Читать книгу The ESD Control Program Handbook - Jeremy M. Smallwood - Страница 79

An ion cloud is rarely accurately balanced in terms of ion charge polarity density. One consequence of this is that any surface within the ion cloud tends to reach a state of excess charge reflecting the ion density balance in the ion cloud. If, for example, the ion cloud has excess positive ions, then surfaces within the ion cloud will tend to become positively charged until electrostatic fields are sufficient to prevent further ion deposition. This effect will create residual voltages on insulating surfaces and isolated conductors within the ion cloud. With a well‐balanced ionizer, this effect will be small, and the excess voltage achieved may be only a few volts or tens of volts. This “offset voltage” is an important parameter of an ionizer specification (Figure 2.25).

A poorly maintained ionizer can become seriously out of balance. This can happen in service due to erosion or contamination of the ionizer needles. If this occurs, then surfaces within the ion cloud may become charged to hundreds or even thousands of volts. Monitoring this effect over time and performing appropriate maintenance of an ionizer to prevent the imbalance from reaching an excessive level is an important aspect of ionizer maintenance (Simco Ion TN‐003 2019).