FIG. 5 depicts a detecting circuit

FIG. 5 depicts a detecting circuit which may be used as the detecting circuit 3 of the embodiment of FIG. 4. This circuit 3 comprises diodes D10, D20 for rectifying
voltages e1, e2 derived from respective secondary coils; smoothing capacitors C31, C32; potential dividing resis tors R31, R32 and R33,R34 for potential dividing the voltages across the respective capacitors; amplfiers 0P1 and 0P2 connected as shown and adapted to conduct differential amplification of voltages ed2,ed3 and ed4,e,11 obtained across the respective potential amplifiers dividing resistors; and an OR gate OG which is
connected as depicted and adapted to receive output signals from the respective
amplifiers OPl,OP2. The detecting circuit 3 compares the coil voltages e1 and e; (i.e. determines whether the conditions e1 éKeZ and CQZKC] (wherein K; 1) are met) to detect abnormality in the primary side.