Is a simple LM317-based laser diode driver safe for powering a laser diode? https://www.youtube.com/watch?v=iuMngik0GR8 (the diagram is at 1:37) enter image description here

I've been told the the proprietary drivers offer some extra protection from very short spikes (on power up and shutdown) in the current which laser diodes are very sensitive to and might be damaged from. But isn't the LM317 regulator taking care of that itself already?

I've been told this from people who actually sell commercial laser diodes so I'm thinking they might be showing some bias.

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    \$\begingroup\$ I just LOVE it. circuit diagrams in YouTube videos. Can there be anything in this world more USELESS? \$\endgroup\$ – JRE May 7 '15 at 12:00
  • \$\begingroup\$ I'm sorry, I don't understand what's the issue. \$\endgroup\$ – Leo Ervin May 7 '15 at 12:03
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    \$\begingroup\$ Crappy resolution, scroll through a MOVIE to look at it. Compression artifacts. I must be some kind of dinosaur. I always want diagrams and project descriptions I can print out and work from instead of fuzzy videos that never show things from the right angle to see what I actually need to do. \$\endgroup\$ – JRE May 7 '15 at 12:06
  • \$\begingroup\$ I know where you're coming from but it's very readable in that video. But here's a screencap: i.imgur.com/eaburvP.png \$\endgroup\$ – Leo Ervin May 7 '15 at 12:10
  • \$\begingroup\$ So why not just put the screen capture into your post instead of expecting us to watch through the video to see it? \$\endgroup\$ – The Photon May 7 '15 at 16:01

If you have a regulated power supply, a simple resistor will do to take the laser above its threshold point. More complexity can involve a thermistor to lower resistance as temperature rises keeping the laser current at a slightly higher value at higher temperatures. Here is a typical laser characteristic for a device supplied by Hamamatsu: -

enter image description here

A more complex circuit would be needed if the laser were to be used as a datacomms device but this would involve only one extra inductor and a capacitor.

Going further, to higher powers and several manufacturers make chips that monitor the inbuilt photodiode inside a lot of lasers. They do this to protect the device from excessive currents.

Using an LM317 current limiting supply seems reasonable to me but the devil is in the detail and a circuit would be needed to be looked at for greater confidence (plus the data sheet of the laser).

  • \$\begingroup\$ @Andyaka Do you know any of LD driver chips for 5-100mW? \$\endgroup\$ – Nazar May 7 '15 at 11:56
  • \$\begingroup\$ BTW the circuit diagram in the video is at 1:37 i.imgur.com/eaburvP.png Laser data sheet: drive.google.com/file/d/0B_6-KC5wFXIJZGo3WFpjeWRwdHc/edit Of course the circuit will be modified for the correct voltage and current. \$\endgroup\$ – Leo Ervin May 7 '15 at 11:57
  • \$\begingroup\$ The circuit looks fine and the laser is a beast. I only tend to work with sub 100mW lasers hence I'm calling it a beast! \$\endgroup\$ – Andy aka May 7 '15 at 12:06

The main issue with the circuit you've shared is transients.

Some linear regulator circuits will tend to overshoot on turn-on, or allow input transients to pass through to the load. Here's the ripple rejection curve for TI's LM317:

enter image description here

You can see that the LM317 will do very little to protect the laser from transients with characteristic times below about 1 us. And an ESD event can easily have a pulse width as short as 1 ns.

And many laser diodes are quite sensitive to over-voltage and over-current. They may also respond very quickly to transients compared to other devices that you might connect to an LM317.

A good laser driver circuit should have ESD protection, transient protection, and a slow-start behavior.

These things are more important if you are driving a low-cost CD laser, or a high-speed telecommunications laser than if you are driving a high-power material processing laser. They may also be less important today than they were 20 years ago due to improvements in the robustness and reliability of newer laser designs.

  • \$\begingroup\$ Do you know what is typically used in these drivers for ESD protection, transient protection, and a slow-start behavior? \$\endgroup\$ – Leo Ervin May 8 '15 at 9:18
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    \$\begingroup\$ Unfortunaely, everybody seems to have their own favorite protection devices and circuits, based on experience. I don't know if there's "one right answer" for that. \$\endgroup\$ – The Photon May 8 '15 at 15:54

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