yes... but it dint work...none of the programs are running.. drain of d mosfet was at 17V Dc wen I connected one of the digital out of mbed nxp lpc 1768 to the gate of mosfet. thanx..

Drain shouldn't matter, unless the mosfet was broken, then it can become a problem. In general I am also in favor of a not too large resistor between the mbed and the mosfet, but it still shouldn't break.

By the way, you say Vout is 5V, but that is supposed to be 3.3V.

im sorry I meant vu=5V(pin number 39) Vout=3.3V.. and rest of the input-output ports are at 2.2V

That seems like a problem. The LPC1768 has two voltage regulators, one for Vout, the other one for the internal ICs. And by default it has pull-ups enabled on all pins. So if not a program is running which changes that, all pins should be 3.3V. That it is 2.2V is not a good sign.

oh... so what does that mean? when all the switches are off the output must be 3.3V. if its not den it means switches are not proper right? but that must happen to only that pin which ws given to mosfet why all the pins are at 2.2V?

Hard to say, you could try if you can measure the supply voltage of the LPC1768, but for sure something is not going correctly.

ok. thank you... does mbed board has any fuse kind of thing.. (just like we have a fuse in Arduino mega board)

I just checked what the arduino mega has, and the mbed has a similar protection, but limitting the current to 500mA from USB port (when powered from Vin that limitation does not exist), sadly does not stop most types of damage.

even I was worried about the same thing, if its powered frm vin then it wont have amy protection. its ok.. thank you..