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I have an Arduino Uno R3. At the 5V output port of the board with nothing connected and blinky running while powered through USB, I can measure only 4.5V. Is this normal? Or is there anything wrong with my board?

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  • \$\begingroup\$ If it's directly powered from the USB with no on-board power supply then it should have been designed to run from the full range of USB power and 4.5V is OK. If the board makes it's own regulated power from the USB and that's supposed to be 5V, then it's not OK. Post a schematic of the board. \$\endgroup\$ Jan 3 '12 at 15:02
  • \$\begingroup\$ arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf \$\endgroup\$
    – 0xakhil
    Jan 3 '12 at 16:17
  • \$\begingroup\$ i am a bit scared because I have once powered the board externally with wrong polarity. I don't know if that made any component faulty. \$\endgroup\$
    – 0xakhil
    Jan 3 '12 at 16:22
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Now that you've posted the schematic, a straight answer is possible. That schematic is a sloppy mess, but I know that's not your fault. It doesn't say much about the folks that designed your arduino board though. Yucc. It makes you wonder what else they didn't pay much attention to.

The board can be powered from USB or a external supply. When from USB only, there is a diode from the USB power to the "5V" supply. The USB supply voltage is nominal 5V, but can be lower than what you actually measured in some cases. What you measured is about expected with 5V from the USB then with a diode drop in series.

It looks like there is at least a reverse blocking diode (D1) at the external DC power input. That means if you hook up the external supply backwards the board won't run but nothing will be damaged. You therefore didn't break anything, at least due to providing the wrong external supply polarity.

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    \$\begingroup\$ "It makes you wonder what else they didn't pay much attention to." - you don't want to go there - frankly I'm surprised the Arduino boards even work at all! The number of corners they have cut I'm amazed it's not round ;) \$\endgroup\$
    – Majenko
    Jan 3 '12 at 16:44
  • \$\begingroup\$ @Majenko what are you talking about?? :/ \$\endgroup\$
    – 0xakhil
    Jan 3 '12 at 16:54
  • \$\begingroup\$ @0xakhil The "quality" of the Arduino design. \$\endgroup\$
    – Majenko
    Jan 3 '12 at 16:55
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    \$\begingroup\$ @Majenko: A design may be optimized for cost instead of performance or robustness. That by itself is OK as long as it's not pretending to be what it's not. However, there is no excuse for a sloppy schematic, whether the circuit reflects something "cheap" or not. \$\endgroup\$ Jan 3 '12 at 18:20
  • \$\begingroup\$ I knew there was a reason why I don't like the Arduino :) \$\endgroup\$
    – m.Alin
    Jan 6 '12 at 13:57
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Olin is wrong, there is no diode between USB input and 5V(i suppose he's talking about the internal diode of the T1 mosfet) . when there is voltage from the USB port, the MOSFET will be turned on by the op amp (U5A) and this will bypass the internal diode of the mosfet. depending on the Rds(on) of the mosfet, there will be a series resistance between USBVCC and +5V and that is approxim. 70mOHM according to the mosfet datasheet

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That'll be because your USB is only providing 4.5v.

That is perfectly normal and within tolerance.

In fact, the microcontroller will run happily from 3.3v should you wish it to.

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  • \$\begingroup\$ Correct, USB power bypasses the 5V regulator. If 5V is desired you must provide an input voltage >= 6VDC (I believe) via the barrel jack or VIN header pin. \$\endgroup\$
    – vicatcu
    Jan 3 '12 at 15:20
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I had same issue, dead MOSFET was the reason. I just removed it and solder wire there. Actually it's not good way to fix, but it works, but you should not use external DC in with that fix.

It's just video to show problem (USB 5v in and 4.5 after MOSFET) http://youtu.be/g3qYECB3tc8

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