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In "RL" N6/92 G., P. Mihaylova recommendations under the heading "How to adapt imported phone", mentioned on the protection of foreign phones-handsets from the "inrush current" in domestic telephone lines using a pair of resistors included between the outlet and the machine. However, in my opinion, the author of this recommendation is not quite right in arguing that "sissy tube" fail due to overcurrent. They refused overload voltage, because when dialing the transistors in the phone on the currentless period of the pulse is locked, the voltage drop in the line is not, and between the collector and emitter of closed transistors appears full line voltage PBX - sixty-odd volts! And import "sissy" is calculated, as is known, at 48 volts.

Offer if modifications are made, to protect your phone from overcharging, to break one of the wires of the line to include a Zener diode for voltage stabilization In 12-18, rated current up to 60 mA. Better suited for this Zener type D.

In the gap of one of the wires directly to the telephone wall Jack, you must include two of the Zener diode, as shown in Fig.1. Depending on polarity, one of the Zener acts as a normal diode, and the second "takes" from the line "extra" 12-18 volts. While the variable component (spoken currents) will not be weakened, because the dynamic resistance of the Zener diode little.

Editor's note

Undoubtedly, the main cause of failure of imported phones is exposure to high voltage in the telephone network. Because of the remoteness of subscribers from the PBX and application in the lines of the cable with a thinner cross section, we have taken as a standard voltage of 60 volts, and not 48 as abroad. Although the most dangerous for chips in the import operation is the voltage, and AC voltage magneto ringing of the telephone exchange, which sometimes reaches 100 or more volts. That is why the first and most important method of protection for owners of imported phones should be to not pick up the phone at the time of the call, it is necessary to wait for the pause. This especially applies to calls before long-distance telephone, when the inductor parcel much longer.

For reliable protection conversational circuitry from the high voltage breakdown must be installed for the diode bridge device is a Zener diode with a voltage stabilization In 60-80, as shown in the diagram in Fig.2. If the Zener diode will have a voltage of less than 60 V, when dialing, it will shunt the line, and the set will be impossible. On the other hand, the higher the voltage stabilization VD1, the less effective protection is obtained.

The Zener diode must be connected over a diode bridge, but certainly after lever switch, otherwise nobody will call after the first parcel VOAVA, the caller will hear the busy tone. Incidentally, such a scheme with the inclusion of the Zener diode is applied by the manufacturers and in some domestic telephone devices.

Sometimes to protect import the IC from breakdown used this effective method of protection: to the pulse input circuit IR (most imported chip is pin 9, HM9100Bv, WE9192B - 16, KS5805A, NMA and TR50981AN - 18) is connected a Zener diode with a voltage stabilization 10-12 V. connection Scheme shown in Fig.3.

The Zener diode will limit the voltage at the output of the chip. However, it cannot be selected with the voltage stabilization In less than 8, as the scheme will not work.

Author: A. Savchuk (U05SA); Publication: N. Bolshakov, rf.atnn.ru