Author Topic: motor controller question  (Read 3618 times)

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Offline SomeSabaTopic starter

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motor controller question
« on: October 20, 2007, 12:49:53 PM »
Hello!

I have some questions:

First of all i have 4 GM2 gearmotors from solarbotics 2 for each side of the robot (left/right) each motor consumes 52 mA free running at 5V. That means all together its 208 mA. But im running them at around 10v, so they consume 416 mA now. Also, im replacing the DC motors in the gearbox with better motors provided by solar botics, these new motors apparently consume 4x the power of the previous ones

So if i assume P = I*V     and the voltage is constant, then 4P = 4I*V? which means free running the motors will consume 416*4 = 1.6 Amps?
 If the motors stall, they consume 600mA each (at 5v with the old motors) which translates to 19.2 Amps at stall total. Now i dont plan to run into anywalls or anything with my robot, its for a comepetition where i have to put stuff up in my box

Questions:

1. did i calculate my current draw correctly? lol

i dont wanna have to pay $50 for a motor controller that can support 20Amps for a few seconds.

I want a dual motor controller that can continuously support 2Amps (or 5 considering the load/weight on the chasis) i guess... but i want it to just be an IC, which is only about $6 max.

2. Is that a dangerous choice? What if i put a heaksink on the IC for the rare occasion that my motors draw more than 2 Amps? will this work

Thank you!
« Last Edit: October 20, 2007, 12:52:56 PM by SomeSaba »
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Offline Tsukubadaisei

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Re: motor controller question
« Reply #1 on: October 21, 2007, 12:42:57 AM »
All your calculations and assumptions are fine. And yes, put a heat sink or a fan on the ICs. And you dont need to worry about the current because you will need one IC for each motor so you can release up to 8A(2A for each motor).
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Offline bulkhead

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Re: motor controller question
« Reply #2 on: October 21, 2007, 01:06:48 AM »
Does this box happen to be 30cm cube?

Try this IC http://www.hobbyengineering.com/H1048.html.  $3.25 here, slightly cheaper on digikey (SN754410NE).

It is wired the same as the L293D motor controller.  Solarbotics's motor controller kit uses the L293D.  Read the documentation on that, which shows how to wire the 2 sides of the IC together (parallel) for double the current handling capacity.  Another way would be to stack 2 IC's piggyback so that the pins are contacting.  If done the second way, heat would be an issue though.  Digikey also has IC heat sinks (clip on), see http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=HS179-ND.  The SN754410NE's are $1.88 on digikey http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=296-9911-5-ND.

I used 2 SN754410NE's to control 2 of solarbotics' gear motors with xmod motors (even more powerful than the solarbotics upgrade) on a 5 lb robot and it worked fine.  You might need to use 4 IC's for 4 motors just in case (safety margin).
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Offline SomeSabaTopic starter

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Re: motor controller question
« Reply #3 on: October 21, 2007, 07:26:06 PM »
yes a 30cm cube :)


thanks a lot for the replies they clarified everything <3
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Offline SomeSabaTopic starter

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Re: motor controller question
« Reply #4 on: October 25, 2007, 10:19:00 PM »
oo, the datasheet for the motor controller IC says that its 4 GND pins are GND / Heat Sink... do i have to connect the heat stink to ground or the GND pins of the ic to my heat sink?
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Offline bulkhead

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Re: motor controller question
« Reply #5 on: October 25, 2007, 11:42:13 PM »
No, it's just that most of the heat will be dissipated through those 4 pins so it would be a good idea to solder them to something (don't just plug into a breadboard).  The heatsink doesn't have to be connected to GND (but it can), just make sure it isn't shorting anything.
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