Patent GB-880741-A (GB)
Title: Improvements in or relating to valve voltmeters
Inventor: SUTCLIFFE HENRY
IPC: H03M1/00
Filed: 1958-11-14
Abstract: 880,741. Electric selective signalling systems. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 28, 1959 [Nov. 14, 1958], No. 36726/58. Class 40 (1). In a digital voltmeter, a voltage to be measured, referred to as a test voltage, is applied to a voltage detector where it is registered on a capacitor, and then a plurality of different known voltages are applied to the voltage detector where they are compared with the registered test voltage the results of the comparisons being used to determine the particular known voltages which are applied. As described the comparator (Fig. 5, not shown) passes a constant current through a coil 348, Fig. 6, either in a positive sense, if the test voltage is larger than the known voltage compared with it, or in a negative sense if the test voltage is smaller than the known voltage. The three cores shown in Fig. 6 function independently as saturable transformers receiving 50 c/s. mains voltage on their primary windings 324-326. Because the cores are arranged to be saturated for almost the entire length of each mains cycle only positive and negative pulses are produced in the output windings 329-331 and of these the positive pulses are amplified and applied to terminals 345-347. In the case of the centre core 322, which has no biasing winding, this results in a pulse, shown at V 2  in Fig. 7, being applied to terminal 346 at the beginning of each mains cycle. Core 323, which carries a biasing winding 342 through which passes a constant current, produces a pulse V 3  after pulse V 2  and core 321 produces a pulse V 1  either just before V 2  or just after it depending upon the direction of flow of the current through its biasing winding 348. The pulse pair V 1  and V 2  which always slightly overlap in time are used to advance or retard the discharge in one of four multi-cathode counting tubes (Fig. 8, not shown) (depending upon whether V 1  comes before or after V 2 ) which control a series-connected chain of resistors and pulses V 1  and V 3 , which only overlap when V 1  comes after V 2 , are used to advance the discharge in a controlling multi-cathode counting tube. In operation the unknown test voltage, which can range from 0.000 to 9.999 volts, is applied to the voltage detector and registered on a capacitor. A start switch is then operated and a sequence of comparisons, one for each cycle of the mains voltage applied to the cores, Fig. 6, then take place. At the beginning of the first cycle the known voltage produced will be 0.000 volts which will be less than an unknown voltage of say 3.456 volts for example. The first comparison will then cause current to flow in coil 348, Fig. 6, in the positive sense (unknown larger than known) which will cause the pulses V 1 , V 2  and V 3  to be produced in this order. The pulse pair V 1 , V 2  is effective to advance a units counting tube one stage which results in the known voltage rising to 1 volt, and the pulse pair V 1 , V 3  is ineffective as they do not overlap in time. The second cycle follows during which a known voltage of 1 volt is compared with 3.456 volts and since the unknown voltage is still the larger the known voltage is raised to 2 volts. The third and fourth cycles are similar, the known voltage rising to 4 volts. During the fifth cycle as the known voltage of 4 volts is now larger than the unknown voltage, pulse V 1  occurs after pulse V 2  and the glow in the units counting tube is stepped back one stage reducing the known voltage to 3 volts. Also, since pulses V 1  and V 3  now overlap the control counting tube is advanced one step which results in subsequent comparisons advancing the tenths counting tube. Further cycles follow in which known voltages of 3.1, 3.2, 3.3, 3.4 and 3.5 volts are tried in turn until the unknown voltage 3.456 volts is exceeded, whereupon the known voltage is reduced by one increment to 3.4 volts and control is switched to the hundredths counting tube. Further cycles follow and eventually 3.457 volts is found to be too large, so it is reduced to 3.456 volts and the control tube is advanced to a position where it causes the count reached by the other tubes to be retained until a reset button is depressed.