Patent GB-663548-A (GB)
Title: Electrical circuit testing apparatus
IPC: H03K3/55
Filed: 1948-09-22
Abstract: 663,548. Cathode-ray oscillographs. BENDIX AVIATION CORPORATION. Sept. 22, 1948 [Sept. 27, 1947], No. 24845/48. Addition to 661,087. Class 37. [Also in Group XXVII] In apparatus for testing an electrical circuit, such as the ignition circuit of an internalcombustion engine, there is provided a cathoderay oscillograph 22, the time-base of which is synchronized with an intermittently recurring event in the circuit by means, for example pick-up 34, inductively coupled to the circuit and forming a tuned circuit 35, 36 adapted to be shocked into oscillation in response to the recurring event; the apparatus may be used for producing successive visible patterns indicating the functioning of a cyclically-operating circuit. Either the primary or secondary winding of the ignition system is connected through an amplifier 19 to the Y-plates of the oscillograph 22, the time-base being obtained by a saw-tooth sweep generator 24 connected to the X-plates through an amplifier 25. The generator 24 comprises a condenser 28 connected in series with a resistance 29 across a battery 30 and connected to the amplifier 25 by a gain control 31, which latter automatically or otherwise permits of maintaining the timebase sweep at a substantially constant value irrespective of engine speed. As the voltage builds up on condenser 28, the beam is swept horizontally and only flies back to a horizontal reference position upon the condenser being discharged due to triggering of a thyratron 32. The triggering impulse is obtained from pickup 34 associated with the lead 11 to one of the sparking plugs and is fed to another amplifier 37. The output from amplifier 37 is fed into a condenser 39 in a pulse former 38, the condenser being connected to the negatively biassed control grid 41 of another thyratron valve 40. The anode 45 of valve 40 is connected to a storage condenser 47 which in turn is connected through resistance 53 and a further condenser 57 to the control grid 55 of valve 32. A charge is normally maintained on condenser 57 by connecting it to. a battery 56 thereby supplementing the bias of grid 55 by battery 56. Since condenser 47 is connected across battery 30 it discharges through valve 40 whenever the latter is triggered so that a momentary potential difference across resistance 53 is conveyed to grid 55 to render valve 32 conductive. The pulse former 38 is so designed that condenser 47 can trigger valve 32 once only during each cycle irrespective of the condition of valve 40. The resulting trace on the screen will indicate in firing order the voltage or current changes in, and hence the condition of, the ignition circuits and the components thereof. For test purposes the ignition system may be loaded by a resistance 9 inserted in the lead to amplifier 19. Values of circuit constants are given. The Specification as open to inspection under Sect. 91 comprises also a statement that the apparatus may be used so that the signals directed to the Y-plates may be representative of the gaseous pressure in the engine cylinders, oil pressures or the like, which are indicative of proper engine operation. The pick-up may be of the capacity or voltage divider type and the oscillograph may be of any known kind. This subject-matter does not appaar in the Specifiation as accepted.