Circuit impedance, control

For large feeders and FIT system.s the ground fault current would be controlled naturally through the ground circuit impedance and a smaller ground conductor may suftlce. 

The designer must also consider the impedance of the control grid circuit Primary grid emission can result in operational problems if the grid circuit impedance is excessively high. Primary grid emission, in the case of an oxide cathode tube, will increase with tube life. 

For special applications, compliance may be achieved by other means. For example, the insertion of a 1 1 safety transformer in a mains voltage system would isolate the system on the secondary side from the earthed neutral system on the primary. Earthing one pole of the secondary side through an impedance so as to limit the potential earth fault current to no more than about 5 mA would create a system that would prevent eleetric shock injuries for a phase-to-earth fault. It is usual to employ a circuit that detects the flow of fault current and trips a circuit breaker controlling the supply. This type of system is often used in test areas. 

Rq = power regulator output circuit impedance R = heater load resistance S = thermometer sensitivity, eraf/deg Tq - heat-sink temperature T = controlled temperature T controller set-point temperature, steady state V power regulator output emf Vq output emf at steady rate, T - Ti. ... 

To contain the start-up inrush current, as a result of low start-up impedance and to control the same as needed through external resistance in the rotor circuit. 

A transformer is not a source of supply (it only transforms one voltage to another) but it is considered so, in terms of fault level calculations. In fact, it provides a means to add to the impedance of a circuit on the lower voltage side, and limits the fault level of the network to which it is connected. One will appreciate that the capacity of the actual source of supply, on the higher voltage side, will be much larger. On the LV side it is controlled by the impedance of the transformer. It is customary to consider this impedance to determine the fault level on the LV side. The fault level is measured as the dead... 

The rule of thumb to determine the ground loop impedance is to consider the ground fault current as one and a halftimes that of the overcurrent setting of the circuit breaker for breaker-controlled systems (a fault condition for a breaker) or three times the rating of the fuses, for fuse-protected systems (an overcurrent condition for the fuses). Based on this rule. Table 21.2 suggests the optimum values of ground loop impedances for circuits of different... 

Let us refer to equation (23.13) for the inrush current, / on parallel switching. If we can increase the impedance of the switching circuit by introducing a resistance R or inductance L, or both, we can easily control this current to a desired level. 

NOTE Do not use snubber values or snubber elements intended for silicon-controlled rectifier (SCR) circuits in switching power supplies. The impedances and parasitic values of these circuits are much lower than within switching power supplies. They will create far too much loss in switching power supply circuits. 

Since the ion transfer is a rather fast process, the faradaic impedance Zj can be replaced by the Warburg impedance Zfy corresponding to the diffusion-controlled process. Provided that the Randles equivalent circuit represents the plausible model, the real Z and the imaginary Z" components of the complex impedance Z = Z —jZ " [/ = (—1) ] are given by [60]... 

Figure 1. Diagram of apparatus (M) monomer reservoir (F) flow meter (VG) vacuum gage (mercury manometer) (E) electrode (T) liquid nitrogen trap (P) mechanical pump (V,) needle valve (Vt) stop valve (Vs) pressure control valve (OSC) discharge frequency oscillator (AMP) amplifier (1MC) impedance matching circuit...

AC impedance measurements were also made in bulk paints. A Model 1174 Solartron Frequency Response Analyser (FRA) with a Thompson potentiostat developed ac impedance data between 10 KHz and 0.1 Hz at the controlled corrosion potential The circuit has been described in the literature( ). 

Noble metal connections can reduce the corrosion to an "acceptable" level. This assumption is not true for leads which enter the package from sensors such as micro-electrodes which are characterized by relatively high impedances. The trend for neuroprosthe-tio devices is towards closed-loop control in which the use of high impedance bioelectric sensors will be common. In addition, differing potentials within multi-circuit cables can result in corrosion even when the conductors are fabricated from highly corrosion resistant materials such as MP35N. 

In practice, the limit at high frequencies is controlled by the inductance in the circuit, (0L. The influence of this on the impedance (in contrast to that of the capacitance) increases with an increase in frequency. The difficulty is that the inductance that becomes significant when the frequency exceeds, say, 104 cps, is often more an irrelevant inductance, not one caused by the electrode process. Thus, it may arise because of some contribution from the wire connections to the cell and their interaction with the surroundings. Hence, very short leads to the cell should be used. It is possible to build circuits that compensate for the inductance effects, but usually the practice is to keep the frequency within the 10 kilocycle/s upper range, so as to make coL negligible. 

An alternative way of measurement is to incorporate the cell in a bridge circuit as shown in Fig. 16. The configuration of the bridge is identical to the classical Wien bridge [49] except that the a.c. voltage is supplied via the potentiostat so that, simultaneously, the mean d.c. potential E can be controlled as usual [21, 22]. The adjustable series combination of the resistor Rs and the capacitor Cs offers an impedance Zs, given by... 

A renewal of interest in the other rate-controlling processes started in those groups who were developing the impedance method [49, 53] and the a.c. polarographic method [12, 25], probably because it was found that, in many cases, Randles equivalent circuit did not hold and also because the appropriate mathematics are more tractable in the frequency domain. Still, it is recommended that the a.c. studies are combined with the diagnostic results which can be obtained from steady-state techniques and/or cyclic voltammetry. 

Figure 7.1 (A) Typical controlled-potential circuit and cell OA1, the control amplifier OA2, the voltage follower (Vr = Er) OA3, the current-to-voltage converter. (B) Equivalent circuit of cell Rc, solution resistance between auxiliary and working electrodes Ru, solution resistance between reference and working electrodes, Rs = Rc + Ru and Cdl, capacitance of interface between solution and working electrode. (C) Equivalent circuit with the addition of faradaic impedance Zf due to charge transfer. Potentials are relative to circuit common, and working electrode is effectively held at circuit common (Ew = 0) by OA3.

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