LM317 - to regulate voltage, sort of, in a pinch?

[rhd]

Quick question -

I'm trying to run a blue and green off the same 2x li-ion power source.
- The green is a module with a stock driver that I'd like to be able to stick with.
- The blue is going to run from a DDL circuit

Since the green driver expects an input of ~3V (basically one cell), and the blue DDL circuit off ~7V (two cells), I'm sort of left with three options:

1) Figure out what current the green module is providing the IR diode with, and just replicate that as a second DDL circuit, ditch the stock driver.
2) Run the green module from a feed coming off just one of the two cells that are running the blue's DDL. Problem here is that this cell will deplete faster than the other.
3) Use an LM317 as a voltage regulator to regulate the 2-cell voltage down to 3V for input into the green's driver. Use another LM317 as current source for the blue.

The first option sucks because I don't want to mess with a module that works. The second option seems lazy and produces a non-optimal result. The third option seems inherently "wrong", sort of like driving a driver.

Am I missing something obvious? Any recommendations one way or the other?

 

[anselm]

Hmmm, OK.

When a green module is made for 3V, the driver will run very hot on 4V from one LiIon.
A side effect is that it will also likely be brighter as most green modules I've seen have
a crappy Vin dependant driver.
Of course, some green modules are made for 3.7~4.2V.
(The ones that come in a 18650 host, duh)

Next, consider that green modules are case positive and the driver regulates the negative side.
I don't know if that will be problem in your particular host, just thought I'd remind you.

To get the voltage down for the green modules level, I suppose you could just put a couple
of "test-load diodes" in series to drop the excess voltage.
but I'm not totally sure on this one, the rectifier diodes being between the battery -
and the green driver in.:thinking:

A lot really depends on if you are going to use the host case as a current path, in which
case you have to figure out everything more carefully, or if you isolate the green module
from case (the blue diode already is, by default), but then you have to run extra wires.

Isolating it from case would also be the way to go if you wanted to step down the voltage
for the green using a LM317, since IIRC it doesn't work on the negative side.

I'm curious, are you trying to build a cyan handheld laser?

 

[Cyparagon]

I would probably run a separate lead to the green, if I absolutely had to do this.

Since the voltage drop you need is roughly constant and the driver can take care of the rest, you can use a series of diodes like mister anselm suggested or a 4-5V zener diode. You'll be dumping a lot of power over it, but the batteries will be at a more similar charge level and it is less complex than a lm317 circuit.

Or the best solution: don't run two components that have different voltage requirements from the same voltage source

 

[rhd]

I ended up de-soldering the stock driver, and wiring up a new one from a 317. Then I was able to run them both from the same battery setup.

And YES! I was trying to build a cyan. And to be honest, it wasn't as awesome as I had hoped for. I think I'll keep it on the shelf for now, and when my LOCs arrive, re purpose this into a poor man's yellow

 

[anselm]

Hey wait a minute you can't just hand-wave this away like this!
I demand moar beamshots.

How did you combine the beams? Splitter cube, dichro, knife-edge?

Any two color handheld is worthy of recognition.
I'm planning on making a pink handheld for my better half.

 

[rhd]

Lol, ok. So -

It was a DX green module and a 445nm diode.

The DX green
- Was running the IR diode at 320mA (~50mW green in the end)
- So I built an LM317 to duplicate that and tossed the stock driver
- It's on the left, bounces into a front surface mirror angling it to the right 90 degrees

The 445nm diode
- Is running on a 1085 DDL circuit, at 320mA also
- It goes into a dichro, where it passes through and mixes with the reflected green

It's cool, and the alignment is pretty good. I used the tiniest bit of tack to keep the dichro upright, while I aligned it all, and then used epoxy to secure it in the chosen position.

Why I'm not super excited about it:
- The aluminum case "flexes" ever so slightly when you close it. It's enough to push the beam out of alignment by 1cm or so at a distance of 20 feet.
- Not terribly bright. I mean, it's a visible beam, and it DOES LOOK like a cyan beam, but the grean and blue beam profiles are so different that it doesn't really fool your eye, even though the ultimate dot is 100% convincing.