HI, Friends . i have a ups which uses oscillator ic cd4047 which gets its power supplied 9 volts ic 7809. from out put of ic which are pin 10 and 11 separate reisistor of value 4.7 are used and after that signal is amplified by transistor d313 which has two big resistor of value 150R and 1k (5 waats). now signal enters a 2.2K resistor after that it goes to fet (irf3710) gate. now problem if when i run fan on this ups fan move slowbut when i check voltage from digital voltmeter they are 240 -250 ( for 220 volts fan) ups is of 500 watts. i am surprised what is going on. i think there is very high thd in the out put of ups . i want to decrease the thd i think it is produced by two amplifying transistors d313 . i saw circuits on the net which only use ic with out amplyfying goes to the power fet gates . as fet are controlled by volts i think if i connect dircet ic out put to the gates of fet thd will be less . am i right??????????????? and how i can check if a fet will work fine at specific gate voltage .for irf3710 i saw data sheet its gate to source volt is +-22 is that volts required to flly on this fet or some thing else. how i can convert this ups into modified sine wave ups ???????? how some body will understand my problem and help me to solve this problem . thanks.