Circuit arrangement for a small PBX system

摘要

The DC voltage required in each case for the electronic units and electromechanical components is derived from the AC mains voltage (Un). In the event of mains power failure while an exchange connection is set up for an extension, this extension is connected directly to the exchange line (AL), bypassing the exchange circuit. The subscriber station concerned is therefore powered in a case such as this from the exchange loop. The actuated switching elements retain their adopted switching position (contacts of a bistable switching relay). A switching means (ZU) which is shared by the connected stations and which indirectly effects the connection to the exchange line (AL) is powered in the event of mains power failure by a miniature battery (NC) until the end of the respective call. The variable exchange line changeover is then cancelled and the exchange line is switched to a pre-defined connected station should the mains voltage failure persist. Once mains power has been restored, the exchange line changeover is cancelled only when no further exchange connection remains set up. A 5-volt miniature battery which is no larger than conventional components and which can be accommodated without difficulty on a printed circuit board suffices to hold an exchange connection which is still set up during mains power failure. The miniature battery is recharged in normal operation from the operating voltage. <IMAGE>