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Topic: Driving Mega2560 from external 5V bricks USB port (Read 10 times) previous topic - next topic


I have an application that wants to use the extended ports and processing power of the Mega2560. Basically the Mega plugs into a backplane where it can have more than one shield , as well as other circuits that plug into the backplane by card slots.

So far, this arrangement works well. But if I power the backplane by way of the 5V supply on the Mega, there is less than 500mA available, which is not enough for some of my additional TTL circuits. I have been supplying 5V for the backplane and the Mega from an external supply. This works fine until I plug a USB cable into the Mega for programming. At that point there is 5V from the external supply on the Mega's 5V pin ( I am not using the Vin pin ) and 5V from the USB port. This works for about a day, then the Mega's USB port will brick. ( I went through 3 Megas before I figured this out. Ouch).The difference in voltage between the 5V supply and the USB port is from zero up to 100mv.

What I think I am going to have to do is adjust my external supply from 5V to 9V and apply it to the Vin pin. That way the internal power switch on the Mega should switch safely between external and USB power. I will then have to provide an additional 5V supply for any additional TTL.

Will this work or am I missing something here?


Dec 08, 2011, 01:36 pm Last Edit: Dec 08, 2011, 02:49 pm by SurferTim Reason: 1
Thanks for that tip!! I was about to do the same thing. I would have been unhappy.

I would use the onboard regulator if it wasn't such a crappy, underrated, linear "POS".

Edit: I guess nobody that builds these got far enough in the voltage regulator datasheets to find this:

My second remark above is a little harsh. I did not mean that as derogatory as it appears. The regulator is fine for a Mega with no shield and a supply voltage less than 10v.

But my Mega with an ethernet shield and a power supply of 9v, and in just a few seconds, the regulator is too hot to touch. At 13 volt supply (minimum automotive) it would get twice as hot.  :smiley-eek:


Is it possible you blew the 500ma fuse on the usb power input? Since you have a couple bad boards, can you try connecting one the bad boards and check the voltage on the fuse. It appears to be the gold colored smd marked "500J" next to the usb port. See if you have 5v on each side of that. If not, the usb port will not get power. According to the schematic, that is the only way the usb chip gets power.

I used the usb case as ground for the volt check.

If it is blown, it appears you could "jump" that fuse with a second fuse.


SurferTim, Thanks for the response. Fuses look good. Both boards I tested are obviously getting power from USB; LEDs are on. One is recognized by XP device manager, but cannot communicate on TX/RX. The other cannot be recognized as a USB device. Both Mega's when powered up run the last sketch uploaded.


I do not see how using two power sources should damage the arduino. The difference between a regulated 5v power supply and the USB port should be minimal and not harmful. 
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

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