Abstract: In this paper,a newly developed transistor power source is presented-auto tungsten inert gas(TIG)welding for all position in pipe pipe and pipe plate butt welding,in which programmable logic controller(PLC)is adopted.Four regions are divided for all position welding and the parameters in all regions can be regulated respectively.This TIG power source can be used for both of pulsed current welding and direct current(DC)welding.It is shown from experiments that the power source is characterized by the intensity of anti disturbance and the stability of arc during transition among the four regions,therefore,the weld being in identical and continuous form.