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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Device for monitoring the integrity of the communication cable. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology Cable communication lines have their own characteristics. These are a large length of the main line (up to several tens of kilometers), a large number of wires in the cable, the presence of signals with an amplitude of up to several tens of volts in the wires adjacent to the tested one, and seasonal changes in the physical parameters of the communication line. A burglar alarm for a cable is usually performed according to the principle of monitoring the integrity of a loop - a pair of wires, at the end of which a resistor of a certain resistance is connected. When the wires are broken or shorted, the input resistance of the loop changes significantly, which is fixed by the signaling device. This solution has proven itself well with a relatively small length of the controlled chain. But when trying to use such a system to monitor the state of a long cable communication line, a problem arose: during transmission over neighboring "pairs" of the inductor call cable (alternating current bursts with a frequency of 20 ... 50 Hz and an amplitude of 80 ... 100 V), false positives are observed signaling, although in reality the integrity of the cable is not violated. In addition, seasonal fluctuations in the parameters of a long cable lead to fluctuations in the input impedance of the loop that are too large for error-free control. The situation is also dangerous when, as a result of cable damage, the high voltage of the ringing message from neighboring wires enters the input of the alarm device. This can damage its input circuit. For example, in the KMG cable (for multichannel sealing equipment), in addition to the usual "twisted pairs", there are also coaxial lines. In them, in addition to a low voltage signal, there is a high direct voltage (up to 2000 V) to power the equipment of intermediate amplifying points. The consequences of such a voltage entering the input of conventional burglar alarm equipment are easily predictable. A variant of control is possible with the transmission of a tone signal of a sufficiently high frequency along the loop. It makes it possible to protect the equipment from unacceptable values of direct or low-frequency voltage. But this option is critical to the fine tuning of the narrow-band filter on the receiving side and to the drift of the control oscillator frequency. In addition, the frequency of the pilot signal must not be chosen too high, so that its influence on adjacent "pairs" in the cable is not noticeable. Another disadvantage of high-frequency control is the possibility of signal penetration to the receiver input through the capacitance between the wires and with a broken loop. With its length of several tens of kilometers, this capacitance can reach tenths of a microfarad. I propose a device for monitoring the condition of a long cable line using symmetrical rectangular pulses. The signal is applied to one of the wires of the pair, and removed for control from its second wire. At the far end of the cable, the wires of the pair are interconnected. The common wire of the generator and receiver is grounded.
The scheme of the device is shown in fig. 1. The master oscillator is made on the elements DD1.1 and DD1.2 in the usual way. Resistor R4 puts the element DD1.1 into active mode. The product of the resistance of this resistor and the capacitance of the capacitor C1 determines the generation frequency. From the output of the element DD1.2, the clock pulses are fed to the counting input of the trigger DD3.1, dividing their frequency by two. From the direct output of the trigger, the pulse sequence through the amplifier, assembled on transistors VT1 and VT2 of different structures, and the capacitor C3 enters the controlled line. The second wire of the line, as already mentioned, is connected to the input of the receiving part of the device. If the line is not broken, then the frequency and duration of the input pulses coincide with the output ones, but they have prolonged fronts and recessions. The degree of distortion depends on the parameters and the length of the line. In the event of a break, the pulses acquire a pointed shape and become bipolar. It is not possible to distinguish between serviceable and faulty lines only by the amplitude of the pulses, therefore, time selection is applied - control is performed in the second half of the pulse duration, when all transient processes have already ended. Pulses from the line through the capacitor C2 and resistor R1 are fed to the input of the shaper on the transistor VT3 and element DD4.1. At the output of the shaper, they have standard logic levels that do not depend on the amplitude of the input signal. Another purpose of the shaper is high voltage protection. It can only damage the VT3 transistor, which is easy to replace. It is also protected by a zener diode VD1. Gating pulses form a node on the elements DD2.1-DD2.3. They arrive at one of the inputs of the element DD4.2, the second input of which is connected to the output of the element DD4.1. At the output of element DD4.3, with a working line, there will be pulses similar to strobe ones, but not with a faulty one. An amplitude detector on the VD4.3 diode is connected to the output of the DD2 element. In the presence of pulses (good line), the output voltage on the smoothing capacitor C5 is sufficient to open the transistor VT4, the HL1 LED is on. If there are no pulses (the line is faulty), the HL1 LED will turn off. Through the capacitor C6, the pulses from the output of the element DD4.3 arrive at the inputs of the installation in the zero state of the counter DD5. Therefore, with a good line, the counter remains in this state, the transistor VT5 is closed, and the HL2 LED is off. If there are no pulses at the inputs of the initial setting, the counter will start to work, counting the clock pulses applied to its input C1. At its output 8 (pin 11), high and low voltage levels will alternate. This will cause the HL2 LED to glow and the sound emitter HA1 to emit a signal. After the fault is removed, the device will return to the low level mode at the output 8 of the counter. The described device is not critical to changing the frequency of the master oscillator, since both control and strobe pulses are generated from it. Since the generator and receiver are located side by side at one end of the controlled cable, the problem of synchronization of these pulses does not arise.
If it is necessary to increase the efficiency of the device, K561 series microcircuits can be used in it with minor changes in the circuit. Capacitors C2 and C3 should be selected for a voltage not lower than that possible in an emergency. For example, if the ringing voltage reaches 80 V, these capacitors must withstand at least 100 V. It is desirable to use not oxide, but film capacitors, although this will lead to an increase in the dimensions of the device. The signaling device is assembled on a printed circuit board shown in fig. 2. All parts are installed here, except for the transistor VT6 with a sound emitter HA1 and LEDs HL1, HL2. These elements are placed on the front panel of the case from a small-sized radio receiver, in which the board is placed. On the walls of the housing there are clamps for connecting a controlled line and a power connector.
The power supply, the circuit of which is shown in fig. 3 is made of electronic ballast from an "energy-saving" lighting lamp, according to the recommendations given in the article by V. Stryukov "Small-sized power supply - from electronic ballast" ("Radio", 2004, No. 3, p. 38, 39). A defective block from a 20 W lamp was subjected to alteration. To restore its performance, it was only necessary to replace the capacitor C2. According to the mentioned article, the ballast choke was converted into a T1 transformer. Its winding I contains 400 turns of PEL 0,1 wire, and winding II is wound with PEL 0,6 wire almost until the frame is filled. Particular attention must be paid to the quality of the interwinding insulation, since the safety of working with the signaling device depends on this. It is best to isolate one winding from the other with two or three layers of varnished cloth. A voltage stabilizer is connected to the output of the rectifier on the diode VD6 on the zener diode VD7 and the transistor VT3. The power dissipated in this transistor is small, so it can operate without a heat sink. The presence of voltage at the output of the unit is signaled by the LED HL1. The power supply board is located in a separate housing (from the power supply unit of the "Electronics" microcalculator). If you add decoupling diodes, then in case of a power failure, you can organize uninterrupted power supply of the signaling device from the battery. The signaling device should, first of all, be connected to a loop that is open at the end, and a steady glow of the HL2 LED should appear (hereinafter, the designations of the elements according to Fig. 1). When the loop is closed at the far end of the line, the HL1 LED will turn on. The resistance of the closed loop should not exceed 1,2 kOhm. The capacitance of capacitors C2 and C3 can be changed downward. High-frequency harmonics will be filtered by the cable itself due to its significant self-capacitance. But if the cable length is short, you can connect a capacitor between the output of the device and the common wire. Its capacitance is selected to minimize interference in adjacent channels while maintaining reliable control of the integrity of the cable. If it turns out that in neighboring communication channels the control signal is heard at a too high level and interferes with the conversation, it is necessary to replace the R9 resistor with a trimmer, and apply a signal to the line from its engine. The signal level should be set only slightly above the level at which the HL1 LED turns on. You can also lower the frequency of the pilot signal by replacing the capacitor C1 with another, larger capacitance. When the device is initially connected to an open loop, the simultaneous illumination of the HL1 and HL2 LEDs is sometimes observed. This indicates that the insulation resistance between the cable wires is not high enough or the capacitance between them is too high. In this case, try to select another one of the free pairs of wires in the cable for monitoring. You can try to use wires from different pairs. The device has been tested on cable communication lines up to 40 km long. It works both when the controlled wires are broken, and when any of them are grounded. Author: A. Dolinin, Baikonur; Publication: radioradar.net
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