GB574665A - Electronic voltmeters and ohmmeters - Google Patents

Abstract

574,665. Thermionic valve voltmeters ; resistance measurements. STANDARD TELEPHONES & CABLES, Ltd. April 21, 1944, No. 7409. Convention date, April 24, 1943. [Class 37] A voltmeter circuit comprises two thermionic valves, a resistance connected between the cathode of the first valve and the plate of the second valve, a resistance in the cathode circuit of the second valve, and a current measuring instrument connected from a point between the two valves to earth. Fig. 1 shows a circuit for measuring direct voltages and Fig. 2 shows a modification for measuring alternating voltages. In both arrangements the cathode 7 of a valve 6 is connected to the plate 24 of a second valve 10 through a resistance 9. The cathode 25 of the valve 10 is connected through a bias resistance 11 to the negative terminal of a D.C. source 21, to which terminal the grid 12 of the valve 10 is also connected. The positive terminal of the source 21 is connected to the plate 14 of the valve 6. Cras-filled tubes 15, 16 are connected in series across the source 21 to maintain a constant voltage between the points 17, 18. Two resistances, 19, 10, which may be equal, are also connected between the points 17, 18, the common point 34 being earthed. The voltage to be measured is applied between earth and an input terminal 1 over a potential divider 2, an adjustable contact 4 on which is connected to the grid 5 of the valve 6. For measuring direct voltages a D.C. measuring instrument M is connected in series with a resistance 23, Fig. 1, between a movable contact 22 on the resistance 9 and the point 34. The resistances 19, 20 form the ratio arms of a Wheatstone bridge of which the valve 10, resistance 11, and the lower part of the resistance 9 form the standard arm, and the valve 6 and upper part of the resistance 9 form the unknown arm. The bridge is balanced by adjusting the contact 22 to obtain balance when no voltage is applied to the terminal 1. If a direct voltage is applied to the terminal 1 the meter M then gives a corresponding reading. For measuring alternating voltages the cathode 7 of the valve 6 is connected through a blocking condenser 26, Fig. 2, and a resistance 30 to one terminal of the meter M, the other terminal of which is earthed. A rectifying diode 27 has its plate 31 connected to the junction of the resistance 30 and the condenser 26, the cathode being earthed. A compensating diode 29 is connected so as to send its emission current reversely through the meter M to compensate for the emission current of the diode 27 when no alternating voltage is applied to the terminal 1. The meter M may be calibrated to read direct or alternating potentials. The arrangement may also be used as an ohmmeter for D.C. or A.C., in which case the meter may be calibrated in ohms.