Patent GB-1125902-A (GB)
Title: Improvements in or relating to drive circuits for glow-discharge tubes
Inventor: SKOYLES DEREK ROBERT
IPC: H03K3/284
Filed: 1965-09-21
Abstract: 1,125,902. Measuring speed electrically; voltage measurement. MULLARD Ltd. Sept. 21, 1965, No.40182/65. Headings G1N and G1U. [Also in Divisions H2, H3 and H4] The value e. g. amplitude or frequency of an applied signal is indicated by repeatedly charging an inductor and discharging it into a linear glow discharge indicator tube at a frequency dependent upon the signal so that the length of the glow discharge is proportional to the frequency. The input signal may be derived from a tachogenerator coupled to a vehicle so that the length of glow indicates the vehicle speed. The Specification describes two circuits for feeding the discharge tube, the first a blocking oscillator as claimed in Specification 1,125,903 and the second a monostable multi-vibrator as claimed in 1,125,904. In the blocking oscillator (Fig. 4) when transistor Trl becomes conductive current builds up in inductor L1 and also in the square loop cored transformer abc. This latter has a feedback winding d in the emitter or base circuit blocking oscillator fashion and the current build up continues until the transformer a b c saturates whereupon the transistor is cut off. Inductor L1 now discharges into the transformer resetting its core to the opposite state of saturation. A D. C. signal Id is applied to the base to charge a capacitor C1 but this is charged in the opposite polarity during conduction of the transistor so that the transistor remains nonconducting until the capacitor is recharged. Accordingly the frequency of operation depends on the magnitude of the input signal. The pulses occurring during the cut-off period are applied through a rectifying circuit to a glow discharge tube and the length of glow depends upon the repetition frequency of the pulses and thus of the input signal. In order to ensure that the core is completely reset to its opposite state a resistor R1 is included so that current builds up in L1 more quickly than in a b c and the resistor is by-passed by a diode so that the whole of the energy in L1 is utilized in resetting the core. It is stated that D1 may be replaced by a capacitor, that Tr1 may be replaced by a gate controlled rectifier and that the upper end of the glow tube may be connected to the positive supply rail or to the base of the transistor. The multi-vibrator circuit (Fig. 5) comprises crossed coupled transistors T11, T12. The application of an input pulse to the base of T11 causes it to be cut off and T12 to be rendered conducting for the unstable duration of the circuit so that inductor L1 is first charged and then allowed to discharge into a rectifying circuit feeding the glow tube G. The length of these discharges depends on the repetition frequency and thus the input pulse frequency. In order to make the amount of energy stored in inductor L1 during the conducting period of T12 independent of the HT supply potential, the base of T11 is connected through a voltage limiting Zener diode D11 to a potentiometer R14, R15 the potential of the tapping of which also varies with the supply potential to change the "off" time of T11. Further, to compensate for non-linearity of response due to the resistances in the rectifier circuit there is provided a capacitor resistor combination C12, R13 having a time constant comparable with the trigger period at the higher frequencies. The energy stored in the inductor thus tends to increase with frequency. It is stated that the transistors could be replaced by gate controlled rectifiers. To measure the rotational speed of a stationary machine or of a vehicle the input signal may be derived from a tachogenerator driven for example by a drive shaft or wheel of a vehicle. The output pulses therefrom may be applied directly to the circuit of Fig. 5 or in Fig. 4 are smoothed to provide the required direct current control signal. In the latter case the repetition frequency of the blocking oscillator is higher than the output frequency of the tachogenerator.