PURPOSE: To secure the constant ringing tone of the bell by the rise and fall to the input DC voltage via the high frequency transformer after converting it into the high frequency pulse once to minimize the size of the transformer and then changing the pulse duration when the DC voltage or the load has fluctuation.

CONSTITUTION: The DC voltage applied to input terminal 5 is once converted into the high frequency pulse via pulse converter 6 and then raised up or lowered down via high frequency transformer 7. This raised or lowered pulse is rectified through rectifier 8 and then smoothed through smoothing circuit 9 to be supplied to switch circuit 10 and thus to deliver the AC voltage switched alternately to output terminal 11. At the same time, the voltage of the plus and minus polarities given from line C' of circuit 9 are delivered to output terminal 12. Thus the bell of the telephone is rung by the AC power sources of terminals 11 and 12. In case the fluctuation is caused to the DC voltage or the load, the fluctuation of voltage is detected for circuit 9 through detector 13. And this voltage is compared with the reference voltage given from the oscillator at pulse duration alteration part 15. Then the pulse duration of converter 6 is altered by the pulse sent from part 15 to secure the constant ringing tone.