Figure 2 Equivalent Circuit for an AC ionizer.
Ionizers are both capacitively and resistively coupled to CPM plates. The capacitive signal (displacement current) is a field effect and arrives at the plate promptly. The resistive signal arises from the physical transport of ions through the air to the plate. The capacitance of the plate integrates the ion current. If the ion current is constant during the half cycle of the ionizer signal (one polarity of ion output), the response of the plate to the ion current will be a ramp waveform. Technically, the ramp is actually a negative exponential waveform and for very long ionizer switching times (~RairCCPM) the ramp will exhibit the curvature of a negative exponential), the ramp will begin to take a simulation of each of the two currents on the plate voltage is shown in Figure 3. This signal has arbitrarily picked the positive half cycle. The actual signal will end when the ionizer switches off the positive voltage and switches on the negative voltage.
Figure 3. Simulation of both signals on a CPM for half of an ionizer cycle
Figure 3, shows the signal beginning just as the positive emitter is turned on. If the ionizer switches after just one horizontal division of the chart, the maximum plate voltage, in this example, is mostly from displacement current. If the switching occurs at >5 divisions, the maximum plate voltage is determined by the ion current. Note that it is difficult to electronically differentiate between displacement and ion current so it is important to understand which is the major factor and its effect on the product. Also, it is necessary to understand the relationship between the equivalent circuit for the CPM / ionizer system and the product-ionizer system (Figure 2).
Separating the Two Types of Current
One way to differentiate between the two currents is to observe the wave shape. By way of example, Figure 4 shows an ionizer emitter voltage waveform in conjunction with a CPM waveform. The CPM signal shows a displacement current waveform at each ionizer waveform edge. The fast edge of the resulting displacement current is marked with arrows. It is clearly faster than the ion current part of the waveform and as can be seen in figure, represents about 20% of the voltage amplitude in this case. MORE