L293D Motor driver problem: 5v on external power input

Hi.

I am using an Arduino Uno together with a L293D motor driver to power 3 tiny motors.
The problem is that when I connect it with the PC via the usb cable I get 5v on the external power pins. I removed the VIN jumper as instructed.

Is it normal behaviour?

It annoys me because I have a 12v led switch to power the PSU that feeds the external power of the L293D and it is always lit by USB power.

If it is normal behaviour... how can I stop the 'backwards' power?

Thanks

Regards,
Mike

Please create a wiring diagram and post it. The word sallad type is not precise enough.

Yes, the L293D will phantom power from the logic supply if the motor supply is missing - this is not an intended mode of operation and you should avoid doing this. I believe the chips designers expected the 5V logic supply to be derived from the motor supply rail, or at least to some from a common power source.

If your motor supply is 12V it could stay connected to Vin during programming as the Arduino Uno auto-selects its 5V supply I think.

Instructed by what/who?

There is no need for wiring diagram: Arduino Uno with L293D connected has 5v on the external power connector. Nothing connected.

MarkT: so it is a normal behaviour?

As I said the chip phantom powers the motor supply from the logic supply - you are not supposed to do this (as with any phantom-power situation). If it tries to drive a motor in this state you risk the logic supply dropping out, and maybe the chip internals even.