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I was asking myself the same question that Member#606804 had and went on to the Femtobuck’s page to see if I could find some answers but no luck there. I’m looking to create a budget LED grow lamp and I want to use the Picobuck to drive some Cree XP-E LEDs. They are ~3v forward voltage and they consume 330mA - 1000mA. I have a 12v 750mA power supply hanging around and with the Picobuck set at 660mA I’d be fine to drive 3 LEDs per channel in series. What I’d like to do though is to try setting the channels up for 1A operation and drive 6 at 500mA each. A pair of 3 LEDs in series, keeping the voltage demand at 12v and splitting the current of 1A down each series of 500mA each.
The AL8805 Datasheet mentions the obvious advantages of using LEDs in series and plots data using LED series examples for simplicity. It does not warn against using LEDs in parallel nor does any of the Sparkfun pages about the product. Use of common Cathode/Anode LEDs would not work because this product doesn’t support bridging channels.
I think if you know how they are wired and do the calculations correctly you should be fine. You just need to keep in mind that when you choose to wire something in parallel you’re not going to get the full current capability of the channel how you set it up. Thankfully the Picobuck is pretty flexible letting you choose a target current with the sense resistor. One other thing I’d like to mention is that if you do short those pads to enable 660mA operation and later change out the sense resistor keep in mind that there are now two resistors in parallel and you’ll need to either un-short the pads, remove the second resistor or swap them both to the appropriate values for your corresponding target current.
I’d like to verify that the PicoBuck can drive three individual Samsung E072A LED engines. My concern is that the configuration of the engine consists of 4 LED’s, wired in an “H” configuration, with two in series, those series parallel, with a common cross-connector. I believe the comment concerning not being able to drive common cathode LED’s is to avoid connecting the outputs of the PicoBuck to one another, which is not what I would be doing. It looks like this could do the job, just wanted to make sure.
Oooooh, I don’t know for sure about that. I’m trying to imagine how that would be hooked up normally.
I don’t like the fact that you’ve got LEDs in parallel with each other; what happens if one of them has a slightly higher bias voltage than the other? Maybe the H wiring offsets that; I don’t know.
How is it normally driven? By a constant current driver?
I’m looking to build a battery pack that can handle up to 12V, does Sparkfun provide batteries I can put together in a series to do this?
Yes, we do, but charging them is another matter.
We sell quite a few different flavors of Lithium Ion Polymer batteries, and those have a nominal voltage of 3.7V per cell. You can stack 3 of them to get close to your 12V output, but we don’t currently sell an easy solution for charging them. We do sell a multi-cell, multi-chemistry charger but you’ll have to provide a sufficiently high voltage power supply (>12V, for sure) to run it, and you’ll need to look into how the cells should be wired for it (it wants to sample the voltage of each cell individually to make sure they’re properly balanced).
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