current measurement

Current measurement

The measurement of the rate of passage of electric charges in a circuit. The unit of measurement, the ampere (A), is one of the base units of the International System of Units (SI). It is defined as that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 × 10^-7 newton per meter of length.

In order to establish an electrical unit in accordance with the SI definition, it is necessary to carry out an experimental determination. The ampere cannot be realized exactly as defined. Electromagnetic theory has to be used to relate a practical experiment to the definition.

Since January 1, 1990, working standards of voltage and resistance have provided the foundations of practical electrical measurements. The standard of voltage is based on the alternating-current (ac) Josephson effect, in which voltage is related to frequency. By international agreement the value 483 597.9 GHz/V for the Josephson constant is now used throughout the world. The working unit of resistance is maintained through the quantum Hall effect, with an agreed value of 25 812.807 ohms for the voltage-to-current ratio obtained under certain defined experimental conditions. These values have been chosen to provide the best known approximations to the SI units and have the advantage of reproducibility at the level of 1 part in 10⁸. The working standard of current is derived from measurements of voltage across a known resistor. See Electrical units and standards, Hall effect, Josephson effect

The moving-coil (d'Arsonval) meter measures direct currents (dc) from 10 microamperes to several amperes. The accuracy is likely to be a few percent of the full-scale indication, although precision instruments can achieve 0.1% or even better. Above 1 milliampere a shunt usually carries the major part of the current; only a small fraction is used to deflect the meter. Since the direction of deflection depends on the direction of the current, the d'Arsonval movement is suitable for use only with unidirectional currents. Rectifiers are used to obtain dc and drive the meter from an ac signal. The resulting combination is sensitive to the rectified mean value of the ac waveform.

In the moving-iron meter, two pieces of soft magnetic material, one fixed and one movable, are situated inside a single coil. When current flows, both pieces become magnetized in the same direction and accordingly repel each other. The moving piece is deflected against a spring or gravity restoring force, the displacement being indicated by a pointer. As the repulsive force is independent of current direction, the instrument responds to low-frequency ac as well as dc. The natural response of such a movement is to the root-mean-square (rms) value of the current. The accuracy of moving-iron meters is less than that of moving-coil types. See Ammeter

For radio-frequency applications it is essential that the sensing element be small and simple to minimize inductive and capacitive effects. In a thermocouple meter the temperature rise of a short, straight heater wire is measured by a thermocouple and the corresponding current is indicated by a d'Arsonval movement. In a hot-wire ammeter the thermal expansion of a wire heated by the current is mechanically enhanced and used to deflect a pointer. Both instruments, based on heating effects, respond to the rms value of the current. Above 100 MHz, current measurements are not made directly, as the value of current is likely to change with position owing to reflections and standing waves. See Microwave measurements, Thermocouple

Above 50 A the design of shunts becomes difficult. For ac, current transformers can be used to reduce the current to a level convenient for measurement. At the highest accuracy, current comparators may be used in which flux balance is detected when the magnetizing ampere-turns from two signals are equal and opposite. Direct-current comparators are available in which dc flux balance is maintained and any unbalance is used to servo a second, or slave, current signal. For the highest accuracy, second-harmonic modulators are used, and for lower precision, Hall effect sensors. Electronically balanced ac and dc current comparators make clip-around ammeters possible, in which an openable magnetic core can be closed around a current-carrying conductor. This allows the meter to be connected into and removed from the circuit without breaking it or interrupting the current. See Current comparator

The obvious method for measuring a very small current is to determine the voltage drop across a large resistor. A sensitive voltage detector having very low offset current is required, for example, an electrometer. Electrometers based on MOSFET (metal-oxide-semiconductor field-effect transistor) devices have overtaken other designs where the very highest resolution is required, as they can have offset current drifts less than 10^-16 A. In order to provide a low impedance to the measured current, it is preferable to use the electrometer device in an operational-amplifier configuration. The input becomes a virtual ground, and so stray capacitance across the input connection does not degrade the rate of response of the circuit as seriously as in the simple connection. See Transistor

current measurement

Current measurement

The measurement of the rate of passage of electric charges in a circuit. The unit of measurement, the ampere (A), is one of the base units of the International System of Units (SI). It is defined as that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 × 10^-7 newton per meter of length.

In order to establish an electrical unit in accordance with the SI definition, it is necessary to carry out an experimental determination. The ampere cannot be realized exactly as defined. Electromagnetic theory has to be used to relate a practical experiment to the definition.

Since January 1, 1990, working standards of voltage and resistance have provided the foundations of practical electrical measurements. The standard of voltage is based on the alternating-current (ac) Josephson effect, in which voltage is related to frequency. By international agreement the value 483 597.9 GHz/V for the Josephson constant is now used throughout the world. The working unit of resistance is maintained through the quantum Hall effect, with an agreed value of 25 812.807 ohms for the voltage-to-current ratio obtained under certain defined experimental conditions. These values have been chosen to provide the best known approximations to the SI units and have the advantage of reproducibility at the level of 1 part in 10⁸. The working standard of current is derived from measurements of voltage across a known resistor.

The moving-coil (d'Arsonval) meter measures direct currents (dc) from 10 microamperes to several amperes. The accuracy is likely to be a few percent of the full-scale indication, although precision instruments can achieve 0.1% or even better. Above 1 milliampere a shunt usually carries the major part of the current; only a small fraction is used to deflect the meter. Since the direction of deflection depends on the direction of the current, the d'Arsonval movement is suitable for use only with unidirectional currents. Rectifiers are used to obtain dc and drive the meter from an ac signal. The resulting combination is sensitive to the rectified mean value of the ac waveform.

In the moving-iron meter, two pieces of soft magnetic material, one fixed and one movable, are situated inside a single coil. When current flows, both pieces become magnetized in the same direction and accordingly repel each other. The moving piece is deflected against a spring or gravity restoring force, the displacement being indicated by a pointer. As the repulsive force is independent of current direction, the instrument responds to low-frequency ac as well as dc. The natural response of such a movement is to the root-mean-square (rms) value of the current. The accuracy of moving-iron meters is less than that of moving-coil types. See Ammeter

For radio-frequency applications it is essential that the sensing element be small and simple to minimize inductive and capacitive effects. In a thermocouple meter the temperature rise of a short, straight heater wire is measured by a thermocouple and the corresponding current is indicated by a d'Arsonval movement. In a hot-wire ammeter the thermal expansion of a wire heated by the current is mechanically enhanced and used to deflect a pointer. Both instruments, based on heating effects, respond to the rms value of the current. Above 100 MHz, current measurements are not made directly, as the value of current is likely to change with position owing to reflections and standing waves. See Microwave measurements, Thermocouple

Above 50 A the design of shunts becomes difficult. For ac, current transformers can be used to reduce the current to a level convenient for measurement. At the highest accuracy, current comparators may be used in which flux balance is detected when the magnetizing ampere-turns from two signals are equal and opposite. Direct-current comparators are available in which dc flux balance is maintained and any unbalance is used to servo a second, or slave, current signal. For the highest accuracy, second-harmonic modulators are used, and for lower precision, Hall effect sensors. Electronically balanced ac and dc current comparators make clip-around ammeters possible, in which an openable magnetic core can be closed around a current-carrying conductor. This allows the meter to be connected into and removed from the circuit without breaking it or interrupting the current. See Instrument transformer

The obvious method for measuring a very small current is to determine the voltage drop across a large resistor. A sensitive voltage detector having very low offset current is required, for example, an electrometer. Electrometers based on MOSFET (metal-oxide-semiconductor field-effect transistor) devices have overtaken other designs where the very highest resolution is required, as they can have offset current drifts less than 10^-16 A. In order to provide a low impedance to the measured current, it is preferable to use the electrometer device in an operational-amplifier configuration. The input becomes a virtual ground, and so stray capacitance across the input connection does not degrade the rate of response of the circuit as seriously as in the simple connection. See Transistor