Steve Iveges, a design engineer with Telecor Inc. in Mississauga, Ontario, Canada, received a BSEE in electronics and telecommunications from the University of Belgrade, Yugoslavia. Email address: siveges@telecor.com
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April 14, 2005[Ideas For Design] Telephone Line-Feed Detector Meets FCC Regulations
According to FCC regulations, a telephone line in the on-hook state must not be dc loaded by less than 5 MV of resistance up to 100 V. Also, it must be galvanically isolated from the local ground. In the case of a standard dc line-feed of 48 V, this means that the dc load must be less then 10 µA. Furthermore, the tip and ring must be isolated by a transformer in case the telephone circuit must be locally ground referenced. The figure shows one way to detect the presence...
