DC Current Measurement Research Articles

Compensation for light intensity variation by superposing AC magnetic field in optical measurement of DC current

AbstractOptical voltage and current measurement generally necessitates the compensation of light intensity variation which occurs in the optical measuring system, because it is possible the light intensity variation may give rise to a measuring error. Therefore, such compensation methods as dividing the ac component by the dc component of the light intensity are usefully introduced in optical ac voltage and current measurement, but those methods are not useful exactly for the optical measurement of the quantities including dc components.This paper deals with a new useful compensation method for light intensity variation in the optical dc current measurement. Linearly polarized light passes through two kinds of Faraday sensors, in which one detects dc magnetic field caused by dc current and the other detects externally applied constant ac magnetic field. Then, the compensation of the light intensity variation is carried out by measuring the dc and ac components of the beam. The principle of the compensation method using the ac component is described in the paper. Experimental results of this method using flint glass as the Faraday effect cells are reported and it is clarified that this method can decrease the measuring error of the modulation depth for dc current and compensate light intensity variation validity.

Rectification of current in quantum point contacts made from AlGaAs-GaAs heterostructures

We report a conductance asymmetry of quantum point contacts (QPC) at near-zero applied current biases. The asymmetry is measured at low temperatures as a rectified dc voltage response to an applied small amplitude ac current. When measured as a function of negative gate-voltage, the rectified signal oscillates. For some of the samples this oscillatory behavior scales linearly with the applied ac current amplitude, which indicates only one step-like non-linearity in differential resistance vs. current. The value of the step is of the order of a few tens of Ohms on a background of several kOhms, and embedded in intrinsic random telegraph signal (RTS) noise. The step could be verified also by a direct measurement of differential resistance vs. dc current. We believe a two-level impurity configuration near the constriction is responsible for the step-like differential resistance.

Observations of Schottky and Poole-Frenkel emission in lead phthalocyanine thin films using aluminium injecting electrodes

DC current density-voltage measurements have been performed on vacuum evaporated aluminium-lead phthalocyanine-gold sandwich structures for both electrical polarities. The electrical characteristics differed markedly depending on whether the positive hole-injecting electrode was gold (forward-bias) or aluminium (reverse-bias). An extensive study of such characteristics revealed two distinct regions in the forward-bias characteristics. In the first of these a power-law dependence of current density (J) on voltage {V) was observed with exponent n∼4, and this was identified with space-charge-limited conductivity (SCLC) controlled by an exponential distribution of trap levels of approximate concentration 1025 m−3 in agreement with the authors’ earlier work. In the second region a square-law dependence of J on V was observed, which was identified with SCLC controlled by traps of approximate concentration 1026m−3 and situated at a discrete energy level. Under reverse-bias, current density was considerably lower...

Corroborating cutoff frequency measurements with DC gain measurements

A test for the correct measurement of the cutoff frequency of bipolar transistors is described. The method tests the equality of the low frequency, small signal, current gain measured by a network analyzer, and by DC current measurements. The method is described, and sample data are shown to demonstrate its practical application.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A computerized system for beam measurements and control at the KFUPM 350 kV ion accelerator

In this paper the computerized measurement and control system which is recently developed for the 350 kV ion accelerator at KFUPM is described. Waveforms from a beam profile monitor are captured by the facility's VAX 11/785 data acquisition and analysis computer. Derived beam parameters allow accurate measurement of beam stability and more precise beam focusing and steering compared to schemes relying on dc current measurement at a Faraday cup. Measurement and control processes run simultaneously with experimental data acquisition. Hardware and software aspects are described and effects of simultaneous acquisition of experimental and waveform data are discussed. Results of beam stability runs are presented together with measured transfer function characteristics of focusing and steering elements.

Sensing and measurement of DC current using a transformer and RL-multivibrator

A circuit for sensing and measurement of DC current using a transformer and RL-multivibrator is proposed. The bridge multivibrator circuit is obtained, connecting the primary coil of a nonlinear transformer to an operational amplifier. The DC current that has to be measured is applied to the secondary of the transformer. It is shown that the DC voltage measured at the inverting node of the operational amplifier gives the information on polarity and value of DC current in the secondary. The transfer characteristic of such a DC current sensor is linear for small values of the measured DC current. >

DC and AC transport current measurements up to 140 A in melt-grown YBCO superconductors

Transport currents of up to 140 A have been injected into melt-grown YBCO to measure AC and DC critical currents. Values of critical current densities of 16000 A cm-2 (zero field), 2000 A cm-2 (1 T) and 550 A cm-2 (5 T) were obtained at 77 K. The 50 Hz AC critical current densities are also reported, with values of 23000 A cm-2 and 12000 A cm-2 (RMS) in applied fields of 1 and 5 T respectively at 77 K. Silver contact pads enabled steady-state currents of up to 7000 A per cm2 of contact area to be injected into the sample without any observable heating effects in applied fields up to 5 T. It should be noted that the 50 Hz results are believed to correspond to the onset of flux depinning in contrast with the DC measurements where a 3 mu V cm-1 detection level for flux creep was applied.

Nature of convection-stabilized DC arcs in dual-flow nozzle geometry. I. The cold flow field and DC arc characteristics

An experimental investigation of the steady-state low current air arcs in a dual-flow nozzle system is presented. The cold flow field with no arc was determined for various nozzle geometries, i.e. two- and three-dimensional and orifice nozzles, and nozzle pressure ratios. Supersonic flow separation and oblique and detached shock waves were observed in the flow field. Using a finite-element computer program, the Mach number contours were determined in the flow field for various nozzle-gap spacings and pressure ratios. In addition, the DC arc voltage and current measurements were made for an electrode gap spacing of approximately=5.5 cm and current levels of I approximately=25, 50, and 100 A for the three nozzle geometries. The arc voltage and arc power increased rapidly as the flow speed increased from zero to sonic velocity at the nozzle throat. The shock waves in the converging-diverging nozzles resulted in a decrease in the overall resistance by about 15%. >

Electrooptic methods for measurement of small DC currents at high voltage level

Electrooptic methods for measurement of small DC currents at high voltage level

Electrooptic Methods for Measurement of Small DC Currents at High Voltage Level

Two electrooptic methods for measurement of DC currents at high voltage level, extending from the nA range and up to the milliampere range have been developed. First, by switching the polarity of a measured DC voltage into a Pockels cell, DC currents can be measured and transmitted along an optical fibre to an electrooptic converter. Second, by use of an electronic circuit the measured signal can be converted into a modulated frequency form for transmission along an optical fibre. These systems are described, measurement results are presented and improvements to be made in the future are outlined. The measuring methods can be used both for development and supervision of electrical insulating systems. For DC measurements a system wherein the voltage is applied (across the Pockels cell) not directly but via an electrooptic circuit was developed. This circuit periodically inverts the polarity of the voltage across the cell, effectively applying a square wave voltage with amplitude equal to the DC voltage to be measured. The switching circuit is based around two high voltage transistors TA, TB, with the Pockels cell electrodes being each connected to one of the transistor collectors. The transistor collectors are connected via resistors RA and RB to the protective side of the voltage to be measured and the emitters to the negative side. The currents flowing in to the bases of the transistors are independently controlled by the light levels following on the two photodiodes PDA, PDB.

DC Current Measurement Using Magnetic Coaxial Pipe

In a method previously reported by the authors for detecting high-frequency currents using a magnetic coaxial pipe, it was found that although the permeability of the amorphous ribbon comprising the magnetic pipes changes with the magnetic field strength, the terminal voltage of the outer pipe remains nearly sinusoidal. If similar operation could be obtained when a dc current bias is added to change the level of flux in the pipe, the permeability at the operating point changes and then noncontact dc current detection with a sine wave output should be possible by using magnetic coaxial pipe. Based on this consideration, the authors conducted various studies. Their results are presented.

Rapid diffusion of cesium through metal foils: Diffusion limiting at high current densities

The surface ionization of Cs on foil samples of Ir and some other materials was investigated at high temperatures and large atomic Cs beam flux densities. Rapid diffusion through the foil was shown to exist, at current densities up to approximately 5 mA cm -2. The size of the ion current from both sides of the sample was diffusion limited and reached a temperature and flux independent value, for both pulsed (1–10 μs) and DC current measurements. There was no indication of a space charge limited behaviour under the conditions of the experiments. The behaviour observed is not compatible with the equilibrium Saha-Langmuir description of surface ionization. Most of the results, however, agree with our recently published model for coupled surface ionization and diffusion. Diffusion in the metal crystallites and desorption above the vacuum level over a small or vanishing desorption barrier are important processes in the model.

Rapid diffusion of cesium through metal foils: Diffusion limiting at high current densities

The surface ionization of Cs on foil samples of Ir and some other materials was investigated at high temperatures and large atomic Cs beam flux densities. Rapid diffusion through the foil was shown to exist, at current densities up to approximately 5 mA cm −2. The size of the ion current from both sides of the sample was diffusion limited and reached a temperature and flux independent value, for both pulsed (1–10μ) and DC current measurements. There was no indication of a space charge limited behaviour under the conditions of the experiments. The behaviour observed is not compatible with the equilibrium Saha-Langmuir description of surface ionization. Most of the results, however, agree with our recently published model for coupled surface ionization and diffusion. Diffusion in the metal crystallites and desorption above the vacuum level over a small or vanishing desorption barrier are important processes in the model.

Portable instrument for noncontacting dc current measurements

In practice it is frequently necessary to carry out similar measurements in dc circuits. Several methods are known that are suitable for measurement without direct intervention into the measured circuit [l]. The Hall generator method [2] is based on measuring the intensity of the magnetic field set up by a current-carrying conductor. The accuracy of this method is relatively low. The design of portable instruments based on saturated chokes is impractical since they require an ac current supply and are rather large and heavy. Meters based on nuclear magnetic resonance [4] are complicated, expensive, and require line power. Other devices are not mass produced [5, 6]. The application of each of the above-mentioned methods is thus seen to be limited by the difficulties associated with their practical implementation.

The kinetics of the reduction of protons at polycrystalline and monocrystalline gold electrodes

The reduction of H + from 1 M HClO 4 and 1 M NaClO 4 solutions at polycrystalline and single crystal faces of very pure gold electrodes is studied by determining the forward rate constant k t as a function of potential and of H + concentration. The techniques applied are dc current and impedance measurements both with a step-wise variation of dc potential (duration 4 s), and dc current measurements with a continuous potential variation. The consistency of the results is extensively tested and found to be quite satsifactory. Plots of 1n k t vs. potential are curved and exhibit limiting slopes corresponding to values for the operational transfer coefficient α = 1 positive and α = 0.5 at negative potentials. This behavior is discussed in terms of mechanistic models described in the literature and also an altenative mechanism is tentatively proposed. An increase in the rate constants is observed when the purity of the gold is less. The slight differences in the rate constants observed at single crystal faces of the same purity but with different crystallographic orientation are discussed.

A study of the reduction of tris-oxalato Fe(III) from aqueous 1 M oxalate solution at a polycrystalline gold electrode

The reduction of the Fe(III)-oxalate complex at a polycrystalline gold electrode from a solution of 1 M K 2C 2O 4+0.05 M H 2C 2O 4 has been studied by dc current and impedance measurements. The results indicate a multi-step mechanism of the CEC type, i.e. the electron transfer is preceded and followed by a heterogeneous chemical reaction. An EC mechanism (preceding reaction not present or fast) can also fit the experiments if the individual transfer coefficient of the electron-transfer step is allowed to be 0.4 instead of 0.5. The conclusions are compared with earlier results obtained at the dropping mercury electrode.

Experimental and Theoretical Study of a D.C. Arc in a Constant Diameter Nozzle Flow

The cold air flow field for a 1.27 cm constant diameter nozzle was determined for subsonic and transonic flow velocities. The dc arc voltage and current measurements were made for an arc gap of 5.52 cm and a current of 100A. With a channel flow model with constant arc temperature and the energy integral for the convective cooling, analytical expressions were derived for the arc radius, electric field strength, and arc voltage as functions of the cold flow properties, current, and axial distance. Calculated arc properties agree well with measured values.

Absolute Load Detection with Microwave Gunn Oscillators (Short Papers)

The change in the dc I-V properties of Gunn-flange microwave oscillators with a change in microwave load are shown to provide a method of measuring the physical properties of dielectric samples that only requires the measurement of dc voltages and currents. A phenomenological equivalent-circuit model has been developed that predicts a dependence of detection sensitivity on bias resistance that agrees closely with experiment and that explains the restrictions on such a bias resistor's maximum allowed value. Properties of a prototype system capable of measuring sample size with 8-percent accuracy are presented.

A New Digtal Method for DC-Current Measurement with Floating Input

This paper describes a new method for direct-current measurement without direct connection to the circuit under test, producing a time-coded indication. A magnetizable ring core with three separate windings is driven to positive and negative saturation by a triangular alternating current. The voltage induced in the detector winding is differentiated. The time interval between two zero-axis crossings of the differentiated voltage is a measure of the current in the measuring winding. Investigations on an experimental setup showed that high resolution and sensitivity and small error of linearity can be achieved. Furthermore the internal resistance can be kept small.

Classified abstracts

The surface ionization of Cs on foil samples of Ir and some other materials was investigated at high temperatures and large atomic Cs beam flux densities. Rapid diffusion through the foil was shown to exist, at current densities up to approximately 5 mA cm-2. The size of the ion current from both sides of the sample was diffusion limited and reached a temperature and flux independent value, for both pulsed (1–10 μs) and DC current measurements. There was no indication of a space charge limited behaviour under the conditions of the experiments. The behaviour observed is not compatible with the equilibrium Saha-Langmuir description of surface ionization. Most of the results, however, agree with our recently published model for coupled surface ionization and diffusion. Diffusion in the metal crystallites and desorption above the vacuum level over a small or vanishing desorption barrier are important processes in the model.