Patent GB-956913-A (GB)
Title: Improvements relating to electrical measuring instruments
Inventor: CHU TSUNG YIN
IPC: G01R27/02
Filed: 1962-05-15
Abstract: 956, 913. Electric measuring instruments; resistance voltage measurement. EVERSHED & VIGNOLES Ltd. Oct. 19, 1962 [May 15, 1962], No. 18704/62. Headings G1Q and G1U. A crossed coil ohmmeter incorporates a battery powered transistor converter giving a high direct voltage, and a three position selector switch arranged so that in one position the test terminals are supplied with high voltage from the converter, in the second position the terminals are connected to the battery with a Zener diode in parallel with the deflecting coil to block the battery current flow, and in the third position the Zener diode is connected in series with the deflecting coil across the battery to allow current to flow, with a resistance connected across the series combination. A transistor converter TC (Fig. 1) energized by battery 11 through presser switch PB and in position a of the 8 way 4 position selector switch SW1 to SW8 a high voltage generated by the converter is applied to terminals 12 over deflection coil D of a crossed coil instrument movement and limiting resistance R 2 , while control coil C is shunted across the converter output over series resistances R 1 , R 3  so that current in coil D depends on the value of a high value test insulation resistance across terminals 12 and the instrument deflection indicates the resistance value. Alternatively coil C may be energized directly across the battery in series with resistance R 3  only. In position b of the selector switch, converter TC is disabled and battery 11 is connected to energize the control coil C over resistance R 3  as well as deflection coil D over resistance R 4 , fuse 14 and resistances R 5 , R 6  in series, while Zener diode Z1 shunts coil D so as to block current flow from the battery. A low test resistance across terminals 12 produces a current in coil D dependent on the resistance valve so that the latter is indicated by the instrument deflection, while the diode protects coil D from any high voltage across terminals 12 by forward conduction or reverse breakdown. In position c of the selector switch, coil C is energized from battery 11 over resistance R 3 , while coil D is connected across the battery in series with the forward sense of diode Z1 introducing a volts drop of 0. 6 to 1v., to the junction of R 5  R 6 ; R 5  being returned to the battery and R 6  shunted across Z1. Fall of the battery voltage below normal reduces the current D to a proportionately greater extent than that in C so that the deflection referred to a datum mark indicates the state of the battery voltage. In position d of the selector switch, control coil C is connected to battery 11 over resistance R 7  and is in series with resistance R 3  across Zener diode Z2 to maintain constant current therein. Coil D is energized from a voltage source connected to terminals 12 over a fullwave rectifier 15 enabling A.C. to be measured. Zener diode Z1 in series with D gives a quasi-logarithmic voltage scale on the instrument scale. Resistances R 8 , R 9  in series with the rectifier having R 9  capacitance shunted give form factor correction.