AC Shunts Research Articles

Precison Automated Measuring System for Accurate Comparison of Resistance Standards and Shunts

In this paper we will present a development of Measurement system for accurate comparison of low resistance standards and AC shunts ranging from 0.1 mΩ to 10 Ω as well as system testing and use. The system consists of a specially built current source, range selector, current reversal module and low cost analog to digital converter. The entire measurement procedure is automated and controlled by LabVIEW program. Subsequent testing has shown that the realized precision resistance measurement system has achieved precision comparable to more expensive commercial devices. The system or system elements can be used both in smart grid applications and in military laboratories for testing new technologie.

Flux modulation scheme for direct current SQUID readout revisited

The flux modulation scheme (FMS) is the standard readout technique of dc SQUIDs, where a step-up transformer links the SQUID to the preamplifier. The transformer's primary winding shunts the SQUID via a large capacitor while the secondary winding connects it to the preamplifier. A modulation flux having a frequency of typically 100 kHz generates an ac voltage across the SQUID, stepped up by the transformer. The SQUID with FMS is customarily operated in the current bias mode, because a constant dc bias current flows only through the SQUID due to the capacitor isolation. With FMS, however, the transformer ac shunts the SQUID so that in reality the operating mode is neither purely current-biased nor voltage-biased but rather nominal current-biased or “mixed biased.” Our objective is to experimentally investigate the consequences of ac shunting of the dc SQUID in FMS and the transformer's transfer characteristics. For different shunt values we measure the change in the SQUID bias current due to the ac shunt using another SQUID in the two-stage readout scheme, and simultaneously monitor the SQUID output voltage signal. We then explain our measurements by a simplified graphic analysis of SQUID intrinsic current-voltage (I–V) characteristics. Since the total current flowing through the SQUID is not constant due to the shunting effect of the transformer, the amplitude of SQUID flux-to-voltage characteristics V(Φ) is less as compared to the direct readout scheme (DRS). Furthermore, we analyze and compare V(Φ) obtained by DRS and FMS. We show that in FMS, the transfer characteristics of the SQUID circuit also depend on the isolation capacitance and the dynamic resistance of the SQUID.

Effect of Current Heating on Accurate Measurements of AC Shunt Resistors at DC to 3 kHz

To propose a structure of high-accuracy shunt resistors, the frequency and current dependencies of ac shunt resistors with different rated current are evaluated both of ac resistance and phase angle. To evaluate these characteristics, modified current-bridge method was developed. The ac resistances of the shunts do not vary considerably with the frequency, up to 3 kHz. The phase angle of ac shunts varies proportionally to the frequency. The current dependence of the ac resistance of shunts seems to change with current value. The current dependence of the phase angle of the shunt remain stable up to 10 A.

AC shunt calibration using a current‐bridge method and its validation

An AC shunt calibration system using a current‐bridge method is developed, and its measurement uncertainties are estimated. A comparison of the calibration results obtained using the current‐bridge method with the results using DC resistance and AC–DC difference methods was performed to verify the system. These comparison results show that the current‐bridge‐based system performs well with small measurement uncertainties for both the AC resistance and phase angle of the AC shunts up to 1 kHz. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Characterization of High-Accuracy, Wideband Transconductance Amplifiers up to 100 kHz

A highly precise and accurate system comprising several ac shunts and an inductive voltage divider (IVD) is proposed for characterizing high-accuracy, wideband transconductance amplifiers (TCAs). Frequency characterization of the shunts and the IVD confirms the wideband (up to 100 kHz) performance of the proposed system. Evaluation of two TCAs up to 100 kHz with the proposed system reveals frequency-, current-, and load-dependent properties as well as phase shifts between the input voltage and the output current. The results for the load dependence show that the load differences have a limited impact on the phase shifts up to 10 kHz.

Development of an extremely precise buffer amplifier for AC shunt standards at audio frequencies

AbstractThe National Metrology Institute of Japan (AIST, NMIJ) launched a project for the development of AC shunt standards. AC shunts allow the most precise measurement of current. A high‐precision AC shunt is an important component of nonsinusoidal power standards. The AC shunt calibration system helps improve the uncertainty assessments of the national primary power standards. The calibration system design with uncertainties of up to 10 ppm requires a precise buffer amplifier which has uncertainties of up to 1 ppm in the frequency range from 10 Hz to 10 kHz, with other uncertainty components taken care of. The two buffer amplifiers dealt with in this paper were verified to have errors within 2 ppm and uncertainties within 1 ppm in in‐phase and quadrature error in the frequency range from 10 Hz to 10 kHz. This also implies a possibility of reducing both in‐phase and quadrature error at 10 kHz within 1 ppm, by taking advantage of both features. This paper describes the circuit design and evaluation results of two different buffer amplifiers. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 178(4): 24–31, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21218

DC Characterization of AC Current Shunts for Wideband Power Applications

The direct-current (dc) properties of a set of recently developed wideband alternating-current (ac) shunts in the range of 30 mA to 10 A are presented. The characterization includes the long-term drift, the temperature coefficient, and the power coefficient, which all appear to have effects on the level of 1-10 μΩ/Ω. After corrections for these effects, the total uncertainty in the shunt resistance is between 1 and 3 μΩ/Ω. These results prove the suitability of the shunts for wideband power applications where both small ac-dc transfer differences (small phase angles) over the whole frequency range and stable dc characteristics are required.

AC Shunt Calibrations at NRC

NRC offers calibration of ac shunts from milliamperes up to 100 A, in the 10 Hz to 100 kHz frequency band, with expanded uncertainties between 10 μA/A for currents in the milliampere range to 50 μA...

A New Method for Measuring the Level Dependence of AC Shunts

We report on a new method for the measurement of the level dependence of the AC-DC difference of current shunts. The method is based on the use of a binary inductive current divider. A method to compare two AC shunts at common ground is described. Measurement results on two 1-A AC shunts, which consist of different numbers of resistors, indicate that the level dependence between 0.5 and 1 A is less than 2 ?A/A with an uncertainty of less than 7 ?A/A from 1 to 100 kHz.

Development of an Extremely Precision Buffer Amplifier for AC Shunt Standards at Audio Frequencies

The National Metrology Institute of Japan (AIST, NMIJ) has launched the project for the development of AC shunt standards since 2007. AC shunts allow the most precise masurement of current. A high precision AC shunt is an important component of non-sinusoidal power standards which is in progress. The AC shunt calibration system helps improve the uncertainty assesments of the national primary power standards. The calibration system design with uncertainties up to 10 ppm requires a precise buffer amplifier which has uncertainties up to 1 ppm in the frequency range from 10 Hz to 10 kHz with other uncertainty components taken care of. The two buffer amplifiers dealt with in this paper were verified to have accuracies within 2 ppm and uncertainties within 1 ppm in in-phase and quadrature error in the frequency range from 10 Hz to 10 kHz. It also implies that there is possibilities to reduce both in-phase and quadrature error at 10 kHz within 1 ppm, by taking advantage of the both features. This paper describes the circuit designs and the evaluation results of two different buffer amplifiers.

High precision calibration of ac current-to-voltage converters

A two-stage transformer, built as a current comparator with a shielded magnetic core, is used to make a novel ac bridge for calibration of ac shunts for large currents and other ac current-to-voltage converters. A fine adjustable reference transresistance is made by current transformation and a non-reactive precision resistor connected to the secondary. By using an auxiliary transformer and resistor, all four two-stage transformer secondary terminals are brought to ground potential and consequently both magnetizing and capacitive errors are kept below one part per million.

Computation and design of ac shunts for testing commutation equipment

The shunt errors were measured on an ac potentiometer type R-56. The voltage terminals of the shunt were connected to the measuring terminals by means of a twisted pair. These tests led to the following conclusions: