Patent GB-634384-A (GB)
Title: Improvements in electronic valve circuit arrangements
IPC: G05B11/01
Filed: 1947-01-21
Abstract: 634,384. Valve voltmeters. DAVIS, N. R., and SUN-VIC CONTROLS, Ltd. Jan. 21, 1947, No. 1900 [Class 37] [Also in Groups XXXVIII, XL (b) and XL (c)] In a measuring amplifier of kind employing negative feedback, in which the 'difference between the feedback and input voltages is applied to a detecting instrument such as a mirror galvanometer, which in turn controls the amplifier by varying the illumination of a photo-cell, diode limiters are used to prevent overload when excessive inputs are applied. The measuring instrument 2, of Fig. 1, is connected in series with resistances 11, 12 as the diagonal element of a bridge, the arms of which comprise a valve 1 and adjustable resistance 9 and batteries 6, 10. The resistance 11 .is shunted by a series circuit comprising the source of potential to be measured in the form of a thermocouple 5, an adjustable zero-setting source of potential 13, a mirror galvanometer 14 and a resistance 15. The valve 1 is provided with cathode feedback over a resistance 7 and its grid is connected to a tap on a potentiometer comprising resistance 19 and a photocell 18 which is illuminated from the galvanometer 14. Any change in the potential of the source 5 tends to produce a corresponding current in the galvanometer 14 and vary the illumination of the photocell. The corresponding change in the impedance of the cell alters the grid-bias of the valve 1 in such a manner as to change the anode current to a value which produces across the resistance 11 a potential sufficient to balance the change in the original applied potential and restore the galvanometer 14 to the rest position. The meter 2 measures the changes in anode current so produced and is protected in either direction against overload by means of a pair of inversely-poled diodes 20, 21, one of which becomes conductive whenever the applied potential exceeds in magnitude the delay bias which is provided by potentiometers 22, 24. The circuit may be additionally stabilized against sudden changes of input voltage by the connection of a condenser across the resistance 19, which introduces a component of control in accordance with the rate of change of the circuit conditions. In a modified arrangement, Fig. 2, the amplifier comprises two stages, each having a stabilizing cathode resistance. The photo-cell 18 is connected in series with resistances 39, 41, 38, across an auxiliary source of potential 36 and controls the grid-bias of a cathode-follower amplifier 1. The potential appearing across resistances 37, 38 is applied between the grid and cathode of the valve 30, which together with resistances 31, 32, forms a pair of bridge arms as in the arrangement of Fig. 1. The sensitivity is increased by means of a regenerative feedback coupling comprising a resistance 40, whereby part of the cathode current of the valve 30 flows through the resistance 41 in the grid circuit of the valve 1. The phase of the feedback is modified by a condenser 42 in order to render the system stable for transient conditions. The meter 2 is protected against overload as in the arrangement of Fig. 1, by means of diodes 20, 21, and the arrangement is also stabilized against sudden changes of input by means of a condenser 35 in the meter circuit. The Provisional Specification describes also a two-stage amplifier of an alternative form and in addition a circuit using a thyratron, in which A.C. is applied to the anode of the thyratron and in quadrature to the grid so as to restrict the conductive period to the latter part of each positive half-cycle of anode potential. The photo-cell is then connected in series with a negative current supply which may comprise a dry-plate rectifier so as to produce an additional unidirectional bias which modifies the period of firing of the thyratron. The current in the measuring instrument is rendered uni-directional by means of a dry-plate rectifier in parallel with the thyratron and is restricted within safe limits by means of diodes as described above.