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Author Topic: Advice about power supply  (Read 519 times)

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Offline CheetofingersTopic starter

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Advice about power supply
« on: May 28, 2014, 09:19:16 PM »
Me and my engineering group designed a maze navigating robot for engineering club last semester.  There were some problems with it and I want to get it running over the summer.

We are using an Arduino Uno, running 2 DC motors (they are Tamiya, 6V i think) with a L293D H-bridge chip, and 3 IR sensors with a 9V battery external power source. It runs fine while the battery is fresh, but the battery drains fast and when it drops below 7.5V, the motors stop spinning.  The motors work just fine when connected directly to the drained 7.5V battery so I believe this is a current problem?
We basically followed this tutorial http://itp.nyu.edu/physcomp/Labs/DCMotorControl.

To fix this problem I want to use a 6V external power source for each motor with transistors and resistors.  I am a little confused about how current is distributed to the arduino pins through the H-bridge however.  Should I be adding resistors to the wires going to motor logic pins?  I think this should help my motors receive more current?  I want to use a set-up similar to the one at the bottom of this tutorial page.
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

I also want to replace my wheels and motors with some better ones from pololu.  Not sure what gear ratio would work best for the gearbox.
http://www.pololu.com/product/1420
http://www.pololu.com/category/60/micro-metal-gearmotors

Any advice or recommendations would be appreciated. Thanks guys.
« Last Edit: May 28, 2014, 09:20:47 PM by Cheetofingers »

Online jkerns

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Re: Advice about power supply
« Reply #1 on: May 29, 2014, 07:04:23 AM »
What kind of 9V battery? The typical "transistor"  battery doesn't have a lot of capacity. LiPo or lead acid batteries last us quite a while in our lab. Look at the amp hour ratings...

" I am a little confused about how current is distributed to the arduino pins through the H-bridge however. "

The Arduino sends control signals (very low current) to the H bridge. The current for the motors goes only through the H bridge and does not go to the Arduino. Think of the H bridge chip as having 4 switches. All the Arduino does is flip the switches - just like you flip switches on the wall without getting mixed up in the current that goes to the lights. 

Resistors in the logic lines between the Arduino and the H bridge are there to limit the current flow on the logic circuit and to protect the Arduino when you blow the H bridge. It won't change how much current is flowing to the motor through the "switches" in the H bridge (unless they are too big and the H bridge doesn't switch completely).

I don't know what you mean by " I want to use a 6V external power source for each motor with transistors and resistors."  the L293 has two power inputs - VCC1 which can get the regulated logic power from the Arduino and VCC2 which gets the (typically) unregulated power for your motors. If you install a separate battery for the motors, this is where it is connected. You keep the grounds common.

Study the data sheet. http://www.ti.com/lit/ds/symlink/l293d.pdf And look at the "typical" application circuits.
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/bachelor-science-robotics-engineering.asp

Offline CheetofingersTopic starter

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Re: Advice about power supply
« Reply #2 on: May 29, 2014, 10:11:36 AM »
Oh okay, that makes a lot of sense. I need VCC1 for arduino power so I can't have external power sources for VCC1 and VCC2. 

How could I supply more current to the motors though? I think upping the voltage at VCC2 to higher than 9V would damage my motors?

Offline jwatte

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Re: Advice about power supply
« Reply #3 on: May 29, 2014, 10:55:22 AM »
What do you mean by "supply more current to motors"?
The motors will attempt to draw the current that they are designed for, for the voltage you supply. This is basic Ohm's law.
If the voltage source is not strong enough (high impedance) then the voltage will drop, and you won't get the current delivered that the motor wants to draw.
If the voltage source is strong (low impedance) then the voltage won't drop (much) and you'll get the current draw that the motors want.

So, what is the problem you want to solve? If you measure the voltage across the motors, is it lower than what the power source is rated for? If so, it's likely that your power source is too weak, and you need a lower impedance power source.
In general, LiPo batteries have very low impedance; NiHM batteries are OK; "regular" batteries are terrible. Plug-in power adapters are good up to their rated max output current, and then hit a brick wall with built-in protection.

Offline CheetofingersTopic starter

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Re: Advice about power supply
« Reply #4 on: May 29, 2014, 06:19:07 PM »
The problem I want to solve is I don't want to have to buy new 9V batteries every time they die out after 30mins-1hour.  So from what I understand I either need a battery with a lower impedence as you suggested, or a motor that draws less current?

Online jkerns

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Re: Advice about power supply
« Reply #5 on: May 30, 2014, 07:38:14 AM »
The solution to that problem is a re-chargeable battery with a larger amp-hour capacity.
I get paid to play with robots - can't beat that with a stick.

http://www.ltu.edu/engineering/mechanical/bachelor-science-robotics-engineering.asp

Offline jwatte

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Re: Advice about power supply
« Reply #6 on: May 30, 2014, 04:21:50 PM »
Yes. Get a LiPo or NiMH battery, and a suitable charger.
Also, you may be able to power the bot for development from a power supply rather than batteries.

 


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