can no longer power the UNO from motorshield

Hi,
Used to have the UNO powered (via link on Rev 1 motor shield) from the 12V to motor shield.
Had very strange problems with intermittently having the L293 bridge chip run RED hot (as in Ouch, I burnt my finger type hot!!) , other times all was good.
Found out that the PLUS socket solder contact for M1 was resting on the CASE of the USB connector of the UNO and when the motor turned clockwise with the board properly seated. In reverse, no problem.
Swapped some H Bridge chips etc. and all is good again EXCEPT that whereas before, after writing the program to the UNO via USB, I would disconnect the cable, set the power link on the shield and all would be fine, whereas now the UNO no longer can feed from the shield.

Have tried to suss out the connections but got hopelessly lost.

Anyone have any idea as to what happened and how I might be able to fix it?

All help greatly appreciated.

Aussiewill

Couple of things come to mind and one is that REAL arduinos have been seen to burn out the VREG at 12 volts.

Dont think I would ever risk running an Arduino FROM a motor shield and would always prefer to see it run from its own power supply of between 7-10 volts.

Most motor shields have a jumper or track to cut/bridge to supply power to/from an Arduino.
You may want to check if that is in the correct place before starting up a motor shields power.

Motors can often throw spikes back down the power rails unless suitably protected.
You may want to check what you have for that as you did not include any specs for anything or schematics etc etc etc.

Hi,
Thanks for that reply.
Yes, as far as I can tell you're spot on.
The problem is that where the thingymebob is going to be installed, I have 12V available but NOT anything lower.

I am just curious about one thing, NEARLY always (there are of course exceptions) when a motor shield is used on other than 5V it will be 12V, as these are the 2 most available voltages.

So why put a power link in at all??

Another question:
Is there a quick and dirty way to SAFELY feed the UNO from the shield supply other than putting in a 7805.
The reason I ask is because this project does NOT use a breadboard and I don't really want to directly solder a regulator chip onto the shield pins and down to the UN.
Very messy.

If anything comes to mind, please advise.

Aussiewill

A simple wall mounted supply if its a fixed installation.

If its a mobile installation then see if there is a regulator already on the motor shield you can take off that is less than 12 volts.
BTW if its a clone Arduino then they are more reliable in the voltage criteria so it would be unlikely to harm it as much apart from possible spikes which could harm either clone or real ones.

If the motors only run one direction consider a diode to prevent back EMF or if its CCW/CW then a small disc capacitor would often help. Or even both !

Plenty of clues on google for motors and arduinos and you will see on the better ones they use various filtering techniques...

Again a lot depends on the specs of the motors and such but usually .01 uF would be a good starting point for a capacitor.

Hi,
Thanks again.

The motor is a very basic 12V 16rpm geared DC motor and yes, I run it both ways.

I think I'll probably go the easy way with a 7805, using the 2 pins on the shield link for 12V and 5V and just a wire to earth. Messy but effective.

Once again, thanks for the advice and may much karma come your way.

Aussiewill

Still take a look at a capacitor across the motor terminals as just a precautions against spikes even at low RPM.