Vin works as a power output but not as a power input

So I have an Arduino Uno connected to an MD25 motor controller, which runs on 12v and has a 5v output for powering the Arduino. It has been functioning fine for months, and then in the past few days it has begun to behave oddly.

If I connect the Arudino's USB to my PC it powers up fine (it runs programs fine and will power the MD25 microcontroller through the Vin pin that connects them). However if I then disconnect the USB power and connect the MD25 to its 12v battery supply, the MD25 powers up (it has LEDs on the board) however absolutely no power reaches the Arduino.

In addition, if I connect +5v connection from the MD25 to the 5v pin on the Arduino, SOME (only a small amount, not enough to run off) power reaches the Arduino.

Does anyone know what the hell is going on? :S I have a university competition for this in just four days...

UPDATE: I have multiple Arduino Unos and have attempted to substitute them in, but I’m still getting this behaviour.

UPDATE: The whole robot was taken apart the other day and the circuitry redone. Is there any way some other circuitry is causing this?

I am not sure you aware but you can't power off an arduino that has any GPIO or analog inputs connected to any source of power because it will "backfeed " through the internal protection diodes.

(each I/O pin has the anode of one diode connected and the cathode of that diode connected to +5V. each I/O has the CATHODE of ANOTHER diode connected and the ANODE of THAT diode is connected to GND.

If you power off the arduino with voltage on I/O pins it travels to the arduino, through the anode of the first diode and powers the 5V pin (minus 0.7V diode drop) with 4.3V.

If a negative voltage is applied to an I/O pin it is shunted to GND by the second diode.

Atmel recommends not exceeding 1 mA through these diodes but I have powered an UNO (unintentionally of course) , drawing 55 mA this way with no permanent side affects.

If I connect the Arudino's USB to my PC it powers up fine (it runs programs fine and will power the MD25 microcontroller through the Vin pin that connects them). However if I then disconnect the USB power and connect the MD25 to its 12v battery supply, the MD25 powers up (it has LEDs on the board) however absolutely no power reaches the Arduino.

Standard electronic troubleshooting techniques dictate that the connections be checked with an ohmmeter or continuity tester. Obviously there is a problem with your statement that the arduino Vin and the MD25 Vin are connected and the MD25 has power but the arduino doesn't . The arduino Vin is the input for the 5V regulator. If the arduino works with USB connected then it is NOT operating off the onboard 5V regulator. If the USB is disconnected and the arduino Vin (allegedly) has power , but the arduino doesn't , that would prompt one to measure the input and output voltages of the onboard regulator to see if it is damaged. If the symptom persists after replacing the arduino, then one would be inclined to go back and doublecheck that there actually is voltage on the arduino Vin pin.

IMO, it's a normal behavior ;-)

Leads from a battery can be inserted in the GND and Vin pin headers of the POWER connector. The board can operate on an external supply from 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may become unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts. The power pins are as follows:

Vin. The input voltage to the Uno board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this

see that

when you tried powering it via 5V pin, did you check the MD25 5V output ?

You haven't posted a schematic but the connections you describe are simple. Obviously something is wrong or you have overlooked something. You need to make a continuity AND voltage CHECK LIST and post it. It should list EVERY POINT you have measured the voltage and the conditions under which they were measured. It should list the arduino on the LEFT side (column) and the MD25 on the RIGHT side (column). Each pin should be labeled. The table should show voltages on the left wire connection continuity check in the CENTER and voltage on the RIGHT. It should include BOTH Vin AND 5V pins of BOTH uControllers. A WORD file would be my choice .

Just to say that what you are trying to do is perfectly normal and it should work.

however absolutely no power reaches the Arduino.

That suggests you have fried the voltage regulator on the Arduino.

Just connect the 12V to the power jack and if no power light comes on the the voltage regulator is not working.

I reckon I've got a small short somewhere on the PCB that connects my MD25 and Arduino, not solid enough to short it completely, but enough to give a strong voltage drop (that board alone, disconnected from the Arduino was making the LED on the MD25 dim).

When moving some wires on that board I blew the fuse to the MD25...

Post the check list

maybe I misunderstood.... i thought OP tried this :

12V -> MD25 and MD25 5V output -> Vin Uno : arduino is not powered, which is normal

then he tried this :

12V -> MD25 and MD25 5V output -> 5v arduino pin : arduino should be powered ON, even if the regulator is dead

That is correct. But in addition, USBv->Arduino and Arduino Vin->MD25 5V works fine for powering both boards

Regardless of what yiu can or cannot do, you SHOULD NOT power the arfuino from the MD35 5V.
That violates every principle of isolation between equipment. The arduino external dc barreljack and the MD25 should be powered ftom the 12V but I eould recommend getting a buck converter to step down the 12V to 7V for the arduino external dc in.

MerrickKing: That is correct. But in addition, USBv->Arduino and Arduino Vin->MD25 5V works fine for powering both boards

??

As far as I can see, the MD25 5V pin is a regulator output - what makes you think you can power the MD25 from the arduino ?

and, as Raschemmel says, you have a 12V PSU, use it for powering both MD25 and arduino ?

I'm not going to waste any more time with this post until the OP posts the Measurement list that shows all the connections , continuity tests and voltages .