nano only putting out 4.78V

Forum 2005-2010 (read only) Hardware Troubleshooting
1

nano powered by USB is only putting 4.78V on the 5V output. I get a perfect 5V coming in on the usb port. Is it normal? Does USB power go thru the LDO regulator? If so, I know that there will be some drop from that. But my duemilanove board is putting out 4.94V. Much better.

The other weird thing is when using battery power, with USB disconnected, the 3.3V output is working. I thought it was only powered when USB is connected..?

I wonder if I damaged my nano? its possible, I was a bit careless with it.

2

Does USB power go thru the LDO regulator?

No

nano powered by USB is only putting 4.78V on the 5V output

Has the pin been defined as an output? Is there any load on it? Having said that it is a perfectly acceptable value for a logic output to be at.

I thought it was only powered when USB is connected

What happens is the 5v feeds the regulator inside the USB / serial chip and so it produces an output.

I wonder if I damaged my nano?

No it's fine.

3

That is quite normal. It's what they call the "Auto +5V selector".

There is a Shottkey diode from the USB +5V to the ATmega +5V and regulator +5V. This diode gives about 0.3V voltage drop.

On the larger boards there is extra circuitry (2 opamps and a P-mosfet) to reduce this drop to nearly zero. The Nano doesn't have this.

4

(by 5V output, I meant the output pin from the regulator. not a gpio)

I see. Interesting litte difference that the nano has there. thanks!