Using Arduino > Installation & Troubleshooting

Driving Mega2560 from external 5V bricks USB port

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bobprescott:
I'm glad that I saw this thread  before I hooked up my Mega, it saved me a ruined board. I'm running off of a 80's vintage backplane, with a typical 5v supply. The supply runs hot, about 5.25 volts. Thanks to seeing these posts I disconnect the Mega2560 from the external supply while programing, then disconect the USB when installed. Works so far. The one time I left the USB conneced when attached to external supply, the Mega USB IC got hot fast.
I also use a modified USB cable with the 5v line cut for when I need to get data while running.

Since the problem is known and can break boards, how about a sticky, or a warning.
Get the word out.

Thanks
Bob Prescott

smeezekitty:

--- Quote from: bobprescott on May 04, 2012, 06:25 pm ---
I'm glad that I saw this thread  before I hooked up my Mega, it saved me a ruined board. I'm running off of a 80's vintage backplane, with a typical 5v supply. The supply runs hot, about 5.25 volts. Thanks to seeing these posts I disconnect the Mega2560 from the external supply while programing, then disconect the USB when installed. Works so far. The one time I left the USB conneced when attached to external supply, the Mega USB IC got hot fast.
I also use a modified USB cable with the 5v line cut for when I need to get data while running.

Since the problem is known and can break boards, how about a sticky, or a warning.
Get the word out.

Thanks
Bob Prescott

--- End quote ---

0.25 is alot of differential between two power supplies. Enough to dissipate several hundred milliamps in the board.
With < 0.1v differential, I don't really see why it would be a problem but with .25, I would avoid it.

vanduino:
In the original post, I mentioned that the differential  between my external 5V supply and the USB power was < 100mV.  Generally less than 25mV, every time I checked. I still bricked the USB ports of several Megas. I wish I had checked the temperature of the chips, but it took me a while to localize the problem, because they did not appear to fail at once. It was only when I tried to reload sketches that I realized I had a problem.

bobprescott:
.25 is a hugh differance, I know, and old power supplies designed for TTL usually had some kick. But it really only matters if power supplies are shorted together, which is unexpected.   I'm sure I can open the supply and adjust it down. Might have to change a resistor.

What does irk me a bit, is that I picked the Arduino Mega2560 specifically because it could run off of an external 5v supply.
Now I have to kludge a wall-wart, and redo my shield to separate supplies? I guess  I'll live with it, not the end of the world.

Bob





vanduino:

--- Quote ---.25 is a hugh differance, I know, and old power supplies designed for TTL usually had some kick.
--- End quote ---


My experience  was with a  differential of 0.025V . That does not seem to me a so big as to account for the failure of an IC. I suspect something else is involved. Could the unplugging and plugging of the USB cable while connected to an external 5V supply generate some kind of spike or reverse voltage?

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