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[on:]

Hey guys,

I have an arduino Nano board and I power it via USB/computer. When I measure the 5V output pin using a multimeter, it reads 4.74 volts instead of 5.0 volts... Can anyone explain?

I also measured 3.3volt output pin, and it showed 3.34 (which is much better than the 4.74 volts).

[Reply #1 on:]

The USB port does not provide 5V precisely. The specification says it can be as low as 4.4V.

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The Quick Shield: breakout all 28 pins to quick-connect terminals

[Reply #2 on:]

I also measured 3.3volt output pin,

That is because it is being regulated from what ever is on the 5v rail. So that can dip without affecting the 3v3 regulation.

[Reply #3 on:]

The USB port does not provide 5V precisely. The specification says it can be as low as 4.4V.

But I measured the voltage of the 1st pin of the mini-USB on the Arduino, and it reads 5.0 volts exactly. Why doesn't the 5.0v output pin also supply this voltage level?  

My problem is, I have a sensor, which needs exactly 5.0 volts to function! and I really need the 5.0v pin (maybe I can solder a wire from the 5.0v USB pin to my sensor, but I don't like the idea of wires sticking out of my project  :)

[Reply #4 on:]

The auto power select circuit on the Nano uses a diode between V-USB and 5V. This explains the difference you measure.

You can either bypass the diode (connect a wire between V-USB and 5V) if  auto-select is not needed or you could recheck your 5V requirement to see if it is not as absolute as you seem to think it is.

[Reply #5 on:]

My problem is, I have a sensor, which needs exactly 5.0 volts to function

If this is actually the case, then you probably don't want to rely on USB anyway.  You should be looking at a solution that provides a more stable voltage reference (like a voltage regulator.)

Just because your USB port is 5.0v today, which is surprising, it will not always be exactly 5.0v.  The USB Spec allows it to vary, at any time, between 4.75v and 5.25v.

[Reply #6 on:]

Consider using a 7805 Voltage Regulator.

[Reply #7 on:]

Consider using a 7805 Voltage Regulator.

This requires you to have more than 5V because it needs a couple of volts in excess of the regulated value to wok.
If you need exactly 5V for a sensor (and I am not sure you do because sensors don't usually work like that) then you need an external PSU and you can't use USB power.

[Reply #8 on:]

If you need exactly 5 for the rest of your circuit, you might consider a boost regulator like this
http://www.pololu.com/catalog/product/799
Use the onboard pot & tweak it to just where you want it.

The ATmega itself is not so fussy as the others have noted. I was testing a remote control for quite while with 3 AAs on VIN. Started at 4.5v, was still going well when got down to 4.25V.
Now have a 3.3V/8MHz Promini running on 3.7V Li ION battery on VIN and that is working well also.