Adaptation of imported phones. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Adaptation of imported phones. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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Adaptation of imported phones

In "RL" N6 / 92, in the recommendations of P. Mikhailov under the heading "How to adapt an imported telephone", it was said about the protection of foreign handsets from "current surges" in domestic telephone lines - using a pair of resistors connected between the socket and apparatus. However, in my opinion, the author of this recommendation is not quite right, arguing that "sissies-tubes" fail due to overcurrent. They refuse from voltage overload, because when dialing a number, the transistors in the phone are locked for the period of a currentless pulse, there is no voltage drop in the line, and the full voltage of the PBX line appears between the collector and emitter of closed transistors - more than sixty volts! And imported "sissies" are designed, as you know, for 48 volts.

When altering, in order to protect the phone from overload, I propose to include a zener diode for a stabilization voltage of 12-18 V, rated for a current of up to 60 mA, in the break of one of the wires of the line. Zener diodes of the D815 type are best suited for this.

Two zener diodes must be connected to the break in one of the wires directly in the telephone socket, as shown in Fig. 1. Depending on the polarity of switching on, one of the zener diodes works like a regular diode, and the second "takes away" the "extra" 12-18 volts from the line. In this case, the variable component (talk currents) will not be weakened, because the dynamic resistance of the zener diode is small.

Editorial note

Undoubtedly, the main reason for the failure of imported telephones is the effect of increased voltage in the telephone network. Due to the remoteness of subscribers from the PBX and the use of cables with a thinner section in the lines, we adopted 60 volts as the standard, and not 48, as abroad. Although the most dangerous for microcircuits in imported devices is not this voltage, but the alternating voltage of the inductor call of the telephone exchange, which sometimes reaches 100 or more volts. That is why the first and main method of protection for owners of imported phones should be the condition: do not pick up the phone at the time of the call, you must definitely wait for a pause. This is especially true for calls before long-distance communications, when the inductor bursts are much longer.

For reliable protection of the conversational circuit from breakdown by high voltage, it is necessary to install a zener diode with a stabilization voltage of 60-80 V behind the diode bridge of the apparatus, as shown in the diagram in Fig. 2. If the zener diode is at a voltage of less than 60 V, then when dialing a number, it will bypass the line, and dialing will become impossible. On the other hand, the higher the stabilization voltage VD1, the less effective the protection is.

The zener diode must be connected behind the diode bridge, but certainly after the lever switch, otherwise no one will get through to you - after the first sending of the video, the subscriber will hear short beeps. By the way, a similar scheme with the inclusion of a zener diode is used by manufacturers in some domestic telephone sets.

Sometimes, to protect an imported microcircuit from breakdown, such an effective method of protection is also used: a zener diode with a stabilization voltage of 9 -9100 V. The connection diagram is shown in Fig.9192.

(click to enlarge)

The zener diode will limit the voltage at this pin of the microcircuit. However, it cannot be selected with a stabilization voltage of less than 8 V, as the circuit will not work.

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

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