Electronic – Driving a laser module from 5V supply


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.

Best Answer

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.