Hi,
I'm trying to power my mega2560 from an external source for the first time.
Using a motor shield the USB power wasn't enough to power the one motor I had plugged in, so I hooked up a 7V battery to vin and gnd and everything was dandy.
Only it doesn't work without the USB connection.
I read a few horror stories of fried boards, any way to test it and see if it is dead?
Motors shouldn't be run off the on board regulator. It doesn't provide much more current than USB. Sometimes less depending on the input voltage.
I'm running the motor from this: Module controleur de moteurs (2x1A)
Powering the whole lot from a 7v battery on the VIN port. It seems to work, just that the board won't boot without USB power.
If I read it correctly the motors run off the battery when both power supplies are present right?
Do you have a multimeter? Measure Vin when the battery is connected.
VIN is at around 7.2V same as the battery, however the 5V out stays at 0V.
memoi2001:
VIN is at around 7.2V same as the battery, however the 5V out stays at 0V.
Is that with anything external wired to the 5v pin like a motor? Measure without anything else wired to the arduino and if you still have Vin at 7.2 but 5V pin at 0vdc then you have a bad on-board voltage regulator.
Dang, looks like it's the regulator 
Thanks for the help guys
memoi2001:
Dang, looks like it's the regulatorThanks for the help guys
Or possibly something causing a direct short circuit from +5vdc to ground on the board, as the voltage regulator will go into 'self-protection mode' and turn off it's output voltage in the presence of a shorted output condition.
Lefty
Would that condition also applie when both USB and VIN are powered?
memoi2001:
Would that condition also applie when both USB and VIN are powered?
I believe so. The auto-voltage switching logic just shuts off the USB +5vdc via a MOSFET transistor is there is a valid Vin voltage applied to the board. But then again the auto-voltage switching logic requires +5vdc to operate.
So better to review your symptom, does the board work with USB only power attached (with everything disconnected from the board)?
Lefty
As long as the USB is connected everything works fine.
The blink example runs perfectly and serial is fine too.
The only problem is getting it to work without a USB.
memoi2001:
As long as the USB is connected everything works fine.The blink example runs perfectly and serial is fine too.
The only problem is getting it to work without a USB.
Ok, that means there is no direct short circuit on the board, just a non-functional on board +5vdc regulator seems to be the most likely candidate to look at.
Lefty