An Automatic RMS/DC Comparator

The design of an instrument for the automatic comparison of an ac voltage with a stable dc source is described. A differential multijunction thermal converter is used as an rms/dc converter with an FET-switched input amplifier for ac/dc substitution. The output voltages of the rms/dc converter with ac and dc input voltages are sampled and stored, and the difference amplified and displayed on a panel meter or chart recorder. Accuracy is ±20 ppm of input ranges of 10-200 V at frequencies of 50 Hz-1 kHz, and maximum full scale deflection sensitivity is 0.01 percent of input range. The instrument may be used either as an rms comparator with a linear voltage scale or as a mean-square comparator with a linear power scale.     

Analysis and performance of novel and highly efficient electronic ballast operating at unity-power-factor

This paper presents the electronic ballast, in which the coupling inductors are used to inject the current to the dc-bus capacitors, to boost the dc-bus voltage and to filter out the ripples from input line current. The current injection coupling inductor injects its stored energy to the dc-bus capacitors in every switching half cycle. Whereas, the boost coupling inductor keeps the dc-bus voltage always above the peak of ac input voltage. Therefore, the proposed ballast maintains unity-power-factor and high efficiency with ripple-free input current over wide range of input line voltage in case of worst regulation. A symmetrical half-bridge inverter is used to drive the fluorescent lamps. The experimental results of the laboratory prototype ballast for 2 × 36 W fluorescent lamps operating at 50 kHz are presented.     

Protection of electrical measuring instruments from overloading by means of silicon diodes

Conclusions It is possible by means of silicon diode stabilitrons to protect from overloading dc and ac voltmeters, micro- and milliammeters (with a measuring range of 10–50 mA), ac ammeters with input transformers, as well as equivalent current and voltage circuits in other instruments.     

A low-noise latching comparator probe for waveform sampling applications

A new latching comparator probe is described. The probe is being developed as part of an effort to augment voltage measurement capability in the 10 Hz to 1 MHz frequency range. The probe offers an input voltage range of /spl plusmn/10 V, input impedance of 1 M/spl Omega/ and root mean square noise referred to the input as low as 55 /spl mu/V. The probe's 3-dB bandwidth is approximately 20 MHz. Total harmonic distortion is as low as -93 dB at 50 kHz. Gain flatness is within /spl plusmn/10 /spl mu/V/V from 100 Hz to 100 kHz. Improved step settling performance is achieved using a technique that minimizes circuit thermal errors. The probe's input range can be extended with a frequency-compensated 1-M/spl Omega/ input impedance attenuator allowing measurement of pulses in the microsecond regime up to 100 V. The attenuator can be compensated further with a digital filtering algorithm to achieve gain accuracy better than 100 /spl mu/V/V.     

Electronic Load-Angle Measurement with Unusually Fast Response

A dc output voltage proportional to the phase angle between any two accessible points in an ac system is generated by the circuitry described in this paper. The transient response of the output voltage involves delays of less than one ac cycle, which represents an advance upon established techniques, and the linearity is of the order of 1 percent. The device has been used for alternator load-angle measurement and in sensing for control of excitation of an alternator in response to load-angle changes. Essentially, a flip-flop is turned on by one of the ac input waveforms and is turned off again by the waveform derived from the other point in the system with respect to which the phase angle is to be measured. To achieve the fast transient response there are six such flip-flops and thus six output pulses per ac cycle, which are then passed through a T-section m-derived filter with band rejection at six times the basic ac frequency. Discrete transistors have been used and their number has been kept to a minimum by the use of circuits with three and six stable states.     

An RMS Digital Voltmeter/Calibrator for Very-Low Frequencies

A portable rms digital voltmeter (DVM) has been developed at NBS to support vibration measurements over the ranges of 0.1 to 50 Hz and 2 mV to 10V. A self-contained calibrator provides for self-calibration and may be used for calibrating other VLF voltmeters. The calibrator basically consists of a Kelvin-Varley divider fed by a reference voltage (either dc or sinewave generated by a ROM-DAC combination) A multijunction thermal converter (MJTC) was selected as the sensing device in the rms/dc converter of the DVM since its low ac/dc difference facilitates calibration of the ac calibrator. Factors affecting accuracy and response time are analyzed. The DVM response time is 40 s for the lowest input frequency. Its accuracy (percent of reading) is 0.1 percent above 0.5 Hz and 5 mV and 0.2 percent below these values. The ac calibrator accuracy is 0.02 percent. Measurement accuracy improves by a factor of about 4 for transfer measurements (comparing input voltages with ac calibrator voltages) Means for extending this accuracy to 0.01 percent are discussed.     

H2对彩色AC-PDP中Ne-Xe混合气体放电性能的影响

使用AC—PDP宏放电单元测试了AC—PDP放电单元加入少量H2对Ne-Xe混合气体放电性能的影响。结果表明，Ne-Xe混合气体中加入少量H2时会显著改变AC—PDP的着火电压、维持电压和放电电流等放电性能参数。当输入电压频率一定时，着火电压和最小维持电压都会随着H2含量的变化而变化，并且着火电压和最小维持电压在某-H2含量下存在最小值；当心含量一定时，着火电压和最小维持电压随着输入电压频率的增大而增大；在Ne-Xe混合气体中加入H2后，起始放电电流会随着心含量的增加而增大。但是，当H2含量一定时，起始放电电流会随着输入电压频率的增加而增加，并在某一特定频率下达到最大值；当输入电压频率继续增加达到某一更高的值时，加入H2的Ne-Xe混合气体的起始放电电流反而会降低至未加入H2时相同。     

Probe characterization in ac field measurements of surface crack depth

In the ac field method of crack depth measurement by the Crack Microgauge, the area of the loop formed in the probe gives rise to an induced voltage, which can introduce errors into the depth measurement. In this paper, a method for measuring the probe area is given, and the quality of the probe is thereby characterized. The underlying theory was given previously, and it is applied here to the probe characterization problem. The probe area is determined by two voltage measurements taken on an artificial rectangular flaw machined in an arbitrary metal. By measurements on several such specimens with the same probe, it is confirmed that the area so obtained is a characteristic of the probe and is independent of the specimen material. Thereafter, measurements on various rectangular flaws with probes of different characteristic area were taken, and very good agreement between predicted and real depths was achieved. Both theory and experiments show that probe characterization is of particular importance when this method is used to measure surface crack depths in metals of low permeability such as aluminum.     

High-temperature superconducting SQUID as calibrator of ac voltage dividers and LF voltmeters

The performance of a radio-frequency SQUID made of high-temperature superconducting ceramics as a calibrator of ac voltage dividers and LF voltmeters at frequencies up to 1 MHz is investigated. The measurement technique and errors are analyzed. The obtained results suggest that in principle the high-temperature superconducting device can reproduce the voltage divider scale with an error less than 10^–3 dB within an input signal range from several microvolts to several volts.Translated from Izmerital'naya Tekhnika, No. 9, pp. 47–50, September, 1995.     

NBS Provides Voltage Calibration Service in 0.1-10-Hz Range Using AC Voltmeter/Calibrator

Prompted by the need to support vibration and pressure measurements at frequencies down to 0.5 Hz (with expected future needs to 0.1 Hz), NBS now offers a calibration service for voltage standards and rms voltmeters in the range of 0.1-10 Hz. The means for the service is an "ac Voltmeter/Calibrator," an NBS-developed instrument containing an rms digital voltmeter and ac and dc voltage calibrators. The methods used to calibrate the ac voltage calibrator are discussed; also, application of the ac Voltmeter/Calibrator to the calibration of customers' voltage and voltmeter standards is described. Finally, a multifrequency voltage reference source with frequency-independent amplitude is proposed as a more suitable transfer standard than thermal voltage converters (TVC's) for the 0.1-10-Hz range.     

Evaluation of The Performance of Portable New Design High DC Voltage Measuring System up 40 kV

Portable DC high voltage (HV) measuring system up to 40 kV has been designed and built with DC HV probe and AC/DC voltmeter developed at TUBITAK UME (The Scientific and Technological Research Council of Turkey). The structure and construction of the HV probe and DC voltmeter for evaluating their performance are described. The comparison measurements have been done using the reference DC HV system up to 40 kV. The voltmeter described here was developed for using in the measurement or calibration of DC HVs with high input impedance greater than 100 MΩ. The voltage error obtained in comparison measurements is less than 100 μV/V at 40 kV.     

Harmonic analysis of sideband signals generated in plasmas

When a small ac voltage with two frequencies was biased to a probe in low pressure inductively coupled plasma, sideband current signals were observed. It was found that two frequencies of the small bias voltage are mutually modulated in the plasma, and this modulation results in the sideband current signals. Experiments for measurement of the sidebands were carried out at various pressures, correlations between the sidebands and the plasma state were investigated. The sidebands were not observed when the plasma was not generated; therefore these signals were produced by the nonlinearity of the sheath. The electron temperature could be obtained from the sideband signals, and it was in good agreement with that from a single Langmuir Probe.     

Harmonic reduction in capacitive micromachined ultrasonic transducers by gap feedback linearization

The nonlinear relationship between the electrical input signal and electrostatic force acting on the capacitive micromachined ultrasonic transducer (CMUT) membrane limits its harmonic imaging performance. Several input shaping methods were proposed to compensate for the nonlinearity originating from the electrostatic force's dependence on the square of the applied voltage. Here, we analyze harmonic generation in CMUTs with a time-domain model. The model explains the basis of the input shaping methods and suggests that the nonlinearity resulting from gap dependence of the electrostatic force is also significant. It also suggests that the harmonic distortion in the output pressure can be eliminated by subharmonic ac-only excitation of the CMUT in addition to scaling the input voltage with the instantaneous gap. This gap feedback configuration can be approximated by the simple addition of a series impedance to the CMUT capacitance. We analyze several types of series impedance feedback topologies for gap feedback linearization. We show that for subharmonic ac excitation, although resistive and capacitive impedances result in a trade-off between input voltage and harmonic distortion for a desired pressure output, harmonic generation can be suppressed while increasing the Pa/V transmit sensitivity for proper series inductance and resistance feedback. We experimentally demonstrate the feedback method by reducing harmonic generation by 10 dB for the same output pressure at the fundamental frequency by using a simple series resistor feedback with a CMUT operating at a center frequency of 3 MHz. The proposed methods also allow for utilization of the full CMUT gap for transmit operation and, hence, should be useful in high-intensity ultrasonic applications in addition to harmonic imaging.     

Enhanced Local Void and Temperature Measurements for Highly Transient Multiphase Flows

We introduce a novel combined-temperature-and-conductivity needle probe measuring system for local void fraction measurements in multiphase-flow experiments. The system is able to deal with direct sheathed microthermocouples as well as open thermocouples. It consists of a thermoneedle probe connected to a special electronic circuit using an ac measuring technique for the conductivity measurements and a high-gain instrumentation amplifier with very high common-mode rejection for the amplification of the Seebeck voltage generated by the microthermocouple. After system buildup and calibration, the system has been tested in different experiments. In a dropping experiment, the time constant of the temperature measurement of the needle probe system was determined as 3.6 ms. Additionally, steam-water and gas-steam-water flow experiments have been carried out to demonstrate the system performance.     

The probe diagnostics of free-burning arc in the atmosphere

A procedure of ac probe measurements is suggested for spatially unstable and unsteady-state atmospheric-pressure plasma. The procedure is evaluation tested by way of measuring the distribution of concentrations of charged particles in the interelectrode space of extended arc with graphite electrodes, which burns in free atmosphere of air. This arc is distinguished by an atypical geometry of the discharge channel, significant instability of its position in space, and appreciable current and voltage fluctuations.     

An analogue digital double integration joulemeter for ac loss measurement in superconducting magnets

A technique for measuring ac losses in superconducting magnets is described. Two signals, a compensated voltage integral across the magnet and a magnet current differential, are multiplied in a Hall probe multiplier. The result is integrated and digitally displayed. The loss per cycle in a range of 0.1 – 10³ Js^?1 is measured by an electronic joulemeter based on this technique.     

High performance AC–DC control power supply for low voltage ride through inverters

This paper presents a control power supply (CPS) design which is used to feed power to the subsystems of a power converter. The CPS design presented here maintains a constant 24V _dc output even over a wide (90V _rms to 270V _rms ) ac voltage variation at its input for a High Power Converter (HPC). The circuit design and closed loop control design of such a CPS are presented. During starting, the power supply to the control circuitry of the CPS in turn is obtained using a separate start-up power supply. The various design issues and details of the on-board power supply and start-up power circuit to ensure the reliable start-up and shut down of the CPS are analyzed. The CPS was tested for wide input voltage range (v _in ) and the performance validates the design. The CPS has also been tested for an input side voltage sag and its performance allows the HPC to meet the low voltage ride-through requirements.     

The influence of thermally activated flux creep on the ac response of a hard superconductor

A theoretical and experimental study is presented of the influence of thermally activated flux creep on the ac response of a type II superconductor in the mixed state. The theory describes the ac response of a cylindrical superconductor in the mixed state in an axial dc magnetic field and a superposed parallel ac field of low amplitude and low frequency. Thermally activated flux creep gives rise to flux motion at the peak and the valley of the sinusoidal ac applied field and a frequency-independent phase shift of the voltage induced in a pickup coil wound about the sample. The instantaneous voltage at the peak or valley of the ac applied field is related to the critical-current density J _c and the pinning-energy barrier normalized by the temperature U/kT. The ac loss voltage and the voltage waveform were measured for a cold-worked NbZr sample. For temperatures T below 4.2 K, the measured values of kT/U are nearly proportional to the temperature. Both J _c and U were observed to depend on magnetic field history.Research sponsored by the Division of Materials Science, U.S. Department of Energy, under contract W-7405-eng-26 with the Union Carbide Corporation.     

On the layout of low temperature bushings for high ac voltage

Bushings for high ac voltages may cause a noteworthy heat inleak into cryostats due to dielectric losses and heat conduction and rising at least with the square of the voltage. The overall heat inleak from the bushing insulator and other parts can be minimized for given radial dimensions by using the optimum length. Then the heat exchange with the outside at the warm flange region vanishes. To ensure a low heat input insulators with low product values of the dielectric loss factor and the heat conductivity and with a high dielectric strength should be used. At least the prospects for lowering the heat inleak by precooling with cold gas are hinted.     

Simultaneous measurement of current and voltage by use of one bismuth germanate crystal

An optical fiber sensor is presented that allows current and voltage to be measured simultaneously by use of only one block of bismuth germanate crystal. The polarized light from the sensing crystal is split into two light beams: One beam is utilized for current measurement based on the Faraday effect, and the other one is utilized for voltage measurement based on the Pockels effect. Compared with the existing optical sensors that can measure current and voltage simultaneously, this sensor is simple and inexpensive and allows measurement of electric power. The simultaneous measurements of ac electric current from 0.05 to 10 A, voltage from 1 to 235 V, and power from 2 to 1000 W have been achieved with good linear-response characteristics. The input characteristics and measurement uncertainties that are due to the nonlinear error of the sensing system are also discussed.