3-TERMINAL POWER SUPPLY CIRCUIT FOR TELEPHONE SET

摘要

PURPOSE:To cope well even with the level drop of DC voltage received from a telephone circuit by providing a ripple component eliminating circuit including a charging/discharging element set between the 2nd and 3rd terminals and therefore setting the minimum working voltage at a lower level between the 1st and 3rd terminals. CONSTITUTION:The DC voltage having a level higher than the value obtained by adding the voltage level of the 3rd terminal 23 and the minimum working voltage level of the 2nd current mirror circuit 22 is applied to the 1st terminal 19. Under such conditions, a movable contact bar 24d of a changeover switch 24 is connected to the 2nd fixed contact 24b and therefore the circuit 22 is driven. Thus the output current of the circuit 22 is supplied to the capacitor of a ripple filter circuit 26 and the stable output voltage is obtained through the 3rd terminal 23. Then the capacitor of the circuit 26 starts discharging and the stable input voltage is obtained from the terminal 23 in a state where the voltage of the terminal 19 is set at a level lower than the value obtained by adding the voltage level of the terminal 23 and the minimum working voltage of the circuit 22.