Current measuring circuit

Rm resistance of the current measurement circuit, O, see Figure 21.2 and equation (21.4)... 

Op amps can be used to construct an automatic three-electrode potentiostat, as illustrated in Figure 23F-2. Notice that the current-measuring circuit of Figure 23F-1 is connected to the working electrode of the cell (op amp C). The reference electrode is attached to a voltage follower (op amp B). As discussed in Feature 21-4, the voltage follower monitors the potential of the reference electrode without drawing any current from the cell. The output of op amp B,... 

The current measuring circuit is very attractive instead of introducing a current reading shunt resistor with the necessary (even if small) voltage drop. The voltage drop in this circuit is virtually zero. 

Impedance Analyzers. Component analyzers, impedance analyzers, and ac bridges have built-in current measuring circuits, which are designed to minimize the above problems. These instruments are usually configured for four-wire connection, which minimizes stray series resistance and inductance, the dominant source of error for low impedance samples. 

As described by equation (6.1.5.2), the contribution from the electrode resistance to increases with decreasing electrode radius and the total cell impedance can become very large. The precision with which high-impedance measurements can be made is limited for several reasons including (a) current measurement circuits always have nonzero input capacitance, (b) an infinite input resistance cannot be achieved with real... 

Fig. 6.1 Symbolic equivalent circuit of an electrode wire heated and polarised inside solutioiL Resistor and capacitor symbols drawn stand for elements of differential size Cdl double layer capacity elements along the wire length, Zf faradaic impedance elements, Rsoi solution resistance elements inside bulk solution. REF symbolises the reference and counter electrodes in common, and POL the polarisation and current measuring circuit From [3], with permissirm...

Figure 3.9 shows a sequence current measurement circuit for a solidly bonded cable. The following equations are given from the 6x6 impedance matrix in (3.47) and Figure 3.9 ... 

Current mode A current measuring circuit is placed across the terminals of the detector, and the average current is measured by the system Current mode operation is the most common operation mode It is used for measuring the power of the reactor and also in most portable survey meters (see Section 3 1.2 4)... 

A common known method to get eddy-current informations about material flaws is the measurement of real- and imaginary part of the complex impedance of a coil in absolute circuit. The measurement, shown in this paper, are done with an impedance analyzer (HP4192A). The device measures the serial inductance L, and the serial resistance Rs of the complex impedance with an auto-balance bridge measurement circuit [5]. 

These are employed for the measurement of power circuit currents through an ammeter, kW, kWh or KVAr and power factor meter, or similar instruments requiring a current measurement. They must have a specified accuracy class as in lEC 60044-1 and the secondary current substantially proportional to the primary within a working range of about 5-120% of its primary rated current. They... 

U2 = 0.1 V) and are suitable for current measurement. For smaller currents, sensitive instruments with 5 kQ per fiPc (Uj = 5 mV) are used. Small currents are usually measured by the voltage drop across a fixed resistance (calibrated shunt) using an electronic amplifier-voltmeter. This method has the advantage that the circuit does not have to be interrupted to measure the current. 

Reference electrodes The generally accepted criterion for the effectiveness of a cathodic-protection system is the structure/electrolyte potential (Section 10.1). In order to determine this potential it is necessary to make a contact on the structure itself and a contact with the electrolyte (soil or water). The problem of connection to the structure normally presents no difficulties, but contact with the electrolyte must be made with a reference electrode. (If for example an ordinary steel prol e were used as a reference electrode, then inaccuracies would result for two main reasons first, electrochemical action between the probe and the soil, and second, polarisatibn of the probe owing to current flow through the measuring circuit.)... 

Voltmeters and potentiometers The instruments described here are generally referred to as corrosion voltmeters. As mentioned previously, the current flowing through any potential-measurement circuit must be small to avoid errors due to polarisation. Moreover, if the current flow is too large, errors will be introduced owing to the voltage drop caused by the contact resistance between the reference electrode and the electrolyte. It is thus clear that the prime requirement of a potential measurement circuit is high resistance. 

In many cases it will suffice to include in the circuit a shunt of appropriately low resistance over which 7/ -drop potential measurements can be made for ready calculation of the magnitude of the current flow. This technique permits measurements to be made as required without opening the circuit even momentarily for the introduction of current-measuring devices. It is also possible to arrange instruments in a circuit so that no measuring resistance is introduced in the galvanic current circuit . 

Further, the operator must be able to choose the drop lifetime and the scan parameters, viz., the starting potential, direction (cathodic or anodic), rate and end potential, together with the sensitivity of the current measurement and the amplification in the ohmic cell resistance compensation circuit. Convenient additional facilities are (a) display of the polarogram on an oscilloscope, (b) delivery of hard copy of the polarograms on a chart recorder and (c) repeated recording of the polarographic curve for the same sample. 

If a cell is to be used as a potential standard, then it must be prepared as simply as possible from chemicals readily available in the required purity and, in the absence of current passage, it must have a known, defined, constant EMF that is practically independent of temperature. In this case the efficiency, power, etc., required for cells used as electrochemical power sources is of no importance. The electrodes of the standard cell must not be polarizable by the currents passing through them when the measuring circuit is not exactly compensated. 

Phototubes are vacuum tubes with a large anode coated with a pho-toemissive substance such as cadmium sulfide. A positive voltage of approximately 90 V on the cathode attracts electrons dislodged by photons from the cadmium sulfide. The current measured by an ammeter is proportional to the number of photons entering the phototube. Figure 5.10 represents a schematic diagram of a phototube measuring circuit. 

FIGURE 1.5. a Three- electrode electrochemical cell, b General equivalent circuit, c equivalent circuit of the cell + potentiostat and current measurer (the symbols are defined in the text). 

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