Telephone line holding circuit activating transistor system

摘要

A controllable semicondcutor switch has a conductive path coupled across a line terminal, and has a control element to be selectively coupled to the line terminal via operated contacts of a push button hold key serially connectd with contacts of hook-switch. The control element is coupled to a storage capacitor, in which upon operation of the hold key the storage capacitor is charged by line current and charge voltage renders the controllable switch conducting. The charge voltage is maintained when the hold key is released by the conducting path of a second controllable switch a control element of which is coupled to the conducting path of the first controllable switch. A third semiconductor switch has a conducting path coupled across the line terminal via the contacts and has a control element coupled to the conducting path of the first semiconductor switch and to a second storage capacitor. The third semiconductor switch is rendered conducting when subset is subsequently brought into off-hook mode, thus, rendering the two semiconductor switches non-conducting.