The utility model relates to a mechanical equipment technical field, the concretely relates to transformer. Constant voltage transfomer, including transformer main body, transformer main body is equipped with two wiring ends, is connected with electric capacity between two binding post, and at least one binding post connects the output of a reactor, and the input of reactor is equipped with a power source, including an iron core and the coil of winding on the iron core in the transformer main body, the coil includes side line circle and secondary side coil, becomes a side line ring layer around side line cast of one deck on the iron core, then forms the water pipe layer around the one deck water pipe, and one time the side line ring layer is arranged with the water pipe layer in turn, winds one deck secondary side coil formation secondary side coil layer again. The utility model discloses a reactor makes the voltage of input injectd and makes the flux ratio of transformer reach the saturated high impedance that has before in the range of predesignating, become the low impedance road after the transformer entering is saturated, and the voltage at the both ends of prevention electric capacity is further ascending.