Arduino Components: The Mosfet (field-effect transistor) NDT2955

Mosfets used to switch electronic signals; it has three pins A, B and C, A can be connected to B by applying small current to pin C, but C is isolated from
A and B (this is the true magic).

This is a photo of the Mosfet NDT2955 in Arduino (circled in red)

There is no clear part number on the part, but I get the part number from Arduino EAGLE files and schematic, they are open source and free you can get your copy.

What is the rule of NDT2955 in Arduino, its datasheet clarifies the applications that NDT2955 fits in:

• DC/DC converter
• Power management

But in Arduino Duemilanove it works like a power source selector. In previous versions of Arduino (such as Diecimila or NG) there was a power selector jumper which the user will select which is the power source (external power or USB power). The old jumper location in Diecimila and NG was the same location of the Mosfet NDT2955 in Duemilanove.

In Arduino Duemilanove we do not need to switch the power between USB and external power, the Mosfet NDT2955 will select for us. If there is an external power supply and the Arduino Duemilanove is connected to the computer the NDT2955 will select the external power supply, if we turn off the external power supply the NDT2955 will chose the USB power.