Phone protection. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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In one of the thunderstorms, the author of the proposed article literally exploded an electronic telephone with caller ID (Caller ID). The same thing happened to the neighbors. I had to buy a new phone and start manufacturing a protective device, simultaneously solving the problem of suppressing interference with television reception created by a device with AON.
Overvoltages often occur in wired communication lines caused by exposure to atmospheric electricity and lightning discharges. Especially in rural areas, where communication is carried out by wires suspended from poles. The voltage both between the wires of the line and between the wires and the ground can reach hundreds and even thousands of volts.
Of course, for an ordinary telephone, well insulated from the ground, only direct lightning strikes into the wires are dangerous. Some elements of the apparatus, not equipped with any protection, can simply be destroyed by the current flowing at the moment of discharge. But the electronic "stuffing" of a modern telephone is much less resistant, it can be damaged even by relatively weak impulses induced in a telephone line by distant lightning discharges.
For a device powered by the mains, the potential difference that occurs between the ground and the telephone line under the influence of atmospheric electricity is also dangerous. Applied between the secondary and primary (usually grounded through the mains) windings of a power transformer, it can cause insulation breakdown between windings.
A diagram of the protection device installed in a rural house telephone set is shown in fig. 1. Resistors R1-R4 and capacitors C1, C2 are mounted on a foil fiberglass board.
Areas for soldering parts are cut out in the foil (Fig. 2). The functions of arresters F1 and F2 are performed by metal-free gaps with a width of 0,3 ... 0,5 mm. The board in a metal screen (the author used the case of an automobile relay-regulator) is mounted on the wall of the house at the telephone line input and connected with a cable to a standard X1 telephone socket installed indoors. The screen and the main part of the foil on the board are grounded by connecting them with a wire with a cross section of 2,5 mm2 or more with a metal pin driven into the ground nearby, at least 1,5 m long.
With a direct lightning strike, the arresters F1 and F2 break through the line, closing the current to the ground. The limiting resistors R1 and R2, as a rule, burn out, so they should be mounted in such a way that replacement is not difficult. Chains R3C1 and R4C2 provide additional protection. During a short lightning impulse, the voltage on the capacitors does not have time to rise to a dangerous value.
The described node is quite enough for a conventional phone that does not have electronic nodes, which can be connected directly to the X1 socket. But the electronic device requires additional protection. Two pairs of VD1VD2 and VD3VD4 diodes connected in anti-series do not conduct current under normal conditions. But if the voltage between one or both wires of the line and the ground exceeds the allowable reverse for the diodes, they will breakdown and the static charge accumulated by the line will "drain" into the ground. As a rule, the breakdown is reversible, but the diodes still need to be checked periodically and replaced if necessary. The last stage of protection (zener diodes VD5 and VD6) limits the voltage at the input of the device to a safe value for it. In order not to create a load on the telephone exchange, their stabilization voltage should, with some margin, exceed 60 V for domestic or 48 V for imported exchanges.
Inductors L5 and L6 prevent radio interference generated by the electronic components of the telephone with AON from passing into the telephone line, and inductors L1 - L4 together with capacitors C3 and C4 form a low-pass filter that reduces the level of radio interference penetrating the lighting network.
Capacitors C1, C2 - K15-5. Diodes VD1-VD4 must be of the same type with Uobr more than 100 V. It is not advisable to use diodes with Uo6p above 500 V, this will not give a protective effect.
The device was operated with telephone sets "RUS classic version 27 PRO" and "Spektr-315" both separately and connected in parallel. There were three severe thunderstorms during the summer. As a result of one of them, the resistors R1 and R2 failed. The remaining elements, including the VD1-VD4 diodes, remained intact. - There was no damage to the telephone sets.
It is not allowed to operate a lightning protection device without connecting it to a ground electrode. Do not install or replace components when approaching or during a thunderstorm. It's life-threatening!
Author: A.Ekimov, c.Omutinskoye, Tyumen region
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