Building a laser driver circuit?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

The circuit is correct, I have made it with 6ohm resistor to Adj pin of LM317 and it's working. Can burn some match stick heads.

Answer 4

The third pin is for feedback. It is not necessary in this application. If you are curious, here is a video by Applied Science that is very interesting. https://www.youtube.com/watch?v=MUdro-6u2Zg&ab_channel=AppliedScience

The best way to find out which pins are which, (without the manual) is to look at the back of the diode. The case is usually the ground, so the pin that does NOT have epoxy around it is usually the ground pin. (The case will always be a common anode or cathode/ground. It can't be any other way because there's only 3 pins)

A way to check is by looking in through the glass lens (WITH THE DIODE COMPLETELY UNWIRED, INFRARED DIODES WILL BLIND YOU AND THEY CAN BE ON AND YOU WON'T KNOW IT!!!).

You can sometimes see the wires inside and what they're hooked up to. If not, just use the case as ground and go for the process of elimination on the other two. If you hook your circuit up to the photodiode by mistake, even if you damage it, it won't matter because you don't need it. To be on the safe side, 1.5-3.0V with a 1k resistor should work. Just keep trying until you see a faint red glow. It is a diode after all so reverse polarity with a low voltage shouldn't damage it.

About the current regulation, you are correct. You cannot "regulate" both current and voltage.

However:

Voltage regulation is done by controlling current. Current regulation is done by controlling voltage.

If I want 1mA through a 1k resistor, I just supply 1V. If the resistance changes, you have to adjust the voltage accordingly. This is what the LM317 is doing when you wire it in current-source mode.

That circuit should be sufficient for powering your DVD burner laser. If you want to be on the safe side, just put one 10 Ohm resistor instead of the 2 in parallel. That will limit the max current of the circuit.