Dialer chip protection. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Dialer chip protection. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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Many, especially inexpensive, foreign-made telephone sets are designed for a line voltage of 48 V. And although the sellers of such equipment claim that it is adapted to the domestic telephone network, there are a lot of failures in operation. Most often, the transistors of the so-called "current key" and the dialer microcircuit fail.

Most devices are assembled according to the scheme, a fragment of which is shown in the figure. Usually, the NSI output of the DD1 dialer microcircuit is protected by a zener diode VD1 (indicated by a dashed line in the diagram) with a stabilization voltage of 13 ... 15 V. However, if the transistor VT1 fails, for example, its collector junction breaks through, the zener diode does not save. When you try to dial a number, the entire current of the line passes through the microcircuit and it also fails.

Dialer chip protection

This can be avoided by including a resistor with a resistance of 4,3 ... 10 kOhm and a power of 0,125 W, as shown in the diagram by a dashed line. In an emergency, it will limit the current of the microcircuit, and in a normal situation, it will practically not affect the operation. On the boards of some telephone sets there are even holes for a resistor, but a jumper is soldered into them, which, of course, must first be removed.

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