Constant voltage power supply

The basic elements needed for demineralization experiments are a demineralization cell, solution reservoir and pump, constant voltage power supply, conductivity cell (with temperature regulation) and bridge and voltage and current meters. A typical demineralization assembly is shown in Figure 2. 

All the wiring is done to form a circuit The switch in the circuit is closed so that electricity flows from the constant voltage power supply. All of these things are done after confirmation is received that people have evacuated the testing site. 

To begin with, the constant voltage power supply connected to the isothermal storage testing device is switched on. 

This heat is released into the medium and increases the thermal agitation of all dissolved ions, and therefore the conductance of the system (decreases the resistance). With constant-voltage power supplies, the resultant rise in current increases both protein migration and evaporation of water from the medium. Any water loss increases the ion concentration and further decreases the resistance (J ). Under these circumstances, the current and therefore the migration rate will progressively increase. To minimize these effects, it is best to use a constant-current power supply. According to Ohm s law ... 

The poly(oxyphenylene) was electrodeposited from freshly prepared solution containing 0.23 M of 2-allylphenol, 0.4 M of allylamine, 0.2 M of butylcellosolve (ethyleneglycol monobutylether) in water/methanol mixture (1 1 by volume) by applying a constant potential of 4 V from constant voltage power-supply (Zentro-Elektrik, Type LA 15/156 B) between the cathode (a platinum coil of 1.5 cm area) and the substrate (anode). The current was monitored with a Keithley 177 microvoltmeter and recorded with a Metrawatt, Model SE 780 recorder. The electrodeposited films of poly(oxyphenylene) formed within half of an hour (aprox. 1.5 /xm thick) were rinsed with distilled water and cured at 150°C for 30 min. 

Fig. 9. Experimental setup for impedance measurements with electrochenucal control of membrane impedance platinized platinum electrodes (a) constant voltage power supply, (b) gold minigrid electrode (c) polypyrrole film, (d) 1 M KCl solution (e) constant current ac circuit, (f). At right is a microscopic view of membrane, illustrating effect of membrane potential on ionic resistance (reprinted with permission ft om Ref.

One source of noise of this type is the slow drift in the radiant output of the source. This type of noise can be called source flicker noise (Section 5B-2). The effects of fluctuations in the intensity of a source can be minimized by the use of a constant-voltage power supply or a feedback system in which the source intensity is maintained at a constant level. Modern doublebeam spectrophotometers (Sections 13D-2 and 13D-3) can also help cancel the effect of flicker noise. With many instruments, source flicker noise does not limit performance. 

Power supplies for use with GTAW should be of the constant-current, drooplng-voltage type (Figure 24.22). They may have other optional features such as up slope, down slope, pulsing, and current programming capabilities. Constant-voltage power supplies should not be used for... 

An electrolysis can be carried out by removing the second thermocouple, filling the crucible with a charge, and connecting the leads from the power supply to the cell. An Inconel plug is used to close the thermocouple hole in the heat shield. The d.-c. constant-current power supply is turned on to warm up without any current fed to the cell. The temperature is then raised to the operating temperature of 780°C. at a rate of 200°C. per hour. When the temperature has equilibrated, the voltage is applied, and the current is adjusted to 45 ma. The electrolysis is carried out for 5 days. 

The Mossbauer transmission spectra were recorded in the constant acceleration mode with an Elscint Mossbauer drive unit and a model MFG 3A Elscint function generator, an MVT-3 linear velocity transducer and an MD-3 transducer driving unit, y-ray detection was done with a Reuter-Stokes Kv-CH4 proportional counter driven by an Ortec 401A/456 high voltage power supply. Voltage pulses were introduced into an Ortec 142 PC preamplifier and an Ortec 571 spectroscopy amplifier. Data were collected on a Tracor-Northern NS-701A multichannel analyzer. The data were later analyzed on an IBM 360/370 computer. 

Top scheme of a classical constant-current electrochemical cell, combining a high-voltage power supply with a large resistance. Bottom, an ESI source. The large resistance results here from the ion flow in the air. Reproduced with data from Van Berkel G.J. and Zhou F., Anal. Chem., 67, 2916, 1995. 

For isoelectric focusing (IFF) (see later section), a power supply that provides constant power is advisable. During electrophoresis, current drops significantly because of lower conductivity as carrier ampholytes focus at their isoelectric points and because of creation of zones of pure water. If a constant-voltage supply is used, frequent voltage adjustments may be necessary. Constant-current power supplies are not customarily used in lEF. Pulsed-power or pulsed-field techniques (see later section) require a power supply that can periodically change the orientation of the applied field relative to the direction of migration. 

Recent EC detectors have pulsed-voltage power supplies that maintain a constant current. With no sample, the pulse frequency is very low as the sample enters the detector, the frequency increases to offset the current loss due to the electron-capturing species. The pulse frequency is proportional to the sample concentration and can be used for quantitative analysis. 

The compounds formed often have higher secondary electron emission than metals, so that more of the energy transferred by the ions is used to produce and to accelerate secondary electrons. The increased secondary electron emission, in the case of constant-current power supplies, automatically decreases the cathode voltage for a fixed power setting. It is therefore -better to maintain a constant voltage. In this case the abrupt rate decrease becomes more smoothed out. 

To carry the experiment, an electric power supply is needed to impose a constant current so as to simplify determining the amount of charge that has circulated during the experiment. Since porous materials have high resistance to current flow, a high voltage power supply must be used (about a hundred volts). 

Immediately after the two solutions are in contact, the high-voltage power supply is turned on to induce electroosmotic flow. As the fluid in reservoir 1 displaces the existing fluid in the microchannel, the electric current draw changes. Initially the current is constant and then changes linearly until it reaches another plateau once the initial fluid is completely replaced. The time required for the displacement represents the time required for the solution to travel through... 

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