Line hold prefix. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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In some cases, it becomes necessary to hang up on one of the two parallel telephones and switch to the other in order to continue the conversation. If you do not use special devices, the conversation will be interrupted and the subscriber will have to call back.
There is a simple line holding scheme, which is shown in Fig. 1.4-11. The prefix can be built into the telephone set, its principle of operation is as follows. When the TA1 tube is placed on the device, the KH1 button is kept pressed. Thyristor VS 1 at the same time opens, causing the HL1 LED to glow. When the telephone is off-hook, the line is usually held at a voltage of 10-20 V, in turn, an open thyristor VS1, taking into account HL1 and resistance R1 with a nominal value of 2-3 kOhm, holds the line at a voltage of more than 30 V. In this case, subscribers are not disconnected.
After that, when the handset is picked up on the TA2 telephone, the voltage in the line will again drop to 10-20 V, which will cause a decrease in the current through the thyristor VS1. If this current is less than the holding current of the thyristor, it will close, resetting the circuit. The TA2 phone then functions in the usual way.
The resistances R1, R2 may have to be selected when using the device on various types of automatic telephone exchange, but if the physics of the process is clear, this does not seem particularly difficult.
Each telephone set can be equipped with such a device.
Publication: cxem.net
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