Abstract: A new touch igniting transistor arc welding power source with super-low open circuit voltage (U₀≤20V) has been developed successfully by using large power transistors (I_cm=30A). Mathamatical model for average and constant current control system based on Hall element is established and analysed, and the improvement of the system is propounded. The designed touch igniting control system is suitable for real time control of the open circuit voltage and the output characteristic of the power source. In order to accelerate the divergence from initial micro-arc to stable welding arc, the system has an unstable precipitous arising characteristic, and so that it makes touch igniting convenient and reliable. The presented results showed that this power source has excellent average and constant current properties, stable welding arc, and the touch igniting is convenient, reliable and no contaiminate.