Panrock wrote:...and "PD" cathode (what's that?).
PD=Photodiode. It's used as a feedback element into the driver circuit where absolute linearity, or optical limiting or stabilisation is required. Here they're linear enough without it, just leave it disconnected. The other two don't seem to have that 'feature' so it's not worth thinking about it here.
Panrock wrote:Best isolate all the cases!
Agreed, but without introducing too much thermal resistance between the case and heatsink. I suggest using three smaller heatinks rather than one larger one - just in case the worst happens. If you connect the heatsinks to +12VB, then if a short does happen the laser diode will simply stop working and the driver circuit won't care. But that means insulating the heatsink(s) from the chassis and ensuring shorts to other stuff can't happen.
Steve A.
Hmm, in the case of the red laser, the word 'could' worries me a bit...IF the cases are connected internally it would be better the anode rather than the cathode. The green and blue seem OK from what you've said. I'll have a review of the datasheets..
I think it's a case (pun again) of wait and see, build up the lasers, add the 1N914, and the screened cable then carefully try and determine which, if any are connected to the case. I wouldn't rely on anodising, if any, on the heatsink to provide insulation. It may be that one is totally isolated, the next connected to the anode and the last connected to the cathode - sod's law.
The simplest approach maybe using three smaller heatsinks insulated from each other and anything else. If the case (therefore the heatsink) is connected to anything or nothing - it doesn't matter. The 'slope' of the I/V curve is quite flat in the operating region so any additional stray capacitance this my introduce is unlikely to have any real effect.
I must get back to the driver circuit i.e. the 'threshold current'/black control...