Switch-off time

On removal of the electric field, the ionic movement decreases dramatically and the initial alignment is recreated by elastic forces propagated from the surface boundary layers as well as by conduction. Therefore, switch-off times, /off, are an order of magnitude longer ( 20-100 ms) due to the high degree of disorder caused by the flow of liquid crystal and the absence of a restoring field effect. 

Therefore, switch-off times are independent of the field strength and directly dependent on material parameters, such as viscosity coefficients and elastic constants, and the cell configuration. Therefore, they are often three or four orders of magnitude larger than the switch-on times. However, sophisticated addressing techniques can produce much shorter combined response times ( on + off The nematic director should be inclined, e.g. 1° pretilt,... 

The switch-on time, ton ittd the switch-off time, toff are both proportional to the square of the cell gap, d, as for the TN-LCD, ... 

The dynamics of evolution become somewhat more complicated if the external field is not strictly periodic but rises and falls during a finite switch-on and switch-off time respectively, as will realistically be the case for an atom... 

As noted above, step-scan FT-IR can provide a better time resolution than PA-IR spectroscopy for time-resolved studies, as well as full spectra at the desired resolution. On the other hand, its major limitation is that the phenomenon under study must be perfectly repeatable-information which often is not available before an experiment is carried out. Another problematic aspect to consider is that sufficient relaxation time must be allocated for the sample to return to its initial state between consecutive perturbations. Unfortunately, this parameter is also often not known a priori before the experiment is performed, and may risk artifacts appearing in the data. In contrast, a single perturbation is required in a PA-IR experiment to record the time-resolved data, eliminating the requirements of repeatability and an a priori knowledge of the relaxation time. PA-IR spectroscopy was used to assess directly the repeatability of the orientation/reorientation cycles for 5CB [27]. Table 13.1 shows the switch-on and switch-off time constants determined individually for a series of 300 consecutive reorientation cycles. As expected for this well-studied LC, the time constants did not evolve systematically as a function of the number of cycles. In this case, however, the repeatability was demonstrated experimentally and not only assumed, as is often necessary in step-scan studies. 

Cycle Switch-on time constant (ms) Error ( ) Switch-ofF time constant (ms) Error ( )... 

Here the subscript ON refers to the switch being closed, and OFF refers to the switch being open. Von and Voff are the respective voltages across the inductor during the durations AtoN and AtoFF- Note that very often, AtoN is written simply as toN> the switch on-time. And similarly, AtoFF is simply torr- the switch off-time. 

Note that the induced voltage during the switch off-time has had its polarity reversed. 

We see that a 5 V output is possible only if we have been switching with a constant ratio between the switch ON and switch OFF time, as given by... 

Of course the output rail continues to stay inverted even when the switch turns ON, because the diode then stops conducting, and there is an output capacitor present, that holds the output voltage steady at the level it acquired during the switch off-time. 

From V = Ldl/dt, we also see that to get the increment in current during the on-time to exactly equal the decrement, every cycle, we need the quantity V multiplied by time during the on-time to be equal in magnitude to the same quantity during the switch off-time. In fact, that is the fundamental voltseconds law of power conversion. It leads directly to the expression of duty cycle in terms of input and output voltages. 

After the excitation pulse, the intensity of luminescence emission decays with the rate that is characteristic to the used label. The photons emitted during the delay time, the time between the pulse and activation of the detector, are lost and, thus, a short delay time is preferred in the detection. The background luminescence decays typically very rapidly (within tens of nanoseconds) and long delay times are not required. However, many used light sources exhibit a switch-off delays and afterglow that prevent the use of short delay times. This switch-off time is an important characteristic of a light source and may limit the performance of a time-resolved fluorometer significantly. 

Technology Status Typical switch-off time at room temperature Remarks... 

Potential shift 100 mV shift of rebar potential in the positive direction when system is depolarized Depolarization occurs when CP current is switched off. Time period required for rebar to depolarize is debatable. The potential reading before interrupting the CP current should be "IR" corrected. 

Equations for the switching on time T n and the switching off time Toff under static driv-... 

Section 5.9 introduces the dynamics of the Preedericksz transition in the classical geometries described in Chapter 3 in Section 3.4.1. The switch-on and switch-off times will be defined when flow is considered to be negligible in the usual twist geometry, as detailed in Section 5.9.1. In some instances, however, flow turns out to be quite influential and leads to the phenomena of backflow and kickback y as to be discussed in detail in the case of the splay geometry in Section 5.9.2. Backflow in the bend geometry is discussed in Section 5.9.3. 

The switch-off time can be determined by similar reasoning. Initially, it is assumed that H > He and that the post-Preedericksz transition static solution (t>o z) is known, for example via the results in Section 3.4.1. Prom equation (5.405) (or (3.136)) this post-threshold static solution for H > He satisfies... 

Clearly, the largest valued Tn is linked to the slowest exponentially decaying mode in the solution (5.417). The switch-off time Toff is defined to be the largest valued Tn and is equal to t. Therefore... 

The switch-off time is also called the decay time or relaxation time by some authors. The analogous result for electric fields yields the same value for the full solution z, t) for t > 0 is of course modified and is obtained via (5.416), (5.417) and (5.423) by replacing 0o with the analogous static solution for the electric field. In many practical situations, such as that to be encoimtered below when we discuss the kickback effect, it is often convenient and appropriate to approximate z) by a fixed constant when H He- The form in equation (5.424) for Toff is common for many relaxation processes in nematic liquid crystals when electromagnetic fields are set to zero. For example, the following values... 

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