Duemilanove @ USB -> only 4 Volt output on Pins?


i am a newbie and testing a little bit with the Arduino Duemilanove and find some behaviour which i am not expecting: The voltage of a Digital output pin or the 5V reference pin is 4 Volts. The Board is connected via usb. I was expecting that i can measure 5 Volts on this pins. Why i can only measure 4 Volts on the Pins ? Regards and thx

How are you measuring. No offense, but if you are a newbie perhaps you are not using a DMM correctly (red lead to output pin, black lead to 0V?)

It's not unusual for USB power to be slightly less than 5V if going through a hub. Is this a direct connection or are there one or more hubs between your board and the PC?

It's also possible there's an overcurrent condition and the resettable fuse on the board is dropping voltage. See if you can find the component marked F1 and touch it....is it warm?

-- The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, light sensor, potentiometers, pushbuttons


Measuring with black lead to GND on the board. Red lead to Pin6 ( which is set to output high via SW ) or 5V Pin result : 4.00 Volt.

Measuring 3.3 Volt on the 3v3 Pin.

Measuring 5 Volt in the USB Plug.

No thinks are connected to the board. :-/

which is set to output high via SW

And configured as an output?

yes, configured as output:


int EnablePin =  6;    //

// The setup() method runs once, when the sketch starts

void setup()   {                
  // initialize the digital pin as an output:
  pinMode(EnablePin, OUTPUT); 
  digitalWrite(EnablePin, HIGH);

// the loop() method runs over and over again,
// as long as the Arduino has power

void loop()                     



If the 5V power output pin has only 4V it never mind what is programmed. Please measure the voltage at both sides of the [ch8222]golden[ch8220] fuse near the USB port. If there are 5V then the drop must be caused at the MOSFET NDT2955.

Have a look to this thread and see if it really is a NDT2955 or something else: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1284034373

And if you have a power supply (8....12V) use it for an external supply of the Arduino via the black female plug. Centre pin must be plus!!

Edit: Put your finger tip onto the NDT2955 and feel if it is warmed up.

Yes, that works! I have checked it with an external supply (12 Volt ). With that supply the Voltage on the Pins is as expected ( = 5 Volt ).

But i would like to use my board without external supply ( USB only and always connected to the pc ). Is there a way for an easy and cheap hack on the board that give me the full 5 Volt on the Pins without negativ side effects ( USB Protection still working )?

That is the board i have ( or a clone of it ): http://arduino.cc/en/Main/ArduinoBoardDuemilanove

Best Regards and thx a lot so far !

Ok, now we know that the ICs (components with more than 3 legs) are not damaged. I think that the MOSFET NDT2955 is exchanged for a not full compatible one. Or more probably, the signal comming from the IC LM358D to the gate of the MOSFET hasn't the right level.

What to do? If you like and if you have a steady hand you can solder a wire bridge across the MOSFET from source to drain. But then it isn't allowed to supply the board from extern if at the same time it is connected via USB to your PC. So take care!!

Here is the data sheet of the NDT2955 to find source (s) and drain (d): http://www.fairchildsemi.com/ds/ND%2FNDT2955.pdf

The transistor is on the left side of the board just below the USB connector.

Edit: The 500mA fuse on board will still protect the board and the PC.

I will take care that i not use an external supply if i solder this wire bridge. Thx a lot for this detailed information to you all and especially to WalterM.

I will try the MOSFET wire bridge approach.

Best Regards newnew