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I've got a cheapo 5mW red laser module (datasheet), it looks like its got an internal regulator, probably a thermal cutoff too. I've been having problems driving it.

First thing I tried is I connected it to 5V via a resistor of varying value, (1k down to something like 200r) and the results are plain weird.

The module starts to lase at about 470r, but would be very erratic and would go off at some point. Driving it with a 200r i get a bright enough spot, but the current measures between 17mA and 21mA, voltage across the laser is 2.7v, but after a while it just cuts off like its been blown. The resistance of the laser module drops to couple of Ohms, regardless of polarity. Leaving it to "cool down" also does nothing.

Fooling around I then just tapped it on the 5V rail with no resistor, and it blinked with bright light, and started working with 470r and 200r resistors again.

Am I hitting some sort of a thermal cutoff or does the module need that minimum current/voltage to just lase, and could probably take higher voltage/current combination?

I'd like to drive it so its at its brightest point, within its rated spec.

Yarek T
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3 Answers3

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You linked to the datasheet, but apparently didn't read it. It's not very good, but enough basic information is there. The typical operating point is 3.0 V at 25 mA. If you are using a 5 V supply with a resistor, then the resistor will drop 2 V. 2V / 25mA = 80Ω. How you came up with 1 kΩ, 470 Ω, and 200 Ω as series resistors is completely beyond me.

Directly connecting this laser to a 5 V supply is a really bad idea. The datasheet only lists the typical voltage, which is 3.0 V. It does show the typical current as 25 mA and the maximum as 35 mA. How is directly connecting this thing to a 5 V supply going to ensure 35 mA is not exceeded? You may have damaged the device, even if it appears to operate.

Try the common value of 82 Ω in series with the 5 V supply and measure the voltage accross the module. It should be about 3 V. To not exceed 35 mA, the voltage accross the resistor must not exceed 35mA * 82Ω = 2.87 V, which also means the voltage accross the module when powered thru this resistor and a 5 V supply should be at least 2.13 V.

Olin Lathrop
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  • I did read the datasheet. I started cautiously with resistors of higher values, since this is the only module I have, and i've never worked with them before. The module did produce a really bright spot at 200r (and then turned off completely). Is it really safe to drive it with 80r series resistor from 5V ? – Yarek T May 13 '13 at 15:04
  • Having taken a break, and came back to this with a cool head, I did what you suggested above and of course the module started working reliably (I've used 50r resistor as 2 100r in parallel). I don't know why I didn't approach this pragmatically in the first place. Thanks – Yarek T Feb 23 '14 at 22:05
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The datasheet for your module is somewhat minimialist, but it does include these lines.

enter image description here

There are two ways to read this:

  1. The laser module contains no internal current regulator. You should use a controlled current source providing 25-35 mA. The regulator will need to provide about 3.0 V to produce this current. If you provide more current, the laser output will be brighter. I think this interpretation is less likely because in this case they should have given a "max" value for the operating voltage, but it's hard to trust such a minimal datasheet.

  2. As you say, your laser module could contain an internal current regulator. You should not be driving it with 5 V, you should be driving it with 3 V as specified in the datasheet. You do not need a current limiting resistor, the internal regulator will limit the current to about 25 and no more than 35 mA. You have no control over how bright the laser operates.

In this case, if you really want to use your 5 V supply to drive the laser, rather than use the 3.0 V supply specified, you need to reconsider the resistor values you are using.

You need the 5 V, minus the resistor drop, to be about 3 V. That means you need 2 V drop across your resistor at 25 mA. That means you need about 80 Ohms, not the 200-1000 Ohms you were working with.

Be aware that if you are wrong, and there is no internal current regulator, it will be pretty easy to damage the laser by biasing it incorrectly. Exceeding the 35 mA max current spec for even a microsecond could permanently damage the laser. A proper fixed-current source is strongly recommended in this case.

Edit In reply to your comment, if you use a fixed-current regulator, it won't increase its output voltage any more than it has to to produce the current you set it to produce. If you make a 25 mA fixed-current regulator, and the device only needs 3 V to draw 25 mA, the regulator won't produce more than 3 V.

A simple fixed-current regulator can be made from a '317 regulator:

enter image description here

You'd adjust R1 to reduce the output current to 25 mA. However this would actually require something like 7.5 V at VIN to ensure it can regulate properly. With a modern low-dropout linear regulator you should be able to make a similar circuit that can produce regulated 25 mA at about 3 V load voltage from 5 V input.

The Photon
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  • I've been thinking about having a fixed current regulator, but I would also need to power it at 3V. At the moment the circuit is built around an ATMega328, so i'm hoping to use 5V as a convenience – Yarek T May 13 '13 at 15:11
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My experience is that controlling a laser diode with a variable current is extremely finicky. Much easier to drive it at full voltage, and use a pulse width modulation technique to dim it.

Below is a circuit along the lines of what you want. Change R2 to get the brightness you'd like. I'd start with about 5K for both R1 and R2, and go smaller on R2 to make it brighter.

from www.jensign.com/opto/ledlaserdrivers

from http://www.jensign.com/opto/ledlaserdrivers/

Scott Seidman
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