Phone-line-voltage monitor meets FCC specs.(design ideas)

EDN, August, 2004 by Peeters, Brad

WHEN YOU DESIGN equipment that interlaces to a phone line, it is often desirable to be able to monitor the dc voltage on the line. This ability can be useful, for example, to determine whether a line is in use before attempting to go off-hook and possibly interrupting somebody's phone call. FCC regulations place strict limits on the amount of leakage current an on-hook device can draw from a phone line. The specifications work out to approximately the equivalent of 5 M[ohm] as the minimum leakage resistance. So, you have the challenge of monitoring the line voltage without exceeding the regulatory limits and also maintaining the galvanic isolation required between the phone line and your equipment. The circuit in Figure 1 shows a method of meeting this challenge with...

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