Same-line telephone switching circuit (2)

Same-line telephone switching device (2) The same-line telephone switching device introduced in this article uses thyristor. In addition to having the same functions as the same-line telephone switching device (1), it also has its own characteristics: ① Parallel connection The number of extensions is large; ② the circuit volume is smaller; ③ the device does not need to be divided into positive and negative when connecting to external lines; ④ the installation and wiring are simpler; ⑤ the impact on external lines is smaller during operation. working principle  　　

Diodes VD1-VD4 form a polarity conversion circuit, so that there is no need to distinguish between positive and negative when connecting to external lines. In the static state, each extension is hung up, and the line is equivalent to a DC circuit break, so the thyristor VS is cut off. The figure only shows the circuit of one subunit. In actual use, multiple subunits can be connected in parallel. When a call comes in, the AC ring current voltage breaks down the VD5 Zener diode and triggers the thyristor VS through R2 to conduct it, causing the phone to ring. Assume that phone A picks up the phone first, then phone A forms the DC path of VS, and VS remains in the conductive state, and the light-emitting diode LED lights up to indicate the off-hook status. After phone A picks up the phone, the voltage between the external lines L1 and L2 drops from 60V to about 10V. The thyristors of other extensions remain in the cut-off state because the Zener diodes cannot break down, so picking up the phone has no effect. When each extension phone calls, its working principle is similar. The one who picks up the phone first VS connects first, occupying the line first, and leaving other extension phones in a blocked state (silent). The component selection and production component list is shown in the table below. No. Name Model Quantity R1 Resistor 200Ω 1 R2 Resistor 10K 1 C1 Electrolytic capacitor 100u/16V 1 VD1-VD4 Rectifier diode IN4007 4 VD5 Zener diode 12V 1 LED Light-emitting diode 1 VS Unidirectional thyristor 1A/400V 1 It should be noted that , the voltage stabilization value of the Zener diode is 12V in the table. During specific debugging, it may be slightly adjusted for different switches.