| 摘要 |
PURPOSE:To prevent ''resonance ringing'' and to prevent a drop in voltage to an oscillation circuit by controlling the oscillation circuit by the output signal of a constant voltage detection part. CONSTITUTION:When a normal incoming signal arrives at input terminals 4 and 5, the incoming signal is rectified by a diode bridge 8 and smoothed by a capacitor 9. This voltage is applied to the emitter of a transistor (TR) 12 to generate a Zener diode across a Zener diode 14 and then the smoothed voltage value H corresponds to the collector potential of the TR12; the oscillation circuit 10 is put in operation from a control terminal 18 to drive a speaker 11. On the other hand, when a ''resonance ringing'' signal is inputted, its voltage is lower than the voltage of the normal incoming signal, so the voltage does not attains to the Zener voltage of the Zener diode 14. Therefore, the TR12 does not operate and the circuit 10 does not operate either, so that the speaker 10 is not ringed. |
