Tap-off transformer

A tap-off transformer includes a coil base, a core and wires. The coil base has isolating walls placed outside and using isolating walls to form a first winding slot on a primary side area and a plurality of second winding slots on a secondary side area. Different kinds of wires could be winded inside the first and second winding slots. A wiring start point inside the plurality of second winding slots on the secondary side area is placed on one of the isolating walls, with coils starting from the start point to extend outwards to either sides of the secondary side area and ending at either sides. The coil base has a hollow structure for placing the core to form a transformer structure. The present invention provides isolating walls and a winding design to increase a rated voltage of the transformer and to improve the stability of the transformer.