Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 49

Static electricity can be modeled as a charge generator, and a simple electrical model can be used to understand many practical situations (Figure 2.1).

The separation of charge is effectively a small electrical current represented by a current source I. The capacitor C represents the charge storage properties of the system and could be a material surface or a conducting object with a capacitance to earth. The resistance R represents charge dissipation processes (other than ESD) and can vary from less than 1 Ω to more than 10¹⁴ Ω for good insulators. (See Sections 1.7.3 and 2.3.4 for discussions on the meaning of insulators and conductors.)

If the current is constant (i.e. the effect of capacitance can be neglected), it's easy to see by Ohm's law that the voltage developed is highly dependent on the resistance R. If a charge generation rate of 1 nA (1 nCs⁻¹) is present, with a resistance of 10⁹ Ω, a steady state voltage of 1 V is produced. If, however, the resistance was 10¹² Ω, a voltage of 1 kV would be produced, and for a resistance of 10¹⁴ Ω, a voltage of 100 kV would theoretically be produced! For a charge generation rate of 1 μA, a resistance of 10¹⁰ Ω would yield a voltage of 10 kV. In practice, electrostatic sources rarely generate charge at this rate or on a steady current basis unless there is steady movement involved (e.g. in a conveyor system).

Figure 2.1 A simple electrical model of electrostatic charge build‐up.

The rate of electrostatic charge generation is affected by many factors. Some of the key factors are as follows:

 Relative position of the materials in the triboelectric series

 Rate of separation of contact area (high rates of movement)

 Condition of the surfaces that make contact

 Rubbing of the contacting surfaces

 Ambient humidity and temperature

The many factors involved in triboelectric charge separation make it a highly unpredictable phenomenon.