[SOLVED]Arduino, Stepper and L293D working, but not as expected!

Other Hardware Motors, Mechanics, Power and CNC
1

Hi all,

I'm back again for a little advice/explanation.

I recently ordered some L293D ic's on ebay and was very excited when they arrived (sad I know). I followed the setup outlined here http://tmmbach.net/2010/08/05/driving-a-bipolar-stepper-motor-forward-and-backward-with-l293d-or-sn754410/ and clagged everything up on my breadboard shown here:

Uploaded and ran the first code on the above stated link and all worked as expected!!

HOWEVER: I accidentally knocked the external motor power 9v positive pin out of the breadboard but the motor continued to cycle through the code!

Surely this shouldn't work so quickly cut the power to avoid smoking anything and swapped the ic out thinking it might be a duffer, reinstated everything, fired it up and it ran as before. I was curious to see what would happen if i disconnected the external motor supply so pulled the positive terminal for the external motor supply and the thing still ran!!!

The motor connected is a Matsumi M35SP-9T can't remember where i found this one (4 wires assumed bipolar but could not find a data sheet, but nearest to it runs between 21.6-24v) and has 11ohm stated on the chassis itself. I decided to swap out the motor to one salvaged from an old epson printer marked EM-463 (4 wires assumed bipolar but i can't find any info on this motor either). Low and behold the motor again worked without the external power for the motor!!!!

I have tried re-wiring everything, checked the pinout for the L293D etc but i still get the same result, could any body shed any light on why this is working when I think it shouldn't?

Kind regards,

Ian.

2

There's probably a built-in diode on the chip somewhere that's letting the Vcc1 drive the Vcc2 (this means going outside the abs max ratings of the chip though, so don't be surprised if it get damaged.)

Parasitic diodes like this are quite common in integrated circuit design - so long as they are normally reversed biased they don't affect circuit operation.

3

Hi MarkT,

Thanks for the info. So as I understand it if I make sure i've got external power for the motors I shouldn't damage the IC?

4

Yes and it needs to be at least 5V. But the L293 loses so much voltage anyway you wouldn't want to run it below 6V anyway.

5

Hi again MarkT,

Thanks for your help!

Regards.