The ZCD requires a current-limiting resistor in series (RSERIES) with the external voltage source. If the peak amplitude (VPEAK) of the external voltage source is expected to be stable, the resistor value must be chosen such that an IZCD_MAX/2 resistor current results in a voltage drop equal to the expected peak voltage. The power rating of the resistor should be at least the mean square voltage divided by the resistor value. (How to handle a peak voltage that varies between a minimum (VMINPEAK) and maximum (VMAXPEAK) value is described in the section below on Handling VPEAK Variations.)
