Drive voltage

Of course the details of device and system performance will depend on the particular device or system under consideration. Our discussion will, however, focus primarily on limitations to system performance associated with material limitations rather than that of a particular device configuration. Parameters of particular interest include drive (Vjj) voltage, bandwidth, waveguide propagation loss, total device insertion loss, drive voltage stability, bias voltage stability, and optical power handling capability. 

Measurement of device bandwidths in the order of 100 GHz typically requires heterodyne detection and a stripline electrode configuration such as that illustrated in Fig. 31. The response of polymeric electro-optic modulators is typically flat to 100 GHz. Fall off above that frequency (Fig. 32) can be traced to resistive losses in millimeter wave transmission structures and in the metal electrodes. 

The bandwidth performance of polymer modulators clearly surpasses those of other materials although by clever device engineering lithium niobate modulators have been demonstrated to somewhat above 70 GHz. 

IPITEK (formerly TACAN) Corporation (Carlsbad, CA) has achieved 0.8 V VK values using 3-cm interaction length push-pull Mach-Zehnder modulators, as illustrated in Fig. 33 [6,252, 302]. With 2-cm interaction length modulators, an average VK value of 1.1 V was obtained [6,252,302]. 

Halfwave voltages (Vn) measured by the TACAN Corporation for various push-pull electro-optic modulators. Twelve modulators (six 3-cm and six 2-cm modulators) were fabricated. The poling voltages and waveguide modulator performance are given in the figure 

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How does galvanising work As Fig. 24.4 shows, the galvanising process leaves a thin layer of zinc on the surface of the steel. This acts as a barrier between the steel and the atmosphere and although the driving voltage for the corrosion of zinc is greater than that for steel (see Fig. 23.3) in fact zinc corrodes quite slowly in a normal urban atmosphere because of the barrier effect of its oxide film. The loss in thickness is typically 0.1 mm in 20 years. 

I/r is the rest potential. The difference between the potential of the working anode and the protection potential of the object to be protected is termed the driving voltage U. ... 

In spite of a low driving voltage of about 0.2 V, about 90% of all galvanic anodes for the external protection of seagoing ships are zinc anodes (see Section 17.3.2). Zinc alloys are the only anode materials permitted without restrictions for the internal protection of exchange tanks on tankers [16] (see Section 17.4). 

Galvanic anodes must not be backfilled with coke as with impressed current anodes. A strong corrosion cell would arise from the potential difference between the anode and the coke, which would lead to rapid destruction of the anode. In addition, the driving voltage would immediately collapse and finally the protected object would be seriously damaged by corrosion through the formation of a cell between it and the coke. 

In poorly conducting media or soils, however, the low driving voltage can limit the use of galvanic anodes. Raising the current delivery, which becomes necessary in service, is only practically possible with the help of an additional external voltage. In special cases this is used if an installed galvanic system is overstretched or if the reaction products take over additional functions (see Section 7.1). 

Substrate metal Density (g cm-3) Coating Density (g cm-3) Coating thickness ( m) Anode current density (A m ) max. avg. Allowable maximum driving voltage (maxA ) Loss (mg A a )... 

In such cases basket anodes are frequently used. These have a relatively large surface and work at a low driving voltage due to their special construction. A cylinder of platinized titanium-expanded metal serves as the basket to which a titanium rod is welded. This serves as the current lead and carrier, and ends in a plastic foot that contains the cable lead and at the same time serves as the mounting plate. The expanded metal anode exhibits a very uniform anode current density distribution, even at large dimensions, in contrast to the plate anode. The reason is the many comers and edges of the metal that make the point effect only evident at the outer edges of the anode. 

The current output of galvanic anodes depends on the specific soil resistivity in the installation area and can only be used in low-resistivity soils for pipelines with a low protection current requirement because of the low driving voltage. Impressed current anode installations can be used in soils with higher specific soil resistivities and where large protection currents are needed because of their variable output voltage. 

Cathodic protection, complete or partial (stem and bow), is arranged by the distribution of the anodes so that the desired current distribution is maintained correctly in the relevant areas. Galvanic anodes, depending on their dimensions and current output, deliver a certain maximum current which depends on the conductivity. The calculated maximum current from Eq. (6-12) based on the driving voltage and grounding resistance is reduced in practice on working anodes due to film for-... 

Since docks are usually situated in brackish water, the anodes must have large surface areas to keep the grounding resistance and the driving voltage low. Cathodic protection of the interior is not necessary because the dock is only flooded for a short time and can be otherwise maintained. 

Cathodic protection of reinforcing steel with impressed current is a relatively new protection method. It was used experimentally at the end of the 1950s [21,22] for renovating steel-reinforced concrete structures damaged by corrosion, but not pursued further because of a lack of suitable anode materials so that driving voltages of 15 to 200 V had to be applied. Also, from previous experience [23-26], loss of adhesion between the steel and concrete due to cathodic alkalinity [see Eqs. (2-17) and (2-19)] was feared, which discouraged further technical development. 

To achieve satisfactory current distribution and to avoid cathodic damage to the lining (see Section 5.2.1), the distance between the anodes and the object to be protected should not be too close and the driving voltage should not be too high. In... 

The primary distribution of protection current density (see Section 2.2.5) for a given geometry and driving voltage, can be seen as follows ... 

A dc restoration circuit is needed following the output coupling capacitor to make the drive voltage referenced to the power switch s common. The supply voltage of the driver should be well bypassed so that its voltage does not droop during the drive pulse. 

The last consideration is from which voltage to draw the base current. Since the base-emitter junction resembles a forward-biased diode, the maximum Vbe is 0.7 to 1.0 V. Ideally, a voltage source of 2.5 to 4.0 V is sufficient. If the base drive voltage source is too high, there will be a significant loss associated with driving the base. 

There is not mueh that ean be done about this loss exeept to seleet a MOSFET with lower values for Oss and Crss and to possibly slightly lower the maximum gate drive voltage. 

The power supply is usually a transformer/rectifier that converts a.c. power to d.c. Typically the d.c. output will be in the range 15-100 V and 5-100 A although 200 V/200 A units are not unknown. Thus fairly substantial driving voltages and currents are available. Where mains power is not available, diesel or gas engines, solar panels or thermoelectric generators have all been used to provide suitable d.c. 

Using the impressed-current technique the driving voltage for the protec-ive current comes from a d.c. power source. The sacrificial anode technique... 

Table 10.2 gives electrochemical properties for various generic anode types. It will be apparent that the driving voltages that are available from sacrificial anodes are substantially less than those available from power sources. At best an anode will produce 1 V to steel whereas an impressed current power source may produce up to 100 V. 

Alloy Environment Operating voltage vs. Ag/AgCI/ seawater (V) Driving voltage (V) Capacity (Ah/kg)... 

The driving voltage to bare steel, i.e. protection potential of steel - anode operating potential. 

Anode Operating Potential, Protection Potential and Driving Voltage... 

The driving voltage is the difference between the anode operating potential and the potential of the polarised structure to which it is connected. For design purposes, the driving voltage is taken as the difference between the anode operating potential and the required protection potential of the structure. 

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