MKS Novx 5000 Series, Руководство пользователя

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19-5000-M Version 1 21 Voltage Detection System Series 5000
8.2 Capacitance
Capacitance is a trait of all bodies, human or otherwise. It can be compared to a
reservoir for charges. When charges are stored in this capacitance, a Potential will
develop. As an analogy, when you pour liquid into a reservoir, its Level will rise.
The smaller the capacitance (reservoir); the higher the potential (level) for a given
amount of charge.
The capacitance of a body located totally isolated in outer space is known as the
free space capacitance (self-capacitance or isolated capacitance). When the body
approaches other bodies or ground planes, other capacitance coefficients come
into play: the parallel plate capacitance or mutual capacitance.
For example, a typical human body has a free space capacitance of 118
picofarads. When the body stands on a ground plane on typical insulated shoe
soles, its capacitance will increase to 275 picofarads.
Charge, Potential and Capacitance are related by the equation: Charge = Potential
X Capacitance (Q=ChV).
One can easily understand that the Charge on a human or other body in motion, as
well as its Capacitance, is continuously changing as it interacts with its physical
environment. Each interaction of a shoe sole with a carpet, each step, will result in
a different triboelectrifi-cation. In addition, each interaction of the foot with the shoe
sole, as well as each movement of the shoe sole with respect to the carpet will
affect Capacitance. It would be very difficult to measure either Charge or
Capacitance; however, it is relatively easy to measure Potential. Novx ESC
Monitors are based on this technology of measuring Potential.
8.3 Electrostatic Discharge
Electrostatic Discharge (ESD) is a redistribution of Charge across bodies. This
redistribution can be very sudden, intense and short or it can be protracted and
fickle.
An example of a sudden redistribution is an arc. One can easily imagine that the
sudden flow of a massive amount of electrons can be very destructive to whatever
is in its path. It can also be highly unpleasant when this path includes, for example,
a human finger.
An ESD event can be very slow, for instance when the charge has to flow through
high-value resistors or through a workbench top material with high surface
resistance.