Arduino Uno R3 5V output reads 4.667V

Just got my board and hooked it up and tried the example ReadAnalogVoltage and it did not function as I would have expected. So I got the voltmeter out and tested the 5V output of the board to GND and it reads 4.667V. Tested the 3.3V and it reads 3.302V. Is this an indication of an out of spec board? or is the 5V always weak?


Is is an official board? The official boards (and some but not all clones) use a PFET to connect the USB power to Vcc if there's no external power connected. Some of the unofficial ones may use a schottky diode for power summing instead (it has some advantages - it's what I'd do if I were making a Dr. Azzyduino) - but this would introduce a 0.3v drop.

Also, measure the 5v pin on the USB from the underside. Is that even 5v, or is your computer outputting less than 5v? This is not unusual.

I have also seen a few absolutely terrible USB cables that had like 5 ohm resistance in the wires each way (so 10 ohm effectively) - which at 50mA would be enough to drop half a volt or so... so if you happen to be using a garbage cable like that, that'd explain it too....

Thanks for the input!

This is an official board in theory. Purchased from MP3Car Store.

So first it is not a particularly low end meter... BK Precision 2709B. the USB cable seems to have a residence of .3 to .5, depending on the pin. The USB power source reads 4.758V

I have 3 different power sources: - USB reads 4.667 - 9V AC/DC Adapter reads 4.667 - 12 AC/DC Adapter Reads 4.993

Still seems like the 5V is under powered.

Thanks again.

That looks correct to me - you get a little drop, plus the USB voltage is a little low to begin with.

It sounds like most of the under-voltage is due to your computer putting out ~4.75v instead of 5v. Then you lost another 90mV or so in the wires and power switching circuitry.

I’m surprised the output is so low off the 9v adapter (i wonder if it’s not switching over?) - but it’s still not so low that things won’t work. Output from 12v adapter power is as expected - the Arduino board does not use a precision regulator, the voltage is not going to be exactly 5.000v from the regulator.

USB is 5V +/-5%, yes? So 4.75V out of the USB connector is legit.
You lose some across the PTC also.

Thanks for the help, much appreciated.