Prefix to the telephone set for paired lines. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Prefix to the telephone set for paired lines. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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Currently, telephones with touchpad (push-button) dialing and a melodic ringing signal have become very popular. A large number of such devices are produced in the countries of Southeast Asia and are now sold at affordable prices. The proposed device allows it to be used on paired telephone lines.

At telephone exchanges, where the automation elements are step finders, electromagnetic relays and multiple coordinate connectors, station telephone pairing devices are used, and diode circuit separation devices are installed at subscribers. The use of blockers was caused by a shortage of subscriber lines and made it possible to connect additional telephone sets (TS) to the PBX.

When sending a call from the PBX, depending on the polarity on the wires of the common subscriber line, the diodes corresponding to the selected line open, and the telephone call works. To discharge the capacitor in the bell circuit, diode blockers are equipped with a discharge circuit, since the capacitor cannot be discharged through the PBX station devices due to the diodes. These interlocks are designed for connecting telephones with electromechanical bells. As a rule, they do not provide sufficient discharge of the coupling capacitor in the ringing device circuit of electronic telephones. A charged capacitor creates additional resistance in the ringing device circuit and the latter does not work.

The prefix, the diagram of which is shown in the figure, will help to solve this problem.

Prefix to the telephone set for paired lines

At the moment the voltage of the unipolar ringing signal decreases to its lower level, transistor VT1 opens due to the voltage on the separating capacitor, therefore, transistor VT2 also opens, which closes the TA terminals. The coupling capacitor is completely discharged through transistor VT2, and the ringing device is ready to receive the next bell signal pulse.

The device can be mounted in a telephone jack housing.

The set-top box allows you to connect answering machines, radio extenders, faxes, and modems to a paired telephone line.

Author: A.Poroshenko, Chistopol, Tatarstan

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