L293D works with 9v but not 12v AD DC wall wart

Hi all,

I've built a circuit to control a motor with arduino/L293D and everything works as expected connecting a 9v/1A wall wart to L293D pin 8, but when I connect a 12v/2A wall wart to pin 8 the motor does some jerky movements and then stops moving all together. If I'm powering arduino through my computer's USB, it actually crashes the computer at this point (12v but not 9v connected to L293D pin 8).

If I power arduino with the 12v wall wart and pin 8 with 9v the motor works as expected. In all cases L293D pin 16 is powered by arduino's 5v output.

My layout is similar to the below example: https://learn.adafruit.com/adafruit-arduino-lesson-15-dc-motor-reversing/breadboard-layout

Thank you in advance!!

Is the 12V supply regulated?

Hi MarkT,

There's sparse documentation with the unit. Here's the product: https://www.amazon.com/Adapter-100-240V-Transformers-Switching-Adaptor/dp/B019Q3U72M/ref=sr_1_1?ie=UTF8&qid=1494856291&sr=8-1&keywords=le+dc+12v+2a

Would an unregulated wall wart cause this behavior?

Thank you so much!

Well that looks like a proper regulated supply - perhaps its not happy with rapidly changing load and cutting out. I'd try adding 470uF 16V electrolytic across the motor supply to smooth out rapid current changes - this might help.

The USB being affected would be because the L293D steals power from the logic 5V supply if the motor supply is removed (a nasty feature of the chip).

Thanks MarkT,

I did have 100uF cap x2 (parallel) between the 12v and ground, but are you recommending placing caps between the terminals of the DC motor, as well?

Hi MarkT, I figured out the problem:

I had an analog input to arduino measuring the current to the motor, and the voltage went too high when connected to the 12v wall wart. I threw in a resistor and it worked.