Hi,
I need to power a Panda from 2 AA Nimh cells - 2.4V. What is the best way? Boost to 3.3V and connect behind regulator or maybe 6V in front of first regulator?
Rudie
can you take the loss you’ll incur by going to 6v then back down? That will most likely cut your runtime. That’s the most effective way if you need 5v or if you can’t be sure your boost will put out a clean 3v3, but it also will waste some juice.
The batteries will drop their voltage as they are depleted. So, if you want to get to 3.3V you will have to go to a higher voltage than 3.3V and regulate down 3.3V.
Check the Panda II schematic. If you put 4V into the 5V input, which is behind the 5V regulator, it will be input to the 3.3V regulator on the board.
You can use a boost type DC-DC converter to go from 2.4V to 3.3V.
Sample ready made dc-dc converters:
http://www.robotshop.com/sfe-ncp1400-5v-100ma-step-up-breakout.html
http://www.robotshop.com/dimension-engineering-de-swadj.html
If you need 5V, I recommend Adafruit’s MintyBoost kit. Circuitry and batteries all fit into an altoids tin. It is USB out, and is too big for an onboard solution, but nice to have for emergency charge of a cell phone or other gadget. [url]http://www.adafruit.com/products/14[/url]
I’ve got my Minty hooked to their solar LiPo charger and a 6V 3.7W panel. The panel puts out a little over 500mA in bright sun, which is enough to run my Panda and charge a LiPo. Fun stuff!
Excellent suggestions - thanks!
I think I’m going to design an all-in one supply/booster/charger aimed at the Panda with LiPo. Like the minty box booster + lipo charger in one.
@ realiser - By design do you mean a PCB? Over at SparkFun a bunch of forum folks asked why they didn’t have such a thing, so there may be (a little?) commercial interest. I’ve seen a ‘lipo backpack’ shield for the arduino out there somewhere. I had the same idea and was thinking a through hole kit, or small SMD board would really be sweet. Would be even sweeter with a little LED battery charge meter.
I’m a software guy, so am still on a learning curve with designing (simple!) boards in Eagle. If you are serious about designing such a thing, I’m down for one, and would be glad to help test a prototype. I think (forum member) Skewworks may also be doing something similar for Gadgeteer.
We have been using these Li-Ion cells to power our USBizi based boards (and other projects): [url]http://www.batteryjunction.com/tenergy-37-2200-pcb.html[/url] . They work great, and all you would need to do is build a charging circuit. Unfortunately Battery Junction has them on backorder (though they don’t tell you that when you order) and we are still waiting for about 40 cells.
Hmm, I have been working on the charging/boosting/fuel gauge for my board. Maybe I should first build a shield to test it before doing to whole USBizi board. Nice idea… 
@ Errol - The combination doesn’t seem to exist in shield form or otherwise anywhere commercially that I can find (easily). Give it a catchy name and with a little marketing the uC crowd will snap them up 8) Keep us posted.
Sparkfun and Pololu 5V boost circuits:
http://dangerousprototypes.com/2011/08/23/sparkfun-and-pololu-5v-boost-circuits/
I am busy putting a spec together for a shield that does the whole lot.
Errol: Please contact me at fez(at)globalpro.Co.za if you want to collaborate or just connect with a fellow South African