Motors are not running properly

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ssyed_mcu
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Posted by ssyed_mcu: Thu. Jun 17, 2010 - 09:57 AM
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Hi,

I am running 2 motors simultaneously using ATmega16 through a L-293D H-Bridge motor driver. The motors were working fine for some time, but after a while they have stopped. I am running the motors using a 12 volts battery pack. I had read somewhere that when the power is switched off the collapsing magnetic field induces a back emf that can cause problems in digital circuits. Is this what is happening in my case?

To offset this I have connected a 0.1 micro farad capacitor across the terminals of each motor to bypass any ripple voltage. Still the problem persists. Any thoughts or suggestions will be helpful.

Thanks,
Sumair

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Learning and Information, General Electronics, ATmega16
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kherseth
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Posted by kherseth: Thu. Jun 17, 2010 - 10:07 AM
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The H-Bridge does have snubber diodes itself, so the back EMF is absorbed there.

What is the current draw on the motors? Do the h-bridge get hot?

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valusoft
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Posted by valusoft: Thu. Jun 17, 2010 - 10:16 AM
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Can you look at the AVR signals driving the L-293D? Are the signals what you expect?

Ross McKenzie, Melbourne Australia

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ssyed_mcu
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Posted by ssyed_mcu: Thu. Jun 17, 2010 - 10:21 AM
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Hi,

Yes the AVR signals driving the L-293D are correct as my motors were running correctly initially. The problem started after about 15-20 mins.

Yes I have observed that the L-293D does get hot sometimes. I am using a development board that has 2 H-Bridges, and none of them has a heat sink connected.

I'll try and measure the current drawn by each motor and post an update.

Thanks,
Sumair

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kherseth
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Posted by kherseth: Thu. Jun 17, 2010 - 10:38 AM
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It might be the automatic temperature shutdown in the h-bridge that kicks in. The circuit is rated to 600mA continous current.

If using PWM, also note that the datasheet says "This device is suitable for use in switching applications
at frequencies up to 5 kHz." So Make sure PWM-frequency is lower than that.

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Bronze Markjs
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Posted by js: Thu. Jun 17, 2010 - 10:25 PM
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Is the battery flat?

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Heihopp
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Posted by Heihopp: Sat. Jun 19, 2010 - 02:15 AM
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Make sure you have a good common ground for all parts of the circuit. Connect the load and digital logic as close to the power supply as possible. Ideally every ground wire should be soldered to the same spot.

Last Edited: Sat. Jun 19, 2010 - 02:22 AM
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tlucas
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Posted by tlucas: Sat. Jun 19, 2010 - 02:21 AM
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ssyed_mcu wrote:
Yes the AVR signals driving the L-293D are correct as my motors were running correctly initially.
This doesn't confirm correct input signals... best to 'scope it.

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Heihopp
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Posted by Heihopp: Sat. Jun 19, 2010 - 02:45 AM
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Another issue about motors is that they do not only send back small emf spikes during switching, it may also come long bursts of energy back to the transistor bridge during braking of the motor. The protecting diodes in the bridge pass this braking energy back to the power supply and the voltage increases to levels that the electronics was not designed for.

Professional equipment use braking resistors in such cases.

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ssyed_mcu
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Posted by ssyed_mcu: Tue. Jun 22, 2010 - 08:48 AM
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Hi,

Thanks for all your replies.

I have a single battery powering 3 different modules as shown below:

1. Microcontroller
2. PS2 Controller to control the robot movements
3. DC Motors

I think over time the battery is getting drained and as a result there is not enough juice to power the motors through the H-Bridge driver.

I know that ideally the microcontroller and motor supply should be different, but since I am using the development board itself as the robot platform, I am forced to use a single power supply.

I may have to develop a PCB later which separates out all these, but don't have the time to do so. I might consider doing that sometime in future.

Thanks,
Sumair

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Bronze Markjs
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Posted by js: Tue. Jun 22, 2010 - 11:13 PM
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Quote:
think over time the battery is getting drained
Quote:
Is the battery flat?
Do I get a prize for guessing correctly then?

John Samperi

Ampertronics Pty. Ltd.

https://www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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tlucas
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Posted by tlucas: Wed. Jun 23, 2010 - 03:23 AM
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