| 摘要 |
<p>PURPOSE:To prevent ringing of a bell at polarity inversion by applying polarity inversion through a constant current feeding mode or a high resistance feeding mode. CONSTITUTION:A charging switch cg and a constant current circuit CC are provided in the voltage polarity inverting system of a talking line of a telephone exchange subscriber circuit. Before the polarity inversion, polarity inversion switches r1, r2 and the charging switch cg are restored and a voltage of a power supply VB is connected to them. In activating the polarity inversion switches r1, r2 and the charging switch cg. the constant current circuit CC is connected to a capacitor CB, a current flows to a coil LLB of the bell, but the current value is too low, therefore the bell does not ring. When the capacitor CB is charged to a voltage value of the DC power supply VB with the opposite polarity to that before the inversion and the charging switch cg is restored, the DC power supply VB is connected to the capacitor CB, but no current flows because the capacitor CB is charged already.</p> |
