GB640257A - Improvements in or relating to discharge tube circuits - Google Patents

Abstract

640,257. Valve circuits. STANDARD TELEPHONES & CABLES, Ltd. March 31, 1947, No. 8667. [Class 40 (vi)] In a circuit arrangement comprising a valve the space current path between the anode and cathode of the valve includes the heater which is maintained at an adequate operating temperature by the space current flow through it. In the circuit of Fig. 2, relay 7 is normally released, current flowing through the heater of the diode 1 from the supply 8, 9. When the current flow in the anode-cathode path reaches a predetermined level, relay 7 operates opening contact 11 to break the heater current supply, the heater then being maintained at the correct operating temperature by the passage through it of the normal space current of the valve. If the anode voltage supply to the valve falls below some predetermined value the current flow is insufficient to maintain the requisite cathode temperature and the valve is rendered non-conducting, contacts 13 opening and actuating an alarm system whilst simultaneously the heater supply to the valve is reconnected via contact 11. The relay will then follow a pulsating cycle of long period if the supply volts remain below the critical value or the circuit may be used to operate a relay after a pre-determined time lag. The rectifier 10 prevents chattering of the relay and limits voltage surges in the circuit. In a modification, Fig. 3 (not shown), a pair of change-over contacts associated with the relay, connect the cathode to either one or other of the heater terminals thus preventing the heater from carrying both the heating and space currents in the initial period before the current flow is such that the relay operates. In a further modification, Fig. 4 (not shown), the heater is connected to the anode instead of to the cathode. A triode valve may be used, Fig. 5 (not shown), a small positive bias being applied to the control grid during the initial heating period by connecting it to a tapping on a resistor in series with the heater circuit.