The utility model discloses a protective transformer, including coil, casing, binding post and protection circuit, the coil is equipped with the iron core in locating the casing in the coil, the wire of coil adopts bimetal level wire, and the wire center is solid aluminum wire core, and the outer parcel of aluminum wire core sets up copper coil layer, the casing in be full of heat dissipation oil, be equipped with the radiator on the lateral wall of casing, the casing both ends all are equipped with binding post, the output terminal S1 of binding post connect protection circuit, load terminal S2, the protection circuit includes output terminal S1, load terminal S2, electric capacity C, resistance R1, resistance R2, diode D1, diode D2, zener diode D3, triode Q, temperature detect switch K, relay J2, contactor J1 and current transformer L, the utility model discloses accelerated the radiating effect of radiator, the radiating efficiency improves, and overall cost is low, and the loss is low, and whole circuit simple structure effectively prevents to burn out equipment, and the longevity is used in the extension.