Modem and phone protection. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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You can protect the modem and phone from the possible consequences of nearby lightning discharges using the protection unit assembled according to the scheme shown in Fig. 1. Despite its extreme simplicity, it made it possible to forget about frequent failures and subsequent repairs of the input circuits of the ZYXEL OMNI 56K modem.
Fig. 1
Although this modem is specifically recommended for rural and other telephone lines subject to interference, it is not equipped with overvoltage protection. And impulse overload is often observed, for example, during a thunderstorm near the route along which the line is laid. For a year of operation without protection, the modem failed twice for this reason. One accident occurred as a result of an accidental ingress of mains voltage into the telephone line.
The main protection is provided by high-speed limiting diodes (suppressors) VD1 and VD2. According to their current-voltage characteristics, they resemble zener diodes, but they have high speed (of the order of 1 nsec). For comparison, the response time of gas dischargers intended for the same purposes is 1 ... 2 μs, and varistors - 25 ns. The pulse power dissipated by a protective diode of the type indicated in the diagram can reach 1500 W (with an average of not more than 5 W), and the pulse current can be 100 A.
Resistors R1 and R2 limit the current flowing through the protective diodes at the time of the overload pulse. And the fusible inserts FU1, FU2 break the circuit if the duration of the overload is too long. The SA1 switch disconnects the modem from the telephone line during work breaks. This is its kind of additional protection against damage by high-voltage impulses.
Fig. 2
The protection device is assembled by surface mounting in a plastic housing with dimensions of 60x60x33 mm from a standard wall switch (Fig. 2). The fuse holders are from an old TV. You can repeat this design in just a couple of hours.
Two "one-sided" diodes 1.5KE150A with a response voltage of 143. .158 V can be replaced with one "two-sided" 1.5KE150CA. Radio amateurs often use protective diodes soldered from various used devices, often without clear markings. Often such diodes turn out to be faulty. They must be checked before reuse.
Fig. 3
The scheme of the test setup is shown in fig. 3. Here VD2 is the diode under test. It is checked by gradually increasing the voltage supplied from the engine of the laboratory autotransformer (LATR) T1 to the diode bridge VDi, and observing the readings of the voltmeter PV1. LATR can be replaced with a variable resistor with a resistance of 100 kOhm and a power of at least 1 W.
Of course, the proposed device does not provide protection from the direct impact of a lightning discharge. This requires a much more complex multi-stage protection. In fairness, it must be said that the energy of a lightning discharge can be so large that it is impossible to ensure one hundred percent guaranteed safety of the protected apparatus with the current level of technology.
Author: A. Zyzyuk, Lutsk, Ukraine; Publication: radioradar.net
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