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Author Topic: power supply and pin current output questions  (Read 533 times)
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Italy
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i have an Arduino Mega2560 board.
i must build an external board with other TTL logics, so i have a unique 5V power supply, for either my board either Arduino Mega.
can i connect my power supply directly to 5V pin of Arduino Mega board, without pass through external power connector X1 ?

i must use usb connector to communicate with PC while the board is working, what's happen to usb supply ?
usb ground jumper must be closed or leave open ?

ATmega2560 have 40mA max current output per pin, but how many outputs pins can give this current simultaneously ?
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Can I connect my power supply directly to +5V pin of Arduino Mega board, without pass through external power connector X1 ?
Yes.

i must use usb connector to communicate with PC while the board is working, what's happen to usb supply ?
It should not be a problem.

usb ground jumper must be closed or leave open ?
I don't know what jumper you mean but I expect it should be unchanged from normal.

ATmega2560 have 40mA max current output per pin, but how many outputs pins can give this current simultaneously ?
It's in the datasheet:

31. Electrical Characteristics
Absolute Maximum Ratings*
DC Current VCC and GND Pins................................. 200.0mA

I guess that means you can have 5 pins at max current.
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Italy
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i must use usb connector to communicate with PC while the board is working, what's happen to usb supply ?
It should not be a problem.
are you sure ?
if i well understand the electric schematic (http://arduino.cc/en/uploads/Main/arduino-mega2560_R3-schematic.pdf) , mosfet T1 should be ON if i power the board directly from +5V instead from connector X1, because wire VIN, i suppose, remain at 0V.
i don't want to burn the PC usb port....

usb ground jumper must be closed or leave open ?
I don't know what jumper you mean but I expect it should be unchanged from normal.
in the schematic is the pad called GROUND between usb connector and atmega16

Quote
I guess that means you can have 5 pins at max current.
ok thanks a lot.

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I am not sure if I understand your question, but if you are asking if you can power the board from +5V, it is not recommened.

See here for more details:
http://arduino.cc/forum/index.php/topic,82046.msg758170.html#msg758170
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Italy
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I am not sure if I understand your question, but if you are asking if you can power the board from +5V, it is not recommened.

See here for more details:
http://arduino.cc/forum/index.php/topic,82046.msg758170.html#msg758170
yes, you well understand my question.

i think the problem is the control of mosfet T1, it depend on state of VIN, that is 0V when the board is powered from +5V pin directly.
i read the thread you link, so there is also another problem due to voltage regulator that in this condition, is reverse powered.

i think that with 2 little modification on the board the 2 problem can be solved (i will test ASAP).
1- a diode with anode to +5V and catode to VIN to protect the voltage regulator IC1 from reverse powering.
2- remove resistor RN5D so also with 4,4V ( 5V-Vak of new diode) can trigger the comparator IC7B

modify the board in this way, let all functionality as before, it can be powered either from usb, X1 connector, and +5V, but when the last 2 method are used, usb power will remain isolated from board.

what do you think ?
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Italy
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i replay by myself smiley

can't work !
if RN5D is removed, usb power continuosly disable itself. smiley-sad
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Power the Mega from 5V on header pin; install a diode from 5V to Vin so that the regulator is not reverse driven. See page 10:
http://www.ti.com/lit/ds/symlink/lm1117-n.pdf
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