Is this an acceptable way to drive a Laser Diode?

[wholder]

I bought a cheap laser "engraver" on eBay (the kind that uses a GRBL-based controller to drive two stepper mechanisms from salvaged DVD players to control the X and Y axes) and noticed that the beam power varies inconsistently even when drawing with the same PWM power level.  So, I decided to reverse engineer the circuit that's used to driver the laser diode (see attached schematic.)  I haven't worked with laser diodes before, but I do know that they require a constant current driver and the circuit I found seems to be nothing more than a 12 volt, high side switch.  The laser diode and lens is in a small cylindrical tube which I can't seem to open, so it's possible that there might be some type of current limiting circuit inside it.  Has anyone else seem something similar to this?  And, if, so, can you explain how it functions?  Or, is this just a shoddy design that will sort of work for awhile and then burn out?

Wayne

[NiHaoMike]

I think it was intended for a laser module with the driver integrated.

[Psi]

Keep in mind that they are super static sensitive.
And they will die instantly if you push too much current into them. (Unlike an LED which survives for a little while)

For example, you can connect an LED up to a lab PSU set to 12V with 20mA current limit. The PSU limits the current to 20mA and that limits the voltage to ~3V for the LED.
 
But if you connect a laser diode to a lab PSU set to 12V / 20mA the laser diode will die instantly.  The psu is too slow to bring the voltage down from 12V, that small burst of current at 12V before the limit kicks in, or from the PSU output caps, is too much for the laser diode to handle.

[ejeffrey]

I strongly suspect that the module has a constant current regulator in it, and the 12V is just the input power.  Because of the ESD, overcurrent, and overvoltage sensitivity of laser diodes it is really preferable to put the driver as close to them as possible,  Long leads from the diode are just more places to pick up ESD, extra inductance the driver needs to deal with, and potentially a way to pick up damaging EMI transients.

[magic]

Are there 2 or 3 pins on the diode?

[pwlps]

The laser diode and lens is in a small cylindrical tube which I can't seem to open, so it's possible that there might be some type of current limiting circuit inside it.

Maybe just a resistor in series? I don't know for DVD engravers but the laser pointers I disasembled had a series resistor.  You can test for this measuring the I-V curve.

[Psi]

Are there 2 or 3 pins on the diode?

That doesn't always tell you much, often you find the 3 pin diodes packages that don't have the photocell on the extra pin.

[wholder]

I finally managed to open the cylindrical case and, as some of you suggested, I found a small PCB inside that was attached to all three pins of the laser diode, but two are soldered to the same point on one side of the PCB and the remaining pin is solder to a pad on the other side.  I've attached photos if anyone is curious.  I guess my next step is to reverse the schematic for this PCB.  Or, perhaps someone has seen this kind of design before and can point me to a schematic?  It looks fairly involved, from what I can see.  Unfortunately, the part number on the 8 pin IC has been sanded off.  Any guesses as to what it is?  I suspect it is some type of LDO regulator, as that seems to be common in some of the driver circuits I've looked up.  Also, is this kind of circuit suitable for use with PWM control?

Wayne

[ejeffrey]

The laser diode and lens is in a small cylindrical tube which I can't seem to open, so it's possible that there might be some type of current limiting circuit inside it.

Maybe just a resistor in series? I don't know for DVD engravers but the laser pointers I disasembled had a series resistor.  You can test for this measuring the I-V curve.

That is common for battery powered devices because the battery voltage is very clean, voltage limited, and chosen appropriately for the diode Vf.  Its not so good/common with a regulated 12V supply.

[StillTrying]

The inside looks like a switching regulator to me.

"the beam power varies inconsistently even when drawing with the same PWM power level."

Could the PWM frequency be aliasing with the regs switching frequency.

[Psi]

Could the PWM frequency be aliasing with the regs switching frequency.

yeah, that would definitely cause power fluctuations like he is seeing.

[wholder]

That makes sense.  I hooked the module up to 12V DC using my bench power supply and the total draw is about 130-150 mA, which a about 2 Watts.  I'm guessing that a lot of the power is being consumed by the circuit and I have no idea what the typical conversion efficiency is for a blue laser diode.  It was sold as a 300 mW laser, but I think that's suspect, as I have another laser engraver with a 500 mW laser (at least that's what the label on the module says) that puts out significantly more power.

So, I'm thinking I should just replace the laser with something designed for TTL control and that can run at 12 volts.  Anyone have any suggestions for a good replacement module in a cylindrical case that will fit (diameter is 12 mm) in the existing holder?

Wayne

[ptricks]

I used a 5 watt laser on an engraver and use an sxd driver.
You can put the driver in the case but I don't recommend it because the driver generates heat and the diode does as well so you would be using a heat sink for the driver that gets hot.
The go to for most people is the SXD drivers , search on ebay for  SXD Laser Driver

[VE2UM]

...Also, have the switching transistor / MOSFET connected in PARALLEL with the Laser diode.

Energetically, this is less efficient, since current is always on, but, the MOSFET will short-out the Laser diode to 0v when switched on. and let the resistor reapply bias to the laser when turned-off, reducing Laser overvoltage to nearly zero.

This method also works for constant-current supplies. The only difference being that the MOSFET must be ON before turning-on the CC supply to protect the laser diode.