser diode using a single constant current driver (LM317) ?

How to individually control multiple laser diode using a single constant current driver (LM317) ?

  • Using a microcontroller to calibrate a resistor in series with the laser

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  • Using a microcontroller to calibrate a resistor in series with the laser

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Thread Starter

Sekou Junior Camara

Joined May 31, 2016
7
(Excuse my English)

I'm currently designing a laser diode driver using the LM317L constant current circuit. I've 7 different laser diodes. I was able to put them in parallel by adjusting the load of every string in order to maintain the highest possible voltage and a constant current flow. The below circuit is a single driver operating multiple similar LEDs. I adapted this circuit by adding a resistor in series to operate laser diodes (to keep a constant current flow in each string). Now that you know what I'm doing, here's the feature I want to add: I wish to control the current of every string independently in order to control the brightness of the laser diodes. The output current of every string depends on R_sense: I_out = (1.25+V_sat) / R_sense. If you reduce (for example) the value of R_sense in a specific string, the current will increase and it will also affect the current in the other strings and I don't want that. Do you have any idea about how to add this feature ?

Thank you.
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ronv

Joined Nov 12, 2008
3,770
Thank you for your reply. That's exactly my problem. I need to control the current in every string individually, perhaps there're some changes to make to this circuit, any suggestions ?
You don't say how much current the lasers take or the supply voltage, but it can't be to much if it works with 1 lm317.
I would use a separate current source for each string and drop the 317.
 

dannyf

Joined Sep 13, 2015
2,197
Do you have any idea about how to add this feature ?
Fairly easy:

Rewire the constant current design into a constant voltage design driving multiple indecently controlled constant current drivers no your choice.

Your question can be easily answered by thinking through it logically: how do you independently control a few current sinks so that their sum of current is a constant?
 

AnalogKid

Joined Aug 1, 2013
8,487
Since one low power 317 can supply all three strings, I recommend using three 317's, one for each string, and eliminating the three 2222's. Your net component count will decrease by eliminating R4, R5, R6.

ak
 

Thread Starter

Sekou Junior Camara

Joined May 31, 2016
7
You don't say how much current the lasers take or the supply voltage, but it can't be to much if it works with 1 lm317.
I would use a separate current source for each string and drop the 317.
I already designed it using separate LM317 for each circuit. I was given a challenge to drive all the laser diodes with the same LM317 with adjustable current in each strings (and of course constant voltage).
 

Thread Starter

Sekou Junior Camara

Joined May 31, 2016
7
Since one low power 317 can supply all three strings, I recommend using three 317's, one for each string, and eliminating the three 2222's. Your net component count will decrease by eliminating R4, R5, R6.

ak
I can do that but what if they can all be controlled by a single one. I use the transistors to protect each diode in case of an open circuit problem in a string. For example, if, for some reason, a diode acts as an open circuit (critical state), the current will be divided between the other strings. To prevent that, I use a transistor as a switch.
 

Thread Starter

Sekou Junior Camara

Joined May 31, 2016
7
Fairly easy:

Rewire the constant current design into a constant voltage design driving multiple indecently controlled constant current drivers no your choice.

Your question can be easily answered by thinking through it logically: how do you independently control a few current sinks so that their sum of current is a constant?
Very interesting idea. Can you please elaborate more precisely ? Thank you.
 

mcgyvr

Joined Oct 15, 2009
5,394
Very interesting idea. Can you please elaborate more precisely ? Thank you.
That idea is the same as everyone elses and that is to use 1 constant current source per diode string..
You can't/shouldn't do it with just one current source

By "rewire" he just meant to get rid of it and not change the LM317 to a constant voltage supply.. "Redo" might have been a less confusing word.

And if by "at work" you mean this is a product you intend to sell then there are much better solutions than an LM317 for a constant current source..
 

Thread Starter

Sekou Junior Camara

Joined May 31, 2016
7
That idea is the same as everyone elses and that is to use 1 constant current source per diode string..
You can't/shouldn't do it with just one current source

By "rewire" he just meant to get rid of it and not change the LM317 to a constant voltage supply.. "Redo" might have been a less confusing word.

And if by "at work" you mean this is a product you intend to sell then there are much better solutions than an LM317 for a constant current source..
Thanks for your answer!
 

AnalogKid

Joined Aug 1, 2013
8,487
I can do that but what if they can all be controlled by a single one. I use the transistors to protect each diode in case of an open circuit problem in a string. For example, if, for some reason, a diode acts as an open circuit (critical state), the current will be divided between the other strings. To prevent that, I use a transistor as a switch.
With three LM317 current regulator circuits, you have independent control and protection. Open circuit, dead short, don't care.

And by the way, you can't have both constant current and constant voltage. Assuming the load is varying and you want to maintain a constant current through it, the way all current regulators work is that they adjust the voltage across the load such that the current remains at the set point. Pretty much by definition, a constant current regulator output has an voltage that varies inversely with the load. Per Ohm's Law.

ak
 
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