Circuit Detects Phone-Line Breaks

Posted on Feb 6, 2014

Alarm system designs often require circuitry that can detect whether a phone line is active or broken. With this type of design, the primary difficulty is drawing less than 5 µA from the phone line over a line-voltage range of 24 to 58 V as the standard dictates. When the phone is in its `on-hook` state, the central office exchange (CO) acts as a

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current source. The circuitry on the line is restricted to impedances that create less than a 5- µA current from this source. When the phone goes off-hook, the phone`s impedance lowers significantly. Consequently, a CO-detected voltage drop is produced. The CO creates the ringing signal by adding a low-frequency signal to this dc bias signal. Shown here is a circuit based on a micropower oscillator biased by the telephone line ( see the figure ). This oscillator generates a differential signal, which is coupled to a detector circuit via high-voltage capacitors. These capacitors supply the required isolation. The detection circuit merely recognizes the presence of the oscillation while presenting negligible output loading to the oscillator itself. The oscillator uses an astable multivibrator. Instead of relying on the traditional collector resistors, which would otherwise become very large, it employs 1- µA current sources (Q4 and Q6). These determine the supply current. Also, capacitor-charging resistors (which establish the oscillation frequency) are based upon current sources of 0. 1 µA nominal (Q3 and Q5). A full-wave rectifier (D1, D2, D3, and D4) and a smoothing capacitor (C6) bias the oscillator from the phone line. C6 has strong impact on the time needed to detect a line break due to the high impedances involved in this circuit. To meet the galvanic isolation requirements for telephones, the oscillator section must...

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