"Noob" Motor Shield Question

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Olly
 
Posts: 4
Joined: Sun May 20, 2012 7:22 am

"Noob" Motor Shield Question

Post by Olly »

Hello everyone.

So glad i found you, before i found your Motor shield FAQ thread I was wondering if my motorshield was completely borked by my cack handed soldering. But now light and even motion!

One quick Q regarding this statement:
I connected a 6v supply but I only see 4.8v at the motor terminals.
There is typically a 1.2v drop across the L293D H-bridge chip. If you need a full 6v to the motor, you should use a 7.2v supply.
Im using, like many newbies im sure, my old lego technics stuff to play with for now.

My motor is the first gen 9V
http://www.philohome.com/motors/motorcomp.htm Motor ref 2838

I need to sort myself an external power supply, a pile of AAs ran them originally, so a pile of AAs will be best for now i guess.
Question: Would i be ok putting 7AAs together and running a 10.5V pack, to account for the 1.2V loss in the H bridge? or would i be better off sticking to 8 and running the motor at a lower 7.8V speed?

does it make a jot of difference?

I pressume i can put pretty much what i want into the external power supply connections (within reason, not a car battery)

Quick bit of feedback: I got my adafruit instruction free last night. I found the soldering instructions ok, but it took me a good hour or two to find that the SERVO_1 and SERVO_2 need to be programmed as pin 9 and 10 (10 and 9 actually).

Is there a datasheet for the motorshield which outlines these things? I havent found it yet if there is!

Many thanks. Im sure i will be back when i come across my next problems.

Olly

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adafruit_support_bill
 
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Joined: Sat Feb 07, 2009 10:11 am

Re: "Noob" Motor Shield Question

Post by adafruit_support_bill »

The motor voltage rating is not a hard number. At lower voltages you will get less torque. At higher voltages you will get more torque, but a shorter motor lifetime. If you get enough torque to drive your mechanism at 7.8v, then I'd stick with the 9v pack. If you need that extra torque, add another cell.
I pressume i can put pretty much what i want into the external power supply connections (within reason, not a car battery)
The H-bridge chips are rated for up to 36v. But if you exceed 25v, you should replace C7 & C8 with capacitors rated for the higher voltage.
The real danger of something like a car battery is not the voltage, but the amperes. A car battery is capable of delivering incredible amounts of current. If you short something out, part of your circuit will be vaporized before you know it. :shock:

Thanks for the feedback on the documentation. We are always working to improve things.

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