Patent GB-545391-A (GB)
Title: Improvements in and relating to electric measuring instruments
IPC: G01R15/08
Filed: 1941-01-20
Abstract: 545,391. Current, voltage and resistance measurements; temperature compensation. MACADIE, H. S., and AUTOMATIC COIL WINDER & ELECTRICAL EQUIPMENT CO., Ltd. Jan. 20, 1941, No. 742. [Class 37] A multi-range instrument for A.C. and D.C. current and voltage measurements is arranged so that the same tapping is used for both A.C. and D.C. on all, or substantially all, of the ranges and the scale shape is substantially uniform for all ranges. A moving-coil instrument 10 is connected, through a swamp resistance (not shown) and a potentiometer 11, to a double-pole change-over switch 12, 13 which in the A.C. position engages contacts 12a, 13a, bringing into circuit a rectifier 14, and in the D.C. position engages contacts 12b, 13b. A shunt resistance 15 byepasses a predetermined proportion of the rectifier output. The contact 12b is connected to the common negative terminal 16 of the instrument, to the input lead 14a of the rectifier 14, and to one end 18 of an auto-transformer 17. This comprises a number of series-connected sections 19 ... 24, one for each current range. Each section comprises a transformer winding 19a ... 24a in series with a resistance 19b ... 24b. Each section of the transformer winding is such as to give in the rectifier circuit of the transformer the standard predetermined current, e.g. one milliampere, for full scale deflection, and the value of the resistance in each section is such as to form a universal shunt to adapt the instrument to measure for each tapping the same range for D.C. as that given by transformer action on A.C. To avoid temperature errors the resistances of the section transformer windings and the section resistances are in the same ratio as that between the resistance of the movement and its swamp resistance and the resistance 11. To the lowest current range terminal 24 is connected the D.C. contact 13b of the switch 12, 13 and also one end of the tapped voltage multiplier resistance 30. The resistances at the tappings 31 on the multiplier are common to A.C. and D.C. ranges with the exception of the tapping 32 for the lowest voltage range, e.g. 5 volts, which is connected through a resistance 33 to the tapping 23. A switch 36 short circuits a part 30a of the resistance 30 when the instrument is set for A.C. measurements, in order to compensate for the resistance of the rectifier. A battery 50 and terminals 51, 52 are provided for resistance measurements. One terminal of the instrument 10 is connected to a switch 55, which is in the free position shown for D.C. measurements and engages a contact 56 for A.C. measurements. For measuring resistances the resistance is connected between the terminals 16, 52, and the switch 55 engages a contact 57 so that the instrument is in series with the resistance under test and with the battery 50 and the resistance 53, the instrument being shunted by a further resistance 58. The terminal 51 is connected to an adjustable tapping 54 on the potentiometer 11 to compensate for fluctuations in battery voltage. The resistance range may be extended by using the 25 volt or 250 volt tappings on the voltage multiplier, the unknown resistance being connected in series with an outside source of voltage and the meter. Low resistances are measured by connecting between the terminals 16, 51, the circuit of the battery 50 being completed by a jumper lead from the terminal 52 to the 2.5 amp socket 19. When the switch 55 is in the A.C. position in which it engages the contact 56 a small current flows constantly from the battery 50 through the instrument 10 and resistances 60, 53 to compensate for the loss in the transformer and rectifier. A range-doubling system comprising resistances 70, 71 and a switch 72, arranged as described in Specification 423,199, may be provided. All the switch contacts may be operated from a single knob with a range indicator. A transformer having a separate secondary winding may be employed instead of the autotransformer 17. The D.C. and A.C. current ranges may be extended by the aid of external shunts used in connection with a milliampere range for D.C. and a current range for A.C. The same external shunt may be used for A.C. and D.C. by arranging that the millivolt drop of the tapping used for D.C. measurement matches that of the tapping used for A.C.