Patent GB-814731-A (GB)
Title: Improvements in or relating to valve voltmeters
IPC: G01R15/12
Filed: 1955-09-07
Abstract: 814,731. Voltage measurement. SANGAMO WESTON Ltd. Sept. 7, 1955 [Sept. 9, 1954], No. 25671/55. Class 37. An A.C. sensitive vacuum tube voltmeter comprises a tube having at least cathode, anode and control grid, with a circuit including a series capacitance connecting the control grid to a first input terminal; a circuit including a direct-current meter and a load resistance connecting the cathode to a junction point also connected to a second input terminal; a rectifier capacitance network interconnecting the cathode and the junction point with the capacitance acquiring a charge proportionate to an applied A.C. input; a circuit including a series resistance connecting the capacitance between the junction point and the control grid; and a power supply developing an operating voltage between the anode and the junction point. A triode V whose heater is energized on closure of switch 15 is connected, in the first or direct voltage measurement position of ganged operation switches S1, S2, S3 in a Wheatstone bridge circuit also comprising anode and cathode batteries 21, 22 with a portion of zero balancing potentiometer R18 determined by the position of a slider returned to the negative of battery 22 over resistance R19; the remainder of the potentiometer being in series with variable resistance R21 and indicating instrument 24 across the unbalance diagonal. The input voltage is applied between terminal 20 of the earth line 26 and terminal 19 connected by switch S1 to the live end of a calibrated resistance chain R1 to R12, whose values are graduated so that tappings corresponding to several predetermined ranges of measurement are selectible by range switch S4, whose output is coupled through series resistance R16 and shunt capacitance C5 (for excluding electrostatic charges); and over switch S3 to the grid of triode V, whose internal resistance changes to unbalance the bridge and produce an indicator deflection representing the input voltage in the. selected measurement range. In the second or peak-to-peak voltage position of the operation switches, the alternating input voltage is connected in series with capacitance C1 to the resistance chain R1 to R12 which is now shunted for correction of calibration at frequencies exceeding 2000 c.p.s. by capacitance C2 across R1 to R3; capacitance C3 across R4 and R5; and capacitance C4 across R6 to R12; the input impedance now being equivalent to 1 m# shunted by 50 pf. The voltage tapped off by range switch S4 is shunted by resistance R16 in series with capacitance C8 and connected by operation switch S3 and capacitance C5 to the grid of triode V, and the amplified cathode alternating voltage is coupled over switch S3 and capacitance C6 to rectifier network CR1, CR2 which develop across capacitance C7 a direct voltage representing the peak-to-peak value of the input voltage according to the selected measurement range. This is applied over coupling resistance R20 and switch S3 to the grid of triode V, which operates to unbalance the bridge and produce a corresponding deflection of the indicator 24. In the third or R.M.S. voltage position of the operation switches, the circuit is similar but for the introduction by switch S1 of resistance R15 shunted by capacitance C10 in series with the input, which increases the input impedance and reduces the sensitivity of the instrument to read R.M.S. voltage values.