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Author Topic: Which battery? 5v system  (Read 1027 times)

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Offline woody_294Topic starter

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Which battery? 5v system
« on: June 04, 2013, 03:02:38 AM »
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!
« Last Edit: June 04, 2013, 03:04:51 AM by woody_294 »

Offline jwatte

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Re: Which battery? 5v system
« Reply #1 on: June 04, 2013, 10:27:36 AM »
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.

Offline woody_294Topic starter

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Re: Which battery? 5v system
« Reply #2 on: June 06, 2013, 02:44:42 PM »
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.

Offline jwatte

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Re: Which battery? 5v system
« Reply #3 on: June 06, 2013, 04:34:50 PM »
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.



Offline woody_294Topic starter

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Re: Which battery? 5v system
« Reply #4 on: June 10, 2013, 04:35:39 AM »
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?

Offline jwatte

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Re: Which battery? 5v system
« Reply #5 on: June 10, 2013, 10:28:05 AM »
Quote
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?


If you have it and don't want to spend the money, it will work, assuming your input voltage is guaranteed to be 7V or more.
If your voltage is going to go under 7V, the 7805 may start sagging on the output.
I'd recommend a cheap switching converter instead.
Pololu has a >1A step-up-down converter with 5V output for $5: http://www.pololu.com/catalog/product/2119
(There's also an adjustable version for $6: http://www.pololu.com/catalog/product/2118 )

Note that the RPi may need more than 500 mA if you're going to be doing heavy graphics and peripherals with it.

Offline woody_294Topic starter

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Re: Which battery? 5v system
« Reply #6 on: June 11, 2013, 03:00:10 AM »
Thanks again jwatte, good points, I'm not in the US unfortunately so can't get one from there but I've found a similar item here in Germany for a similar price so I think I'll take that suggestion and use it on the mk2 robot, with the H-Bridge I'll be building. Mk1 is going to use a 7805 for the Pi and an L298N circuit I bought with it to drive the motors from the PWM'd output.

Am I right in thinking that I could just 50% modulate it to get an equivalent 3v output from 6v or should I be using RMS values? I'm going to look into it but it'd be nice to get some direction. What kind of frequency am I looking at?

A little confused on some of the H-bridge examples I'm looking at too, some have a logic circuit on the front end before the H network, is this some kind of buffer or protection circuit? If it is I'm going to want to do one on my first foray to limit explosions

Also, where is the "Useful" button??
« Last Edit: June 11, 2013, 05:11:58 AM by woody_294 »

Offline newInRobotics

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Re: Which battery? 5v system
« Reply #7 on: June 11, 2013, 07:10:00 AM »
Also, where is the "Useful" button??
Left hand side of the post under username find Helpful?, just under that click [click for yes].
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music." - Kristian W

Offline woody_294Topic starter

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Re: Which battery? 5v system
« Reply #8 on: June 11, 2013, 08:30:43 AM »
Huh, odd. I don't have that button, is it post count limited? That would explain it.

Going to shove a 555 astable output into ENA and ENB on the L298N circuit I have, should give me around 3v output if I understand this, which is possible I suppose, haha.

Thanks anyway newInRobotics :)

Offline jwatte

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Re: Which battery? 5v system
« Reply #9 on: June 11, 2013, 10:20:05 AM »
Quote
Am I right in thinking that I could just 50% modulate it to get an equivalent 3v output from 6v or should I be using RMS values?

For square wave PWM power is square of voltage so to get the equivalent of 3V out of 6V, use a 25 percent duty cycle.
« Last Edit: June 11, 2013, 10:37:35 AM by jwatte »

Offline woody_294Topic starter

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Re: Which battery? 5v system
« Reply #10 on: June 11, 2013, 02:08:45 PM »
Awesome, thanks again. Got some circuits to make!

Offline Sergtov

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Re: Which battery? 5v system
« Reply #11 on: June 21, 2013, 05:18:09 AM »
Quote
I'd use the LiPo, straight into the Arduino input voltage, as the Arduino has an on-board regulator.

I use the LiPo too , and I recommend it you too .

 


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