Help to Run Laser Pointer on 12V

I have a green laser pointer that runs on 3V (2 AAA batteries). It's rated at 5 mW, but I have no idea if that's exxagerated or not, or if there are any start-up transients when the laser comes on. I would like to power the laser pointer from a 12V source. I assume I just need a resistor in series with the laser pointer to step down the voltage, but I'm not sure. I don't want to guess and use one with too low a resistance, allowing too much current to go to the laser diode and blow it out. How could I determine the correct resistor to use? Or is there another method that's quick and simple?

I have available to me the laser pointer, some wire, a soldering iron, $3.00, a full tank of gas, and a Radio Shack around the corner. Go, McGuyver!

To be safe, how about just make a 3V regulator? You can do it with a LM317 (probably available from RS) and two resistors. Consult the LM317 datasheet for details.

Another approach is to model the laser diode as a regular diode such as an LED. If you can, measure the amount of current the laser diode draws at 3V. Then use Ohm's law to determine the resistor necessary to drop 12V to 3V at that current.

My guess is that a 5K resistor would probably work.

Thanks. I will go get the LM317 data sheet you mentioned.

I don't think it would be good to just model it as a simple diode. Green lasers actually include an IR laser diode that pumps a frequency doubler. It's a two stage device with some other electronics to handle startup transients. Since LEDs have very steep current/voltage curves, I would want a pretty exact solution. Keeping the input at 3V will be good.

Hi spetch,

the laser should not draw high current. Best method is measuring the current flow at 3V and use an LM317L as a constant current source, which is pretty much independent of input voltage and temperature changes.

Used as constant current source (100mA max) the required forward voltage will automatically be achieved.

Boncuk

It is difficult to measure current accurately at a supply voltage as low as 3V. The current meter's resistance in series with the battery and the laser might drop the voltage and therefore also drop the current too much.