This is absolutely correct. If you want to get into the gritty details, you may want to read through the ILDA standard for projectors, which talks about all the signals sent to a projector, not just the color modulation signals.
Yup. You've got this down pat.This isn't useful for driving lasers directly, because it has weak drive strength, and because diodes are current-driven. It's just a signal.
Correct again. Laser diode drivers are constant-current drivers. However, in the end the only way to change the amount of current flowing through the diode is to change the voltage across the diode. (Ohms law, basically.) The difference is that a tiny change in voltage can have a dramatic effect on the current. So the driver is designed around maintaining a given current output that is proportional to the input signal.Diode lasers, like other diodes, have a "forward voltage drop" when they're operating. Varying the voltage across them is nonsensical, the voltage is basically flat even across wide variations in current. (This is the flat region after the "knee" in the I-V curve.) So the function of a laser diode driver is to take that voltage signal from the controller, and use it to control a current with some chunky drive strength behind it.
Pretty much. Often mosfets are used, but I've seen darlington transistors used in higher-current drivers. The current sensing resistor is often a .1 or .2 ohm precision resistor, and it needs to be purely resistive; a wire-wound resistor will wreak havoc with the driver due to inductive effects. Beyond that there are tricks to tweak the feedback circuit to eliminate ringing, and some drivers also include safety circuits to protect against excessive input modulation, reverse polarity, and so on.At its heart, a laser diode driver is probably a big fat transistor and sense resistor for feedback, with an op-amp varying the drive current according to the signal voltage.
Surprisingly the color signals from the controller are pretty close to ideal. You can run into issues where you can't get the full 5 volts to the driver due to low input impedance on a poorly designed driver (this makes the losses in the long signal cable more significant), but it's very uncommon to not see zero volts when the laser should be off.neither the analog color signals nor the diodes themselves are ideal. The color signal may not go fully to zero during times when the laser should be off.
A bigger problem is the linearity of the diode. This can be compensated for in software (assuming it's a supported feature), but it is tricky.
Another issue is what to do about the lasing threshold. Most diodes will begin to emit light at very low current levels, but you won't get a coherent beam until you get above the threshold current. Some drivers try to improve the modulation response time by "idling" the diode at just below the threshold current. This results in a diffuse output all the time, which is visible if your projector doesn't have a shutter. The alternative is to completely kill current to the diode below a given input signal level, but this introduces a delay when the diode turns on. Different drivers follow different schools of thought when deciding how to handle these two problems. (Some allow you the flexibility to set it up however you want, idle or no idle.)
You *need* the high-end adjustment for any driver, or else you run the risk of blowing the diode from over-current. Having the low end adjustment is a nice bonus that many drivers include, and being able to set the cut-off voltage on the modulation signal (anything below this level = no output) is another plus, although it's less commonly found.So a good driver probably has adjustments (trimpots?) to scale both the top and bottom of the input signal to match what it gets back from the sense resistor.
This is a high-end feature that is not normally found on hobbyist-level drivers. Some commercial drivers include a thermal feedback input from the diode can, however.It hopefully has some sort of temperature feedback, which down-regulates or outright inhibits the drive current with increasing temperature.
Some of the nicer drivers have an internal thermal shutdown, but most of them just warn you to do the math ahead of time to ensure you heat sink the output transistor appropriately.It might have thermal protection for itself, too, since that big pass transistor will be operating in its linear region and dissipating a bunch of power when the laser is on.
Yes, assuming that you know which diodes are used (so you can set the current limits on your analog drivers properly). Barring that, you'd need to measure the current first, before you replaced the drivers. But yes, several people have upgraded their TTL-only projectors with better drivers to get analog performance. Note that you need to be extremely careful when connecting and disconnecting the leads to the laser diode, as laser diodes can be killed instantly by static electricity. In particular, low power single-mode diodes (*especially* low power red diodes) are super vulnerable to this failure mode. (If you don't mind going down the rabbit hole, look into the Pangolin "Lasorb" device.)When we buy the cheap RGB TTL laser modules, there's a tendency to think of the whole package as a monolithic unit. Four mounting screws, a power connector, 3 color connectors, done. But really, there's nothing special about this cheap 3-in-1 TTL driver board, it can be removed and replaced with better drivers. The lasers themselves are just 2-wire devices, and can be driven as we please.
As for knowing where to set the current, that information is readily available for nearly every modern diode on the market. A conservative approach would be to use the data sheet limit, but most people simply go to DTR's Laser Shop and look at the test results he's posted for each diode. (He posts current draw vs power output in fine steps from threshold all the way *past* the maximum.) And of course, you could always measure it yourself by applying a steady 5 volt signal to the TTL driver and measuring the actual current to the diode...
Also, once you know where you need to set the current you can use a dummy load to set it ahead of time (before you connect the laser to the driver), or you can dial the driver down low, connect the diode, and then ramp it up while you monitor current in real time. Either way works. Just don't get carried away or you'll blow your diode.
Bottom line: you have a very good understanding of how this all works.
Adam