Raising the interrupting capacity of vacuum gap with transverse magnetic field by means of shunting resistor

The possibility is investigated of raising the interrupting capacity of a vacuum gap with constant axisymmetric magnetic field by means of a shunting resistor with a resistance of 0.5–2 Ohm. The investigations are performed at a current increasing in time with amplitude of up to 1000 A. The dependence of clamping current on the resistance of shunting resistor and on the rate of current rise is found.     

Noise Considerations in Resistance Bridges

A signal-to-noise analysis is made of the Wheatstone bridge, where the unknown and standard resistors may be at different temperatures, a situation which occurs in resistance thermometry. The limiting condition is assumed to be dissipation in the unknown resistor. It is shown that the ratio arms should be low in resistance compared with the unknown resistor, while the standard resistor should be high. This is true to an extreme degree when the unknown resistor is at liquid helium temperatures, and the standard resistor at room temperature. This factor is much less important in high temperature thermometry, where the noise in the unknown resistor will predominate strongly. An impedance step-up device (transformer or tuned circuit) is valuable in raising the bridge signal and noise level above the noise of the first amplifier tube. However, as the step-up ratio is increased, two counterfactors appear, namely, noise in the loss resistance of the step-up device, and grid current noise. There is therefore an optimum step-up ratio, which has been found. The step-up device is permitted to have a different temperature from the unknown and standard resistors, to evaluate the advantage of cooling it. With certain assumptions about the noise and grid current of the first tube it is found that the equivalent temperature of a unity ratio (Mueller) bridge used for liquid helium measurements may be 400°K.     

The Measurement of Load Coefficients of dc Resistance Standards Using ac Loading Currents

A new method of measuring the load coefficients of precision direct current standard resistors is described which has many advantages over the presently used direct current methods. Alternating current is used to provide almost all of the load required in the resistor under test while a small direct current is used in the measurement of direct current resistance changes. Problems of measuring circuit design are discussed relating to the simultaneous presence of alternating and direct currents and because of the alternating loading current supply circuit connected to the standard resistor under test. Possible sources of measurement error are considered and comparative curves of resistance change as a function of temperature and load are presented for several direct current resistance standards of differing resistance value and construction.     

New current-feedback amplifier-based universal biquadratic filter

A new universal biquadratic filter configuration using two current feedback amplifiers (CFA's) is presented. The proposed configuration can realize the second-order lowpass, bandpass, highpass, notch and allpass filters. Both the active and passive sensitivities are no more than unity. The natural frequency ω₀ can be controlled by a grounded capacitor. The quality factor Q can also be independently controlled by a resistor. Moreover, the quality factor will be insensitive to the current tracking errors and output voltage tracking errors of the CFA's. Experimental results that confirm the theoretical analysis are obtained. The results are useful in analog filtering applications     

一种新型的交直流差可计算电阻

提出了一种新型交直流差可计算电阻方案,并建立了数学模型,推导出此种结构的电磁场计算公式,进而计算出此种新型电阻在0～100 kHz内的交直流差.与"同轴线型"、"四回线型"和"八回线型"等方案相比,该结构具有较小的交直流差和较好的机械稳定性.此外,文中提出的结构更为紧凑,因而可使用线径较粗的电阻丝,电阻的自发热问题不明显,不需要使用浸入硅油等散热措施,电阻值的细微调整也比较容易.     

A fast-acting superconducting switch for protection of superconducting coils in the persistent current mode

Superconducting switches which are normally used as shorting switches to operate superconducting coils in the persistent current mode are unsuitable for field energy removal from a coil in case of quenching by a parallel resistor because of the low resistance in the non-superconducting state. Moreover these switches have a long turn-off time compared with the characteristic transient time from the superconducting to normal state of high current density coils.The superconducting switch described in this paper permits switch off of current from 20–90 A within a few milliseconds with a normal resistance of about 200 Ω. The switch is activated inductively by a special winding configuration.     

Effects of a parallel resistor on electrical characteristics of a piezoelectric transformer in open-circuit transient state

This paper investigates electrical transient characteristics of a Rosen-type piezoelectric transformer (PT), including maximum voltages, time constants, energy losses and average powers, and their improvements immediately after turning OFF. A parallel resistor connected to both input terminals of the PT is needed to improve the transient characteristics. An equivalent circuit for the PT is first given. Then, an open-circuit voltage, involving a direct current (DC) component and an alternating current (AC) component, and its related energy losses are derived from the equivalent circuit with initial conditions. Moreover, an AC power control system, including a DC-to-AC resonant inverter, a control switch and electronic instruments, is constructed to determine the electrical characteristics of the OFF transient state. Furthermore, the effects of the parallel resistor on the transient characteristics at different parallel resistances are measured. The advantages of adding the parallel resistor also are discussed. From the measured results, the DC time constant is greatly decreased from 9 to 0.04 ms by a 10 k(omega) parallel resistance under open output.     

Determination of Current Transformer Errors at Primary Currents up to 100 000 A

A method is described for testing precision current transformers, particularly devices with very high values of rated primary current and rated secondary currents up to 200 A. It makes use of the well-known differential method (Arnold, Hohle), substituting, however, the common resistor with an electronic operational amplifier, thus yielding an almost negligible resistance and a very high sensitivity of 0.01 ppm. Results of ratio error measurements are reported at values of primary currents between 500 and 100 000 A.     

A Direct-Current-Comparator Bridge for Measuring Shunts up to 20000 Amperes

A direct-current-comparator bridge for calibrating four-terminal resistors or shunts at currents up to 20 000 amperes is described. Measurements can be made at up to full rated current of the shunts so that the effects of the load coefficient are included. The resistor under test is compared with a reference resistor of higher value by measuring the ratio of the currents through the two resistors required to produce equal voltages across them. A comparator bridge with a range of 100 amperes and errors of less than 1 ppm has been described previously. Improvements to this bridge have been made, the main one being a reversing feature, which permits the currents through the resistors to be reversed in a few milliseconds. This makes an accurate measurement easier, particularly if there is a change of resistance due to heating. By connecting a second comparator in cascade, the range has been extended to 20 000 amperes at an overall ratio up to 2 × 106: 1, with only a slight loss of accuracy; the errors may be a few parts per million. Other applications of the measuring system are the accurate measurement of large currents or the calibration of transductors.     

COMMENT Practical voltage/current-controlled groundedresistor with dynamic range extension

Recently, Maundy, Gift and Aronhime presented a voltage/current-controlled grounded resistor which makes use of bisection of the drain-to-source voltage of a FET to produce a linear resistor with wide dynamic range extension. The purpose of this communication is to bring on record, in the context of the above paper, our works on the same topic published more than a decade back which are closely related to the work reported in the above-mentioned paper but have not been cited therein.     

10GΩ在片高值电阻溯源实验

为解决在片高值电阻参数的溯源问题,组建了一套可溯源的在片高值电阻测量系统,并提出针对该系统的量值溯源方案,实现了在片高值电阻到常规同轴形式标准高值电阻的溯源。组件的测量系统通过额外的探针和线缆将同轴形式的标准电阻器接口延伸至探针末端,使用在片直通对接线将对应探针短接,形成同轴-在片-同轴形式的测量回路,从而将在片测量值与同轴端测量值联系起来,同时给出了保守的不确定度评估方法。使用组建的测量系统进行在片高值电阻溯源实验,实验数据显示在10GΩ点的相对扩展不确定度为0.3%(k=2)。     

Techniques for Traceable Measurements of Small Currents

The Italian national standard of direct current is being extended down to the femtoampere range. Two current sources have been developed. The first one, working in the 100 pA to 100 fA range, is based on a low-frequency trapezoidal wave generator, measured with a sampling digital voltmeter at regular time intervals, which charges and discharges a gas-dielectric capacitor. The second, working in the 10-muA to 10-pA range, makes use of a calibrated resistor to generate a known current. Current is traceable to national standards of voltage, resistance or capacitance, and frequency     

The effect of hydrogen on the conductivity of Ag-Pd thick film resistors

The resistance of silver-palladium thick film resistors decreases and their surface conduction type changes upon heating in a flow of hydrogen at temperatures within 50–100°C or hydrogenation in an aqueous acid electrolyte at room temperature. These effects are due to the reduction of PdO (present in the Ag-Pd film) to Pd by hydrogen entering into the resistor material. In the electrolyte, the resistance of samples starts decreasing at the moment of the current being switched on.     

Reply to: Practical voltage/current controlled grounded resistor with dynamic range extension

In response to Prof. Senani comments to our paper entitled 'Practical voltage/current controlled grounded resistor with dynamic range extension', we wish to state that we were not aware of his previous work done in this area as referenced by his supplied list of references. We ourselves had a long list of references as much work has been done in this area. For this, the authors thank him for drawing it to our attention, given his work relies upon the same technique 'Bisection of the drain-to-source voltage of the FET'. As such we would have cited it had we been aware of it. Therefore we welcome his drawing his previous work to our attention.     

Investigation Into Static and Dynamic Performance of the Copper Trace Current Sense Method

This paper investigates the static and dynamic performance of current sense methods, which exploit the resistive voltage drop across the current carrying copper trace. This approach promises very low cost since no dedicated shunt resistor is required, no additional power losses occur and no extra space on the printed-circuit-board (PCB) is necessary. A microcontroller can be used to calibrate the copper trace resistance and implement a temperature drift compensation by means of a temperature sensor. Given that today almost every electronic device has at least one microcontroller that can provide the small computation power required for this current sensing technique, the additional cost of such a technique is small.      

Improving the Frequency Characteristics of RF Standard Magnetic-Field Generator Employing Loop Antenna

In the HF and VHF region, the standard magnetic-field generator having a loop antenna has been generally used for the calibration of a radio receiver or a field-strength meter. Presented are new standard magnetic field generators which are able to produce a constant magnetic field over a wide band frequency range by means of measuring the loop current through a compensation transmission line and a compensation thermination resistor. A magnetic-field generator in which a thermistor was used as both a compensation resistor and a current sensor was constructed and examined over the frequency range 10 to 50 MHz. The measured values were agreed with theoretical values within ±0.1 dB.     

电阻热效应对力式微固态测风传感器测量精度的影响

本文就电阻热效应对基于风的拖动力原理的微型固态测风传感器测量精度的影响进行了研究.微型固态测风传感器通过溅射在悬梁上的铂电阻测量悬梁弯曲应变来获得风速信息和简单的风向信息.风吹过铂电阻时,除了应变引起电阻阻值变化外,由于热传导、对流、辐射等热效应,其阻值也会随着风速的变化而变化.当热效应引起的电阻的变化趋势和应变引起的电阻变化趋势相反时,传感器的输出先减小后增大,风速测量产生较大误差.文中对两种铂电阻在悬梁上的放置连接方式的传感器进行了理论分析,通过对利用MEMS工艺制作的两种传感器的测试,证实理论分析与实验结果相符.     

Electrostatic modeling and ESD damage of magnetoresistive sensors

The response of a thin-film sensor to the excessive current and/or voltage during an electrostatic discharge (ESD) event is studied. An unshielded magnetoresistive (MR)-like recording head structure is analyzed and modeled from the viewpoint of electrostatic theory. An electrical model for the MR head structure is proposed and used in circuit simulations to study the current flow through the thin-film resistor during a Human Body Model ESD transient. A thermal model for the thin-film resistor burnout is compared with experiment and 2D modeling of the fields and voltages are presented. Finally, Maxwell's method is used to calculate the induced charge on the MR structure when a charged external conductor is present     

Leakage current detection in cryogenic current comparator bridges

Several tests have been developed to locate leakage currents in cryogenic current comparator (CCC) resistance ratio bridges used at NIST to measure ratios of 1000 Ω/100 Ω, 6453.2 Ω/100 Ω, and 10 kΩ/100 Ω. The major advantage of the tests is that they can be performed in situ using the sensitivity of the CCC bridge. These test procedures have been used to reduce the leakage error uncertainty of CCC ratio measurements, linking working standards to the quantized Hall resistance (QHR) and to the NIST calculable capacitor experiment. CCC bridges require that the current which passes through a standard resistor must equal the current through the appropriate CCC winding to very high precision. This can be difficult to verify at or below 1 pA because a large number of possible leakage paths exist. Errors due to six important leakage current paths are given, and the calculated changes in the resistance ratio are compared with measurements made with a controlled leakage resistance in a 100 Ω/1 Ω CCC bridge     

The Application of the Direct Current Comparator to a Seven-Decade Potentiometer

The design and construction of a self-balancing direct current comparator for use in a seven-decade potentiometer is described. The comparator generates an output current whose value, as a proportion of a constant input current, is determined to a very high accuracy by the ratio of the numbers of turns of two windings on a magnetic core. A linear, adjustable voltage scale is obtained by passing this output current through a resistor whose value does not vary with current. Since the voltage adjustment is made by varying turns on a magnetic core and not by means of a resistive divider, the usual problems of contact resistance and thermal electromotive forces associated with this adjustment in conventional potentiometers are avoided. The main sources of error in the comparator and the design techniques used to keep the errors less than the smallest step of the output current are discussed. A self-checking feature whereby the linearity of each step of the output current can be checked quickly and easily is described. The performance of the prototype model is given. The normal range of the potentiometer is from zero to 2 volts in steps of 0.1?V.