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Which battery? 5v system

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woody_294:
Firstly, Hi all am new and making my first "robot" without sensors at first.

I'm going to be using a Raspberry Pi as the brain and an L298N as the driver, providing I'm not told this is a terrible idea!

The RPi uses 5v and my motors are rated at 3v (Mabuchi FA-130s) so I'm going to supply 5v VCC to the L298N circuit I'll be using, so I reckon a 5v regulated circuit (LM7805 + extras) will be good for everything.

I'd like to either use 5 A Cell NiMH (6v) cells or a 2S LiPo pack (7.4v) but I'm unsure of which way to go. I have chargers for both so not worried about that but I'm not sure if stepping down the 7.4v of the LiPo would be a good way to go, would I be wasting loads of power that way? The NiMH setup would be heavier and there's less choice of battery packs, charging them separately would be fiddly and I'd have to use battery holders.

I guess what I really want to know is if the 7.4v LiPo would be a bad choice for a 5v system.

Thanks in advance for any help

Woody

*Edit* Apologies, this is probably in the wrong section! Also thanks to the big man for making the tutorials!

jwatte:
I'd use the LiPo, straight into the Arduino input voltage, as the Arduino has an on-board regulator.
I'd use a 3.3V BEC (or perhaps a 5V BEC) for the motor power. Make sure it's rated for enough current for your motors. Something like this: http://www.integy.com/st_prod.html?p_prodid=7223&p_catid=150#.Ua4VaEAcdms
I wouldn't use a linear regulator to power the motors for efficiency reasons.

woody_294:
Thanks for the reply jwatte!

I'm having no luck with this thing at the moment! I agree, I'll probably end up getting 2 UBECs for the Pi (not Arduino) and the H-Bridge and hope it will be enough.

The more I read the less I know! It does seem that my 3v motors are a dumb choice (they came with the gearbox) and some 6v motors would be in order so the L298 doesn't explode and kill my tiny computer.

Nowhere have I found anyone willing to suggest that building my own H-Bridge is a good idea for a couple of 3v motors either.

jwatte:
Building your own H-bridge is a FANTASTIC idea ... if you want to learn how to build H-bridges, and have enough time and materials set aside for that purpose :-)
Pre-built DC motor carriers are so cheap and robust, that building your own for small motors just doesn't make sense for other reasons.

You can feed 6V to your 3V motors, and simply PWM it so you don't burn out the motors.

And, yes, sorry, as you say, the Pi does need its own BEC or similar, because it expects already-regulated USB power. Somehow, I read "arduino" there.


woody_294:
Oh cool, I guess I'll go ahead and learn about h-bridges then! I've done fairly complex electronics in the past, I used to work on vhf and hf radios down to component level back before everything was contract repair and board swapping but it's been a while! I'll look into pwming the 6v then I don't need to regulate the motor voltages, I have some 555s and other bits knocking about I could maybe use.

I'm umming and ahing about just using a 7805 for the pi as it draws less then half an amp, what do you reckon?

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