Please note that driving the power N channel MOSFET requires a better signal than the typical port pin of an 8051. To drive a MOSFET of this type properly requires a swing from GND to well above the ON threshold of the MOSFET with a fast rise time. The weak pullups in the 8051 just cannot provide the kind of drive needed to charge and discharge MOSFET gate capacitance properly.

Specifically the IRF has a gate capacitance of close to 700 picofarads and requires a V_GS of close to 10 volts to fully enhance the device for conducting its rated current of 10 to 15 amperes.

I am not so sure that even the servo amp device shown in the pictures that Charles shows are capable of adequately driving the MOSFET. I suggest you go to the IR web site and look at their various application notes regarding the proper usage of MOSFETs. There are special chips designed specifically for driving MOSFETS. Some of the manufacturers are ST, MICREL, TELEDYNE, and IR.

If you try to run motors and high current loads without properly driving the MOSFETs into full ON and full OFF states with fast rise times, the devices will get very hot indeed.

In addition to that it is absolutely silly to put 500 ohn resistors in the gate circuit of a MOSFET with high gate capacitance. The resulting RC time constant with the gate capacitance causes the device to take forever to turn on and turn off. Just what you do not want in a high power switching application.


Michael Karas