Fixed-voltage sources

Radiation. Protection against high voltage and fixed isotope sources of radiation is usually a matter of shielding and the observance of strict... 

This is Ihe most commonly used inverter for Ihe control of a.c. motors and is shown in Figure 6.28(a). The fixed d.c. voltage from the uncontrolled rectifier converter acts as a voltage source to the inverter. The voltage in Ihe inverter unit is varied to Ihe required level by using a pulse width modulation, as noted earlier. Through Ihe switching circuit of Ihe inverter Ihe frequency of the... 

The design philosophy behind the fixed-base drive circuit is to draw current from a relatively low voltage source (3 to 5 V) which is usually provided by an auxiliary winding on the power transformer. The resistor directly in series with the base (R2 as shown in Figure 3-33) should be on the order of 100ohms. Its... 

Many different types of conductivity cell are available but the principle of operation is the same in the great majority of cases. A small current is passed through the sample liquid between two electrodes using a square wave voltage source (this type of excitation minimises the effects of polarisation) 63 . If the electrodes are of fixed area At and constant distance xe apart, then the conductance is ... 

Fig. 3. Equivalent circuit of amorphous silicon solar cell showing voltage source, photoconductive resistance, and fixed series resistance.

For most applications, an alternative is employed. Recall that, in measuring the resistance of a thermistor, a fixed resistor is normally connected in series with the sensor. If a constant-voltage source ( s) is used, the circuit current is inversely proportional to the total resistance. Then the relationship between the measured voltage drop across the fixed resistor and the thermistor temperature can be almost linear over a range of temperature. The linear part of this curve can be shifted along the temperature scale by changing the value of the fixed resistor. 

Pulse width modulation at a fixed frequency has no minimum on-time restriction. Therefore, current peaking and torque ripple can be minimized. For analysis, a d.c. motor can be modeled as a RL circuit, resistor and inductor, or in series with a voltage source. If the motor current is assumed continuous, then the minimum and maximum motor current can be represented by the following equation ... 

This arrangement of a fixed reference half-ceU and salt bridge (Fig. 23.2) is frequently used in chemical measurements. It is important to perform the measurements with a current as weak as possible (ideally with no current) so that the electrochemical equihbria remain undisturbed. This can be accomplished by applying an equal counter-voltage from an external voltage source in combination with an adjustable resistor. It is much simpler and more precise, though, to use a voltmeter with high internal resistance. 

It should be noted that, if a sample is connected directly to the electric voltage source (fixed potential difference across electrodes), there is no depolarization charges on the external surfaces of the sample. In this case, Ei = 0 and the local field acting on a molecule in the cavity is given by... 

In addition to its influence on r]y p, the effects of series resistance must be properly accounted for when designing an LED circuit. The importance of this effect is partially illustrated in Fig. 9 for two (Al Gai-jjo.sIno.sP LEDs where the series resistance of LED 2 is seen to be greater than that of LED 1, as noted by the smaller slope of the I-V curve for LED 2. As a result, for a fixed voltage, the LED current will vary from LED to LED, due to differences in series resistance. Since the LED brightness is determined by the LED current, it is important to use a current source to drive LEDs, as opposed to the voltage sources that are used for most other electronic devices such as transistors. To further minimize the effect of variations in LED series... 

In the first case, the liquid crystal ceU is disconnected from the voltage source. The free charge on the interface between the liquid crystal and the electrode is fixed, and this case is called fixed charge. The external voltage source does not do electrical work, that is dW = 0. At constant temperature and pressure, the Gibbs free energy. 

Next we consider the second case, where the liquid crystal cell is connected to the voltage source such that the voltage applied V across the cell is fixed. The electric potential in the cell is (z). On top of the cell (z = h), the potential (z = h) = 2 is low. At the bottom of the cell (z = 0), the potential i — 4>2= V- The electric field is in the + z direction. The free surface charge density on the top surface of the liquid crystal cell... 

To calculate the complex impedance, a voltage source was used to fix the voltage at the electrode sites. The current was assessed by integrating the conduction current density and the displacement current density over a predefined face, assuming a harmonic oscillation (Eq. 3-5). 

The cell and electrolyte used in this work are closely related to those used by Van Tamelen and co-workers (36) to demonstrate normal electrolyte fixation of N2. In those previous experiments, an external voltage source was used with either two Pt electrodes or with an A1 anode and a nichrome cathode to fix N. In the photoelectrochemical system, no external voltage source is required to achieve N2 fixation the activation energy for the reaction is provided by light alone. 

The frame with the film is fixed to the translation stage of a reflecting polarizing microscope. Thanks to the x, y motion of the frame, any portion of the film can be positioned above the system of electrodes. The system of electrodes is supplied by two independent voltage sources so that the direction of a dc field can be chosen arbitrarily or a rotating electric field can... 

The photocurrents observed are usually in the range of 1 to 100 pA depending on the power of the light source. A special purification effort is required in order to reduce the dark conductivity below these values (see Section 1.8). Usually, photo current spectra are taken at a fixed voltage. An example is shown in Figure 4. These data must be normalized with respect to the spectral distribution in order to estimate the photoconductivity energy threshold. The ratio of photocurrent to light intensity... 

Through a frequency sweep with a voltage source at 5 mV (peak value) and a fixed resistor R = 120 Q, the gain analysis was carried out. Figure 4 presents the gain-frequency response of V (jo )/V (7a ). ... 

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