Patent GB-806784-A (GB)
Title: Improvements in or relating to voltage-measuring circuits comprising cathode-ray tube indicators
IPC: G01R13/20
Filed: 1955-06-16
Abstract: 806,784. Voltage measurement. PHILIPS ELECTRICAL INDUSTRIES Ltd. June 16, 1955 [June 19, 1954], No. 17403/55. Class 37. [Also in Group XL (c)] A voltage measuring circuit comprises a cathode-ray tube whose anode and deflection electrodes are A.C. energized in similar waveform and phase at a frequency high with regard to that of the voltage to be measured, and whose deflection voltage amplitude is controlled in response to the voltage to be measured by connecting the deflection electrode to the anode of an amplifier responsive to the measured voltage; wherein the amplifier anode is resistance connected and that of the indicator tube is directly connected to an A.C. supply, their cathodes are connected to A.C. supplies of similar frequency and opposite phase, and the remaining terminal of the A.C. supplies are interconnected. Fig. 1 shows a measuring circuit for a direct voltage applied between earth and the control grid of a triode 15, whose anode is energized through coupling resistance 10 from winding 8 of mains transformer 7 and whose cathode is biased in antiphase (or in phase) therewith from winding 9, to establish a working-point accommodating the variable input voltage. The anode is directly connected to the control electrode of a cathode-ray tube 3 of the tuning indicator tube, whose anode-target is energized from winding 4 in phase with the output of winding 8, and whose cathode is energized from winding 5 in antiphase therewith. All the secondary windings are returned to earth, and the input working range is extensible by introducing negative feedback across a resistance 11 in the cathode return of triode 15, which may be shunted by capacitance 12. Variations of the input voltage produce corresponding variations of the internal resistance of triode 15, which together with resistance 10 functions as a potentiometer correspondingly varying an alternating voltage applied to the control electrode of tube 3 in phase with its anode supply; and on positive half-cycles of the latter, the beam deflection is proportional to the input voltage, but due to persistence of vision the display appears continuous. Linearity of response may be improved and blurring of the display reduced by inserting a negative direct bias voltage into the anode circuit of tube 3 or a positive voltage into its cathode circuit, effective to reduce the angle of flow of its anode current below the 180 degrees of each positive half-cycle, so that its display only appears when triode 15 is fully conducting in its linear condition. The angle of flow may be increased above 180 degrees by inserting a positive bias in the anode circuit or a negative bias in the cathode circuit, and the display then comprises a fixed reference indication corresponding to equality of voltage on the anode and deflection electrode of tube 3. Similar results are obtainable by increasing the angle of flow of triode 15 by positive anode or negative cathode bias, or by decreasing it by reverse bias. Positive cathode or negative anode bias for either triode 15 or tube 3 is obtainable from the rectified half-cycles of current therein by anode or cathode resistances shunted by capacitances, such that the circuit time constant is large compared with the period of the exciting alternating voltage. Fig. 2 shows a modification wherein the control electrode of indicator tube 3 is excited from the common anodes of amplifier triodes 15, 24 whose cathodes are supplied with alternating voltages in phase opposition from earth returned windings 5, 25 of transformer 7 and whose anodes are excited over common resistance 10 from the positive double frequency half-cycles of an unsmoothed fullwave rectifier 30 energized from windings 28, 29, which also supplies the anode of the indicator tube whose cathode is returned to earth. Control grids of triodes 15, 24 are excited by measurement voltages at terminals 17, 27, which are alternatively amplified during successive fundamental halfwaves and displayed as two simultaneous traces by tube 3. A reference indication may be produced by reducing the angle of conduction of the amplifier triodes by inserting positive cathode bias derived from a parallel resistance capacitance combination in the common cathode return. Alternatively, a negative voltage bias may be applied to the anodes from an external source or from a resistance-capacitance combination in series with resistance 10. Negative feedback resistances may be inserted in the cathode returns, while the cathode alternating voltages may be supplied from voltage dividers across windings 28, 29. Fig. 3 shows a modification wherein an indicator tube 36 whose anode is supplied from unsmoothed full-wave rectifier 30 and whose cathode is returned to earth has two control electrodes respectively connected to anodes 37, 38 (also supplied from rectifier 30 over resistances 39, 40) of amplifier triodes 37, 38, having their cathodes supplied with equal phase opposed alternating voltage at fundamental frequency from resistance voltage divider 41, 42, 43 across windings 28, 29 of mains transformer 7. Measurement voltages are applied to control grids of triodes 37, 38 over terminals 45, 46, and are amplified and displayed during successive fundamental halfwaves as two simultaneous traces against reference indications corresponding to equality of voltage between the anode and the control electrodes of tube 36. The anodes of diodes 47, 48 having common cathodes with triodes 37, 38 may be connected respectively through resistances 49, 50 to the control electrode of tube 36 connected to the anode of the opposite triode, whereby the anode voltage excursion of each triode during its cut-off period is limited to a value set by the values of resistances 49 50 and the cathode and anode excitation voltages of the triodes, so that the relative voltages between the control electrodes and the anode of tube 36 during cut-off of the associated triodes are variable to control the position of the reference indications. When negative feedback is applied to triodes 37, 38, the diode and triode cathodes are separated; the former being connected direct to the terminals of a constant voltage antiphased alternating supply (or to earth or a constant direct voltage) while the cathodes are similarly connected over their respective feedback resistances. In a modification (Fig. 4, not shown) the cathodes of the amplifying triodes are earth returned over feedback resistances, while the anodes of diodes having common cathodes with the triodes are connected to antiphased alternating voltages derived from the fullwave rectifier supply over a voltage divider, so that alternate positive half-wave pulses are applied to the cathodes through the diodes, and the amplifier triodes are alternately operative and inoperative. Fig. 5 shows a modification wherein the anode of triode 61 and the cathode of triode 62 are connected to the control electrode 18<SP>1</SP> of indicator tube 36, and through load resistance 63 to one side of earthreturned windings 28, 29 of transformer 7 producing antiphased outputs, while the cathode of triode 61 and the anode of triode 62 are connected to control electrode 18<SP>11</SP> and to the other side of the windings through load resistance 64. Measurement voltages are applied to the control grids of triodes 61 62 over terminals 45, 46 while the anodes of diodes 65, 66, whose cathodes are connected respectively to those of the triodes, are connected to earth. The anode of indicator tube 36 is energized from the unsmoothed double frequency positive half-wave output of a fullwave rectifier, fed from windings 28, 29 while its cathode is earthed. Triodes 61, 62 respectively conduct and are blocked on successive halfcycles of the fundamental, while indicator tube 36 conducts on successive double frequency halfcycles to show a deflection corresponding to each measurement voltage. The cathode of the conducting triode is driven negative to earth over resistance 63 or 64 so that diode 66 or 65 conducts to effectively earth the cathode of the conducting triode and the anode of the blocked triode. A negative feedback resistance may be inserted in the diode earth return, while the cathode of the indicator tube may then be connected to the diode anode, so that a fixed reference indication is obtained. Fig. 6 shows a modification for measuring the difference to be obtained of two voltages applied over terminals 45, 46 to the control grids of triodes 71, 72, whose cathodes are connected over common resistance 73 to winding 74 of alternating current transformer 7, while the anodes are connected both over load resistances 75, 76 to winding 77 whose output is in phase with that of 74; and respectively to the control electrodes 18<SP>1</SP>, 18<SP>11</SP> of indicator tube 36, whose anode is connected to winding 77 and cathode to winding 78 in antiphase therewith. Alternating voltages proportional to the two measurement voltages are simultaneously applied on alternate half-cycles in phase to the control electrodes 18<SP>1</SP>, 18<SP>11</SP>, so that the net deflection is proportional to the difference voltage. Resistance 73 provides heavy negative feedback to the amplifying triodes, and eliminates the effect of small unbalances. D.C. bias may be introduced into the supply circuits of polarity such as to reduce the angle of current flow, and to display a reference value on the indicator tube. The alternating supply from windings 77, 78, 74 of transformer 7 may alternatively be obtained from a single untapped winding energizing a bridge rectifier whose negative pole is earthed (Fig. 7, not shown). Antiphased alternating voltages at fundamental frequency for the anodes and the cathode of the indicator tube are derived from the ends of the secondary winding over blocking capacitors and earth return resistances, while the voltage applied to the cathodes of the triod