Ionization smoke detector with increased operational reliability

In an ionization smoke detector containing an ionization chamber operated at an extra low voltage, said ionization chamber having a sensor composed of a measuring electrode and a counter electrode, the ambient air being accessible to the ionization chamber, the ionization chamber containing at lease one radioactive source for generating ions, an electrical circuit for triggering an alarm, the smoke detector being connected by means of lines with a central signal station which delivers to the lines a detector operating voltage, the improvement which comprises:

• converter means provided for said smoke detector;
• said converter means reducing the detector operating voltage to the operating voltage of the sensor such that such sensor operating voltage is at least five times smaller than the detector operating voltage;
• a first circuit element which is at the sensor operating voltage and controlled by a voltage drop appearing across the ionization chamber;
• said first circuit element upon exceeding a predetermined smoke density becoming conductive and reducing the sensor operating voltage;
• a second circuit element which is at the detector operating voltage and controlled by the sensor operating voltage;
• and
• said second circuit element, when the sensor operating voltage falls below a predetermined value, becoming conductive and triggering an alarm signal;
• said converter means reducing the detector operating voltage to the sensor operating voltage such that the sensor operating voltage is at least ten times smaller than the detector operating voltage;
• said first circuit element comprising a field-effect transistor which is non-conductive in its rest state;
• said field-effect transistor having a gate, source and drain;
• said gate being connected with a measuring electrode of the ionization chamber so that upon exceeding a certain smoke density the field-effect transistor is rendered conductive;
• said second circuit element comprising a cold-cathode tube having a control electrode and constituting a bistable switching element;
• switch means cooperting with said cold-cathode tube;
• said cold-cathode tube having a control voltage which is maintained by said switch means in the rest state below the firing voltage of the control electrode of the cold-chathode tube;
• and
• means for actuating said switch means upon becoming conductive of the field-effect transistor such that the control voltage of the cold-cathode tube slowly ascends until there is reached the firing voltage and the cold-cathode tube ignites.