Constant-current sources

The titrant in a conventional titration is replaced in a coulometric titration by a constant-current source whose current is analogous to the titrant s molarity. The time needed for an exhaustive electrolysis takes the place of the volume of titrant, and the switch for starting and stopping the electrolysis serves the same function as a buret s stopcock. 

Time, Cost, and Equipment Controlled-potential coulometry is a relatively time-consuming analysis, with a typical analysis requiring 30-60 min. Coulometric titrations, on the other hand, require only a few minutes and are easily adapted for automated analysis. Commercial instrumentation for both controlled-potential and controlled-current coulometry is available and is relatively inexpensive. Low-cost potentiostats and constant-current sources are available for less than 1000. 

Directions are provided for constructing an inexpensive constant-current source and demonstrate its use in determining the concentration of HGl coulometrically. 

Large L is essential to smoolhen ripples and provide a near constant current source to the inverter. 

For the amplifier pulse to be recognized in the ADC, it must exceed the lower level set by a discriminator, which is used to prevent noise pulses from jamming the converter. Once the pulse is accepted it is used to charge a capacitor that is discharged through a constant current source attached to an address clock typically... 

The constant current source consists of a battery C and a variable high-resistance resistor R ... 

In order to determine the thermal time constant of the microhotplate in dynamic measurements, a square-shape voltage pulse was applied to the heater. The pulse frequency was 5 Hz for uncoated and 2.5 Hz for coated membranes. The amplitude of the pulse was adjusted to produce a temperature rise of 50 °C. The temperature sensor was fed from a constant-current source, and the voltage drop across the temperature sensor was amplified with an operational amplifier. The dynamic response of the temperature sensor was recorded by an oscilloscope. The thermal time constant was calculated from these data with a curve fit using Eq. (3.29). As already mentioned in the context of Eq. (3.37), self-heating occurs with a resistive heater, so that the thermal time constant has to be determined during the cooHng cycle. 

The constant current source keeps the emitter current constant at 10 mA. Since the collector current is approximately equal to the emitter current, the constant current source keeps the collector current approximately constant at 10 mA. Since the base is grounded, the emitter voltage is a diode drop below ground ... 

To demonstrate temperature effects, we will look at two circuits that can be used as constant current sources. One circuit will be greatly affected by temperature and the other is designed to be relatively independent of temperature. The circuits use transistors and resistors. The temperature dependence of transistors has already been discussed in detail in Section 4.F. Before we look at the circuits, we will look at how PSpice handles temperature characteristics of resistors. 

The op-amp constant current source below is designed to eliminate the effects of temperature on the BJT used in the current source. This current source is a very accurate and temperature-independent current source ... 

Even with resistor temperature dependence, the collector current changes by only about 13 pA over the entire temperature range. This circuit is a very temperature-independent constant current source, as long as the temperatures of R1 and R2 are the same and they have similar temperature coefficients. ... 

EXEHC15E 143 The circuit below is a current mirror and is a constant current source of about 50 pA. Display how the collector current of Q2 varies with temperature. Let resistors have a linear temperature coefficient of 200 ppm. The part name of both transistors is LM3046-Q. 

Electrical connections for resistivity measurements usually include separate current and voltage leads and the resistance is measured by determining voltage changes with a constant current source across the sample. This technique avoids spurious voltages at the contact points of a two or three contact connection but introduces a consideration of current path separate from the voltage measurement that can lead to faulty measurements. 

In order to eliminate the uncertainties associated with transient current and field inhomogeneities along the sample dnring TSC experiments, it has been suggested that experiments be performed nnder constant-current conditions and compared with the nsnal constant voltage results. Constant-current sources for this type of test are commercially available. 

A simple definition of a power conversion circuit is a circuit that converts a power source of a certain characteristic (e.g., 110 V AC battery voltage, spacecraft bus) into a power source with a more desirable characteristic (e.g., regulated +5V DC for digital logic, constant current sources). A wide variety of these circuits are presented in this chapter. 

Controlled-current coulometry is also called coulometric titration. An apparatus for controlled-current coulometry is shown in Fig. 5.35 for the case of determination of an acid. It consists of a constant current source, a timer, an end-point detector (pH meter), and a titration cell, which contains a generating electrode, a counter electrode in a diaphragm, and two electrodes for pH detection. The timer... 

Fig. 5.40 Circuits for conductivity measurements with two-electrode cell (a) and four-electrode cell (b). In (a), S AC voltage source D detector I, II, III bridge elements. In (b), S constant-current source POT potentiometer Rs variable resistor C and C electrodes for current flow P and P electrodes for voltage measurement.

Figure 8.2 The digital ohmmeter uses a constant-current source to produce a voltage across the unknown resistance proportional to its resistance, and a digital voltmeter to read that voltage.

A constant-current source can be constructed from a series combination of a battery and a resistance network (see Chap. 6). Such an arrangement is shown in Figure 25.2a. The current with the load shorted is equal to E/Radj. Viewed as an electrical network, a constant-current source should have a very high... 

Figure 25.2 Constant-current source using a battery and series resistor, (a) Dummy (resistor) load (b) Norton s equivalent circuit for part a (c) electrolysis cell as the load.

Figure 25.3 Operational amplifier constant-current source. [Adapted from J. Janata and H.B. Mark, Jr., Electroanal. Chem. 3 11 (1969) reprinted by courtesy of Marcel Dekker, Inc.]...

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