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Old 12-18-2013, 06:40 PM #1
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Default Low-dropout driver for reds

Greetings,

I want to build a red laser using a single Li-Ion (4.2V) battery. A LM317-style driver would work great if the dropout voltage wasn't so high. I'd like for the current to be adjustable from 0-150mA. I'm using an Opnext 638nm diode which has a Vf of 2.6V at 150ma. So the dropout voltage needs to be around 1V.

So basically it has to fit the following criteria:
  • Low-droput driver (Linear or buck)
  • 0-150mA
  • Drops around 1V at 150mA
  • Current adjustable with external pot

Just wondering what my options are. (And yes, I know the Flexdrive will work, but it's expensive and I don't feel like reverse engineering it!)

Peace,
Grant

Edit:

Well, after not building lasers for so long, I totally forgot that I have a whole bunch of Groove drivers in my workshop. Implementing the groove driver might be difficult for my application, so if you guys have any other ideas please let me know!


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Last edited by Jimmymcjimthejim; 12-18-2013 at 07:36 PM.
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Old 12-18-2013, 10:54 PM #2
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Default Re: Low-dropout driver for reds

Quote:
Originally Posted by Jimmymcjimthejim View Post
Greetings,

I want to build a red laser using a single Li-Ion (4.2V) battery. A LM317-style driver would work great if the dropout voltage wasn't so high. I'd like for the current to be adjustable from 0-150mA. I'm using an Opnext 638nm diode which has a Vf of 2.6V at 150ma. So the dropout voltage needs to be around 1V.

So basically it has to fit the following criteria:
  • Low-droput driver (Linear or buck)
  • 0-150mA
  • Drops around 1V at 150mA
  • Current adjustable with external pot

Just wondering what my options are. (And yes, I know the Flexdrive will work, but it's expensive and I don't feel like reverse engineering it!)

Peace,
Grant
Grant;

I have used the the LM1117 regulator for Reds with no problem (Dropout ~1.1 VDC).

Just use a quality name-brand 18650 cell & you will be fine.

LarryDFW
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Last edited by LarryDFW; 01-19-2014 at 06:25 PM.
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Old 12-20-2013, 09:53 PM #3
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Default Re: Low-dropout driver for reds

To elaborate on what LarryDFW said, it has to do with how Li Ion cells act throughout the discharge cycle.

When fully charged, the Li-Ion is at 4.2V potential. If you have a low power load, it will draw a low current, which most cells can easily handle. If you have a high power load, it will draw a higher current. Depending on the quality of the cell, the cell's voltage will almost instantaneously drop in response to the amount of current you are pulling, as the ions have to be able to move fast enough to keep up with the current, and as they fall behind, the voltage sags.

How does this relate to what LarryDFW said? Well, a crappy cell may be at 4.2V full charge, and you could draw 5mA from it without it sagging, but if your load is drawing an amp of current, your cell voltage will sag. How much depends on how bad the cell is and how much current you are pulling from it.

I would probably have to recommend Sanyo UR18650FM, aka the classic sanyo 2600mAh cell, if you want to stick with a linear driver with ~1v dropout. Larry used to carry these, I don't know if he still does or not, since they are so ubiquitous. Larry's got the high capacity cells that you should go with if, instead of a linear driver, you go with a switchmode bucking driver. The UR18650FM will hold their voltage higher at 0.2-0.5A discharges, but a switchmode driver won't care, it'll go until the battery's empty, so you'd be better suited to use a high capacity cell in that scenario.
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