Laser diodes in series or parallel?

[RA_pierce]

My reading suggests that laser diodes need to be run in series in order to achieve a balanced load across both LDs, so as not to overdrive one and underdrive the other - with catastrophic results. This may not be so much of an issue if the diodes are similar (same substrate and electrical characteristics).
However, the blue laser arrays seem to be operating with several strings in parallel, so I'm confused about what is going on here...

I am working on a project where I want to run 2 laser diodes (case isolated) simultaneously. If I can power them both with a single driver, that would be great. So, in order to do that, I need to know if I should be driving the diodes in series or parallel.

Additional context: I want to implement a high/low power setting.
The drivers I have on hand (Flex V5 and Groove V2) are set with tiny potentiometers. I don't want to adjust power settings externally using the pot (bad idea) so my alternative was to use 2 drivers - one set high and one set low - and wire these to a 3-position switch to select between high and low mode.
This is a clunky design, but easy and effective. So if I can power both diodes from a single driver, I only need 2 drivers (one for high power and one for low power). If both diodes need their own driver, I'd need to have 4 drivers which is far from ideal.

The Flex V5 is a buck/boost but only capable of boosting reliably to 5.5V. So, I would prefer parallel operation to reduce the forward voltage and still have respectable drive current (1.5 A max from the Flex, divided among 2 diodes that's a max of 750mA each).
If you have any better suggestions, I'd love to hear them.

What I'm trying to accomplish:

• Run 2 diodes simultaneously (series or parallel?)
• High and low power modes (2 separate drivers?)

 

[RedCowboy]

I wouldn't back feed the switching LD drivers or wire them in parallel.

I always string mine in series, usually with a LM338, you can get high/low by switching in/out additional resistors in parallel to set your current limit of your lm338 between the adj. and output pins.

If your driving two laser diodes needing 5v each plus the lm338 dropout I use 4 x 18650 cells in series for something like m-140's or you can use a dc to dc to power your lm338, take a look at the spec sheet for your regulator.


Also you could use a couple of 2 or 3 mode flashlight drivers, one for each laser diode and you can power both drivers from one switch/battery pack.

Circuit Boards or Driver Boards

Circuit Boards or Driver Boards

kaidomain.com

 

[RA_pierce]

I wouldn't back feed the switching LD drivers or wire them in parallel.

I always string mine in series, usually with a LM338, you can get high/low by switching in/out additional resistors in parallel to set your current limit of your lm338 between the adj. and output pins.

If your driving two laser diodes needing 5v each plus the lm338 dropout I use 4 x 18650 cells in series for something like m-140's or you can use a dc to dc to power your lm338, take a look at the spec sheet for your regulator.


Also you could use a couple of 2 or 3 mode flashlight drivers, one for each laser diode and you can power both drivers from one switch/battery pack.

Circuit Boards or Driver Boards

Circuit Boards or Driver Boards

kaidomain.com

I'm doing this with red diodes, so the voltage and amperage requirements are not so high.
LM338 is a good idea. Maybe AMC7135 would do the trick as well. The dropout voltage of these regulators can be quite high, though...

 

[Mattronium]

From memory, the AMC7135 have fairly low voltage dropout.
If you could get a single driver to give enough voltage to run them in series, would it be possible to simply use a mosfet to switch on another set resistor to get two power levels?
What is the max current you are planning on running them at?

 

[FAP]

The problem with running diodes in parallel using a single driver is, even if the same type of diode, one can hog more current than the other. If in series, they can each only draw what the driver allows, in total. Reason one diode in parallel can draw more and the other far less is because even if the same part number, the voltage drop can vary from one diode to the other. In that case the driver only limits total current, not individual current to each.

 

[FireMyLaser]

I also agree that series is best when running multiple laser diodes for the reason already told. Same applies to LEDs. I don't think the arrays are supposed to be running the strings in parallel. Either all stings are wired in series, or each string gets its own driver. It is still possible to run (very equal) diodes in parallel, but you should then use a a series resistor to help absorb any voltage difference between the diodes. But if one diode fails and goes open circuit (unlikely), then the other diode will get all the current and also fail. Connecting in series removes all of these issues and you can even use mismatched diodes.

 

[RA_pierce]

From memory, the AMC7135 have fairly low voltage dropout.
If you could get a single driver to give enough voltage to run them in series, would it be possible to simply use a mosfet to switch on another set resistor to get two power levels?
What is the max current you are planning on running them at?

Mosfet is a good idea as well. However, it increases the drain on batteries and produces more heat, which I'm trying to minimize. A regular ol' switch seems to be the most efficient for changing resistor values.
Spec sheet says AMC7135 has a dropout of 120 mV. That's quite good.
With AMC7135s in parallel, could I just have a switch that disconnects a couple of the chips to get a low setting? Can't really envision that.
I'm thinking max ~700 mA for each diode.
For 2 diodes, 2 AMCs will get me 350 mA each, which is above threshold of the Sharp 700 mW 638 diode, and 4 AMCs will get me 700 mA to each diode, which is perfect - just below operating spec of 810 mA.

... even if the same part number, the voltage drop can vary from one diode to the other...

Connecting in series removes all of these issues and you can even use mismatched diodes.

Thanks for the comments. Series it is.