Transient current

Introduction
Transient current :-
Transients -- they can be currents or voltages -- occur momentarily and fleetingly in response to a stimulus or change in the equilibrium of a circuit. Transients frequently occur when power is applied to or removed from a circuit, because of expanding or collapsing magnetic fields in inductors or the charging or discharging of capacitors.
MISSION STATNENT General Physiology
is the study of biological mechanisms through analytical investigations, which decipher the molecular and cellular mechanisms underlying biological function at all levels of organization.
The mission of the Journal of General Physiology is to publish articles that elucidate important biological, chemical, or physical mechanisms of broad physiological significance.
Two Fast Transient Current Components during Voltage Clamp on Snail Neurons
Voltage clamp currents from medium sized ganglion cells of Helix pomatum have a fast transient outward current component in addition to the usually observed inward and outward currents. This component is inactivated at normal resting potential. The current, which is carried by K+ ions, may surpass leakage currents by a factor of 100 after inactivation has been removed by hyperpolarizing conditioning pulses. Its kinetics are similar to those of the inward current, except that it has a longer time constant of inactivation. It has a threshold close to resting potential The time constants of the slow process are similar to those of slow outward current inactivation.


Transient current
Electric current is motion of charge and for a closed system the current must satisfy the equation of continuity

(3.8)


or in integrated over the volume

(3.9)


Where is the particle density, the current density and the total current in the volume . In the system we study, is identified by the total charge density , where is the elementary charge. In the continuity equation (3.9) the integration is performed over some finite volume within which the current is calculated, see figure 3.4; here we will consider the volume to be , where is the length in the current flow ( -) direction and is the cross sectional surface area of the cylinder surrounding the lead.


: Volume of integration - the cylinder length is along the -axis and its cross sectional surface area is .

We have already made the approximation to replace by . By defining the left(right) number of charge , and the partial overlap the transient charge current is given by






(3.10)


Suppose that the integration length is entirely in the left lead. Then, since the tail from a right wave function is exponentially small in the left region the integrals and are negligible, which results in



By adding the vector potential to the kinetic energy part of the Hamiltonian) we calculate the current as a response to the electromagnetic field given by . Hence, the system is described by

(3.11)


The non-equilibrium hopping matrix element



Contains the vector potential. Next we replace by its corresponding matrix element whenever belong to the same contact, i.e. same side of the potential barrier, and neglect the differences and . The usual non-equilibrium tunneling Hamiltonian is, thus, obtained as

(3.12)


As discussed earlier the shape of the potential may be arbitrary since its explicit form is never used in the derivations
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Tutorial discussion on Transient assessment
Transients are divided into two categories which are easy to identify: impulsive and oscillatory. If the mains signal is removed, the remaining waveform is the pure component of the transient. The transient is classified in the impulsive category when 77% of the peak-to-peak voltage of the pure component is of one polarity. Each category of transient is subdivided into three types related to the frequencies contained. Each type of transient can be associated with a group of phenomena occurring on the power system.
The impulsive low-frequency transient rises in 0.1 ms and lasts more than 1 ms. Measurement of these types of transients should be useful for all classes of application (benchmarking, legal, trouble shooting and laboratory)
The medium-frequency impulsive transient lasting between 50 ns to 1 ms and oscillatory transients between 5 and 500 kHz are less frequent than the low-frequency types but have much higher amplitude.
Source voltage assessment
. These standards specify an open-circuit voltage Ug which decreases at the terminals of an impedance Zs at the moment the generator injects a current into the equipment under test. This impedance Zs is known as the artificial mains network or line impedance stabilization network (LISN) which is specified as a function of the range of frequencies contained in the transient, as follows:
- (0.4  + 800  H) for frequencies lower than 9 kHz [IEC 725]
- 50  in parallel with (5  + 50  H) [CISPR 16] for frequencies from 9 kHz - 150 kHz
- 50  in parallel with 50  H [CISPR 16] for frequencies from 150 kHz to 30 MHz.

, The source voltages UaS, UbS, and UcS to be compared to the values recommended in the standards for susceptibility tests are
[7]
[8]
[9]
.

Transient over voltage envelope
.
The rms voltage assessment is used to assess the rms voltage envelope for a duration exceeding a half cycle. When the supply voltage U(t) includes a short transient detected at time  , the percent voltage Vp of interval T related to the voltage envelope is given by:
%
% [10]
Where:
VP = rms voltage as a percentage of the declared voltage Vd
VD = rms declared voltage
 = beginning of the interval assessed
T = interval assessed
U(t) = supply voltage involving a short transient.
 t = sampling interval
Rms amplitude-duration decomposition. The variable ISV% is calculated using the following equation:
% [11]
Where:
ISV = instantaneous steady-state voltage calculated in
VD = rms declared voltage.
.. This value in the interval between each half-decade yields a value for the factors of the rms envelope, as follows:
VHFC = root mean square of voltages between 1 µs and 5 µs
HFC = root mean square of voltages between 5 µs and 10 µs
HMFC = root mean square of voltages between 10 µs and 50 µs
MFC = root mean square of voltages between 50 µs and 100 µs
MLFC = root mean square of voltages between 100 µs and 500 µs
LFC = root mean square of voltages between 500 µs and 1 ms
VLFC = root mean square of voltages between 1 ms and 5 ms
MEMBRANE POTENTIAL
Information transmission can be understood in terms of two major components: Electrical signals and chemical signals. Transient electrical signals are important for transferring information over long distances rapidly within the neuron. Chemical signals, on the other hand, are mainly involved in the transmission of information between neurons.

Electrical signals (receptor potential, synaptic potential and action potential) are all caused by transient changes in the current flow into and out of the neuron, that drives the electrical potential across the plasma membrane away of its resting condition.

Every neuron has a separation of electrical charge across its cell membrane. The membrane potential results from a separation of positive and negative charges across the cell membrane. The relative excess of positive charges outside and negative charges inside the membrane of a nerve cell at rest is maintained because the lipid bilayer acts as a barrier to the diffusion of ions, and give rise to an electrical potential difference, which ranges from about 60 to 70 mV.


Vr = -60 to -70 mV.
Being Vr, the resting potential.
The charge separation across the membrane, and therefore the resting membrane potential, is disturbed whenever there is a net flux of ions into or out of the cell. A reduction of the charge separation is called depolarization; an increase in charge separation is called hyperpolarization. Transient current flow and therefore rapid changes in potential are made possible by ion channel, a class of integral proteins that traverse the cell membrane. There are two types of ion channels in the membrane: gated and nongated. Nongated channels are always open and are not influenced significantly by extrinsic factors. They are primarily important in maintaining the resting membrane potential. Gated channels, in contrast, open and close in response to specific electrical, mechanical, or chemical signals. Since ion channels recognize and select among specific ions, the actual distribution of ionic species across the membrane depends on the particular distribution of ion channels in the cell membrane.
. Na and Cl are more concentrated outside the cell while K and organic anions (organic acids and proteins) are more concentrated inside. The overall effect of this ionic distribution is the resting potential.
There are two forces acting on a given ionic species. The driving force of the chemical concentration gradient tends to move ions down this gradient (chemical potential). On the other hand the electrostatic force due to the charge separation across the membrane tends to move ions in a direction determined by its particular charge. Thus, for instance, chloride ions which are concentrated outside the cell tend to move inward down its concentration gradient through nongated chloride channels. However the relative excess of negative charge inside the membrane tend to push chloride ions back out of the cell. Eventually equilibrium can be reached so that the actual ratio of intracellular and extracellular concentration ultimately depends on the existing membrane potential.



The same argument applies to the potassium ions. However these two forces act together on each Na ion to drive it into the cell. First, Na is more concentrated outside than inside and therefore tends to flow into the cell down its concentration gradient. Second, Na is driven into the cell by the electrical potential difference across the membrane. Therefore, if the cell is to have a steady resting membrane potential, the movement of Na ions into the cell must be balanced by the efflux of K ions. Although these steady ionic interchange prevents can prevent irreversible depolarization, this process cannot be allowed to continue unopposed. Otherwise, the K pool would be depleted, intracellular Na would increase, and the ionic gradients would gradually run down, reducing the resting membrane potential.






Summary: The shieding properties of a wire penetrating an infinite planar screen are considered. Time domain results are presented for the case of a transient current pulse propagating along the wire. These results are obtained by first computing numerical solutions for the problem in the frequency domain and then utilizing the inverse Fourier transform. Two double exponential pulses with differing characteristics are considered. Numerical results for the two pulses are compared to determine the effects of the pulse characteristics on the shielding properties of the geometry. Applications to via structures in high-speed circuits are also briefly discussed. It is observed that even for very small apertures, the effect of the screen on the low-frequency pulse is negligible. As the pulse width decreases, the effect of the screen becomes more prominent. For the high-frequency case, the pulse is significantly affected by the screen. Unlike the low-frequency pulse, the amplitude of the high-frequency pulse is dependent on the aperture size. Even for large apertures, the attenuation becomes significant as the current propagates down the wire. It is shown that as the width of the input pulse decreases, the distortion in the pulse shape becomes more pronounced. This effect is especially important in applications related to high-speed integrated circuits
BIBLIOGRAPHY
www.goolge.com/wikipedia
www.yahoo.com/physics fundamental
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