Telephone line controller. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Telephone line controller. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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The telephone line controller (KTL) is designed to protect the telephone line from unauthorized connection. It signals a short circuit or an open telephone line. When an additional load is connected to the line (resistance not more than 100...150 kOhm), the KTL indicates that the line is busy or is listening.

A simplified version of the KTL (Fig. 1) contains a bridge circuit for generating the supply voltage (VD1...VD4). a threshold voltage control device for a telephone line (relaxation pulse generator based on an avalanche transistor VT1), as well as an alarm line indicator made on an analogue of a thyristor (VT3, VT4) and a pulse generator (VT2).

The device works as follows. When the KTL is connected to the telephone line for the first time, the thyristor "latch" is open, the full supply voltage (about 60 V) is applied to the pulse generators, both generators are working, the HL1 (green) and HL2 (red) LEDs are lit. After touching the "Reset" sensor or pressing the "Reset" button, which can be connected in parallel with the R5, C3 chain, the thyristor latch (VT3, VT4) is activated and self-locking. The operation of the generator on the transistor VT2 is blocked, the device goes into standby (working) mode. The potentiometer R1 is adjusted in such a way that when the handset is off (line voltage is about 12 V), the generator on the transistor VT1 produces rare flashes of light. At the same time, connecting a resistance of the order of 100..150 kΩ in parallel with the load should cause generation failure. Of course, if a parallel connection of a high-resistance load to the line was made before the KTL was connected and configured, the device will only be able to respond to the connection of another parasitic load (a second telephone set, high-impedance headphones, a tape recorder, etc.).

(click to enlarge)

When a non-standard device connected in parallel or in series is installed on the telephone line, which is associated with a break in the telephone line or a short circuit in it for a period of more than 3 s, the thyristor latch opens. Voltage is applied to VT2, and the generator starts to generate red flashes (HL2), which indicates an emergency condition of the telephone line. Generation continues until then. until the "Reset" button (sensor) is pressed.

The delay in the response of the device to a short circuit or line break is determined by the time constant R2 C2. A small value of the time constant of this chain increases the sensitivity of the device, but the probability of a false operation of the device when dialing a number also increases.

Author: M. Shustov, Tomsk; Publication: N. Bolshakov, rf.atnn.ru

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