# Building a laser driver circuit?

I have a small laser diode I ripped out of a CD/DVD read write drive. It has 3 pins on it and my first question is what function does the third pin have? Is it a second ground? How do I go about determining the function of each pin safely without damaging it?

I'm attaching a photo of a driver circuit I found online. Would this be a suitable driver circuit for this laser diode? I'm not for sure on the specs of the diode but I'm guessing it's more than 100mW.

Furthermore, in a driver circuit for a laser I need to regulate not only voltage but current as well. Therefore, I'm confused on how I'm supposed to design a circuit that satisfies both of these conditions. For some reason as beginner electronic engineering student I can only make sense of having one of these regulators in a circuit controlling voltage OR current. To me, changing one would effect the other. So how can I control both since controlling one will control the other.

-
Some of your questions were previously answered here: electronics.stackexchange.com/questions/84764/…? – The Photon May 7 '14 at 18:05

It has 3 pins on it and my first question is what function does the third pin have?

Many laser diodes are packaged with a photodiode that receives the light from the laser's back facet. This allows setting up a control loop to drive the laser in a constant output power mode rather than just setting a constant current.

Usually the laser and photodiode are connected in either "common cathode" or "common anode" configuration, so that only 3 pins are needed for the two devices.

How do I go about determining the function of each pin safely without damaging it?

The best way is to read the datasheet for the part. Obviously when you're salvaging parts you might not be able to do that. In that case, you have to basically diode-check each combination of pins to find the laser anode and cathod and the photodiode anode and cathode. You can probably tell one from the other because the laser ought to emit at least a small amount of light when you find its pins. Preferably use a diode tester that is voltage limited to maybe 5 V and current limitted to a couple of mA.

in a driver circuit for a laser I need to regulate not only voltage but current as well.

For a laser diode, you generally want to drive it with a constant current source. However you should design the source to have a maximum output voltage consistent with the laser's maximum ratings to avoid damaging the laser during power up, power down, or in case of a current-control failure.

Even better, if your laser does have a monitor photodiode, is to control the supply current to achieve the desired output power level. Again this circuit should have appropriate current and voltage limits to avoid damage.

You might also want to design your supply circuit to have a "soft start" feature to eliminate high inrush currents when turning the laser on and off. (This is probably the reason for the 10 mF capacitor in the schematic you posted)

Final note: 100 mW is more than enough to cause permanent eye damage if you mishandle the laser. Be sure you understand the risks and take appropriate safety precautions before powering up this device.

-

Your intuition is correct. You can regulate voltage or current, but not both.

But you can have a current regulator with a voltage limit, or conversely, a voltage regulator with a current limit. Many benchtop power supplies can operate in either mode, and switch smoothly between them.

-