Device for limiting the working voltage for mechanical switches in telephony includes terminals for connection to a telephone line, a connection and power supply branch for a control circuit extending from a first terminal, the branch having a first switch, the cathode terminal of a first Zener diode...http://www.google.de/patents/US5602914?utm_source=gb-gplus-sharePatent US5602914 - Device for limiting the working voltage for mechanical switches in telephony
