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I want to drive 24V/5A DC motor with Atmega32. So bought motor driver having 24V/20A rating. But i am facing some issues :

  1. When i connected Atmega Board with Motor driver and connected 24V supply to driver input, motor worked fine. But after that when i turned off power supply to Atmega Board, I observed that Power indication LED on board was still glowing as well as motor was rotating. Is it due to Back EMF ?

  2. To solve this, I connected diodes in series to the connection between Board to driver, to avoid Back EMF. It was a success. But after few seconds driver was blown with fumes coming out.

(Note : I maintained proper input to drive, i.e 24V. Didn't cross that mark.)

So my question is how to stop Back EMF as well as save the driver ?


  • 2
    \$\begingroup\$ Can you post a schematic for how things are/were connected? If you hit Ctrl-M you get a nice in-your-browser schematic editor. \$\endgroup\$ Commented May 4, 2013 at 7:08
  • \$\begingroup\$ how to post schematic ? \$\endgroup\$ Commented May 4, 2013 at 7:16
  • \$\begingroup\$ Edit your post and hit Ctrl-M. Or use the 7th button from the left, the one after the Image button. \$\endgroup\$ Commented May 4, 2013 at 7:20
  • \$\begingroup\$ Where was the LED - on the atmega board or the motor control board? Which line(s) did you connect with the diodes? \$\endgroup\$
    – Andy aka
    Commented May 4, 2013 at 11:21
  • 1
    \$\begingroup\$ Please also show how the LED and the diodes were connected. And the part number of the motor driver. I don't think back EMF made the diode glow -- back EMF happens when the drive to the motor is turned off, due to the inductance of the motor. It sounds as though your glowing LED was some leakage from the motor supply to the atmega supply. \$\endgroup\$
    – Tom Davies
    Commented May 8, 2013 at 6:38

1 Answer 1


I don't think your motor driver is designed to handle the supply +/- connected but VCC/Ground not connected. You should always have both supplies to the motor driver and the ATMega either ON or OFF, an not have one ON and the other OFF (ATMega ON, driver OFF is probably alright, but you would have to be careful.)

If you require it to function like that, you should take specific measures to allow it to work.

  1. The motor driver should be fully powered or not powered at all at all times. So, instead of using the Vcc/Ground from the ATMega, use a 7805 or the like and generate 5V from the 24V supply. Use this to power the Motor driver's VCC (and NOT the ATMega's VCC)

  2. Once you've split the supplies, you need to translate the logic signals from the ATMega's supply to the Motor Driver's logic supply. Since you're talking about 24V/5A, or over 100W, I'll gather these are fairly heavy duty motors. It's worthwhile to go the extra mile and do this using optocouplers. Optocouplers / optoisolators allow you to translate signals from one supply to the other without any copper / conductive attachment between the two sides. This is useful for a number of reasons, but also tends to expensive. Analog devices has a line of iCoupler isolators which don't use optics but use another method instead. Isolation of this kind (usually identifiable by having separate grounds on both sides, not just separate supplies as in level translating buffers) will make sure that you can run your logic signals from one side to the other without worrying about noise coupling back into your ATMega's ground and supply and all that.

  3. You can avoid proper isolation and use a simpler (and cheaper) level translating buffer like TI's SN74LVC2T45 and family, but you may see strange behaviour come from the high currents that are switching on the motor side.


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