Patent GB-636113-A (GB)
Title: Electronic timer
IPC: H03K17/288
Filed: 1946-06-12
Abstract: 636,113. Switching systems for consumption apparatus; condenser arrangements. BENDIX AVIATION CORPORATION. June 12, 1946, No. 17639. Convention date, July 30, 1945. [Class 38 (iv)] [Also in Groups XXXIII and XL (c)] In a condenser discharge time-delay switching system the condenser is charged from a D.C. source through a circuit comprising the anodecathode path of an electronic valve, the winding of a relay and a rectifier; the condenser voltage is applied as negative grid bias to the valve, which eventually cuts off, thereby de-energizing the relay which closes its contacts to complete a discharge circuit for the condenser. When the condenser voltage has fallen sufficiently the valve conducts, the relay opens the discharge circuit and the cycle repeats. Fig. 2 shows the time delay circuit controlling the magnet 12 of a stepping switch 25 which sequentially energizes over its contacts 26, and from source 32, the solenoids 39A-K of inflation valves of de-icing boots of an aircraft. The switch also comprises contacts 27 in series with contacts 26 for reducing sparking at the latter and contacts 28 associated with resistors 51 which form part of a timing circuit determining the period of inflation of each boot. A normally closed pair of contacts 52 are opened when the rotary contacts of the switch are in the positions shown. The time delay circuit comprises discharge tube 56-the anode circuit of which includes the stepping magnet 12, a condenser 79, which forms the other element of the timing circuit, and normally closed contacts 24 which are opened upon energization of the magnet 12 and allow the condenser to charge from the source 32 through a circuit including in series a diode 100, tube 56 and magnet winding, as shown more clearly in Fig. 3. A five-position switch 71, operating contact arms 67-70, provides for the following: (1) system inoperative; (2) single cycle operation; (3) continuous operation with 60 seconds interval between cycles; (4) continuous operation with no interval between cycles; and (5) automatic 50 per cent increase in the period of inflation of each boot. With switch 71 set for single cycle operation, as shown, closure of a press switch 64 completes, over conductor 62, the cathode circuit of tube 56 which fires and energizes the stepping magnet 12. Timing condenser 79 charges whilst switch contacts 24 are open, the magnet releasing and stepping forward its switch to energize inflation solenoid 39A when the charging current falls to a low value. The condenser voltage holds " off " tube 56, which now has its cathode circuit completed over the contacts 52 independently of the press switch 64, until its charge has leaked away through resistor 51A, when the tube re-conducts, recharges the condenser and moves the stepping switch forward to energize inflation solenoid 39B and de-energize 39A. The sequence continues until the contacts 52 are again opened by the stepping switch at the end of the cycle. For continuous operation switch 71 is moved into any of the three lower positions when its arm 67 short-circuits press-switch 64 and contacts 52 and so maintains closed the cathode circuit of tube 56. The interval between cycles is determined by resistor 51L, which is tapped by arm 69 of switch 71 to give intervals of about 60 seconds and zero seconds, respectively. In the lowest position of switch 71 arm 69 connects a condenser 95 in parallel with 79 to provide a 50 per cent increase in each discharge period. The solenoids are supplied through an indicator lamp 38 mounted together with the switch 71 in a control box also containing switches 46-50, by which the solenoids may be manually controlled if the automatic control fails. A separate lamp unit may give visual indication of the energization of the solenoids. A resistor 111 may shunt the tube 56 to control the voltage induced in the charging circuit of the condenser by the magnet 12 when contacts 24 open. In the modified arrangement of Fig. 4 a resistor 112 relieves tube 56 of charging current, a condenser 113 being provided across the magnet 12 for momentarily maintaining the contacts 24 open to allow charging of the timing condenser. In another arrangement the magnet of the stepping switch is replaced by an electric motor, energization of which is initiated by conduction of tube 56 and which is maintained over another parallel path whilst the motor drives the switch from one position to the next, whereupon a cam switch driven by the motor interrupts the parallel path and again places the motor under the control of tube 56. Specifications 621,464 and 621,490, [Group XXXIII], are referred to.