The ZCD requires a current-limiting resistor in series
(R_{SERIES}) with the external voltage source. If the peak amplitude
(V_{PEAK}) of the external voltage source is expected to be stable, the
resistor value must be chosen such that an I_{ZCD_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 (V_{MINPEAK}) and maximum
(V_{MAXPEAK}) value is described in the section below on **Handling
V _{PEAK} Variations**.)

Figure 1. External Resistor

Figure 2. External Voltage Source