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I am using Adafruit's Motor shield to drive a DC motor I got out from an RC car...

Powering the Arduino from USB. And powering the shield from a 12V / 1.5 A Power adapter (the shield is using an L293D)

I checked the datasheet of the L293D and it says max current is 0.6 A and max power dissipation is 5 Watts.

I measured the drawn current from the supply and it was : 0.5 A. (with running motor) As soon as the motor turns on the chip gets hot quickly (withing 1 sec) & the motor's speed drops down.

Another weird thing is that when I connect the motor directly to the power supply it starts turning for 1 second then turns off for 2, then it turns back on & so on... I am suspecting that it's exceeding the power adapter's max current (1.5 A) which is also weird because the motor is small and doesn't look that powerful.

Can anyone please help me here?

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  • \$\begingroup\$ Do you know the motor specifications? \$\endgroup\$
    – JYelton
    Mar 18, 2014 at 16:24
  • \$\begingroup\$ The on-off behavior could also be the thermal limiting in the motor itself, if it has such a thing. I second @JYelton , can we see the motor specs? \$\endgroup\$
    – bitsmack
    Mar 18, 2014 at 16:33
  • \$\begingroup\$ Max current is the max attainable with great heat sinking or fans, etc. NOT the max just freestanding in ambient air. I don't see any obvious attention to thermal design in the motor shield... Do you have the version 2 or the older one? \$\endgroup\$
    – darron
    Mar 18, 2014 at 16:56
  • \$\begingroup\$ @bitsmack Unfortunately I don't have the motor specs, looks like a cheap small generic one though (cheap Chinese RC). and darron : I have the older version of the shield. \$\endgroup\$
    – br4him
    Mar 18, 2014 at 18:57

1 Answer 1

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Maximum power dissipation of the L293D is far less than 5W. The N type package will warm up 67 degC per watt dissipated (pg 4 of data sheet) and 2.5W would likely push it past the limit of 150 degC stated on page 4 also.

It's hardly ever good news with the L293D (and its near-identical twin the SN754410). These are poor choices for H bridges if you are pushing the current over 0.5A and by the sound of it you are. There is also a suspicion that an RC motor probably isn't rated at 12V but somewhat less so you are probably pushing this just too far. Most RC batteries I've seen are about 7.5V.

Back to the L293D (aka SN754410).... When driving a load it doesn't act like a relay contact - it loses about 1.2V on the lower transistor and 1.4V on the upper transistor - take half an amp or so and the power dissipated is already 1.3 watts and has taken the N package device up 87.1 degC above ambient. That's quite hot in an ambient of 25.

Not a good choice of H bridge but, to be honest I'm yet to hear of an application when the L293D (SN754410) is a good choice. I'd look into the DRV8837 from TI. It has a combined high and low side mosfet on resistance of 0.28 ohms meaning, at 0.5 A the total volt drop is 0.14 volts and the internal power dissipated is 70mW. Maximum motor voltage is 11V but, I suspect you may be over-powering your motor anyway. There are probably others from TI that have a higher voltage rating.

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  • \$\begingroup\$ * Yeah the motor is rated @ 6V. * I see, poor choice of the H-Bridge... guess I'll have to throw that shield and use another H bridge instead. * I also need to drive 3 x 4 A Stepper motors (CNC) would you please suggest/recommend a driver for this purpose ? * Can you also please explain the weird behavior when powering it from the power supply directly ? (on for 1 sec then off for few ... loop) * Thanx in advance \$\endgroup\$
    – br4him
    Mar 18, 2014 at 19:09
  • \$\begingroup\$ Sorry about the formatting, am new here this is actually my first question. An asterisk denotes a new line. \$\endgroup\$
    – br4him
    Mar 18, 2014 at 19:11
  • \$\begingroup\$ Look at TI for their offerings - they have probably the best internal search engines for this sort of thing - do take a good look at the DRV8837 though. Weird behavior might be electronics in the motor cutting current out because you put too much voltage on it. \$\endgroup\$
    – Andy aka
    Mar 18, 2014 at 20:07

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