Zero Crossing Detector

A zero-crossing is a point where the sign of a mathematical function changes (e.g. from positive to negative), represented by a crossing of the axis (zero value) in the graph of the function. It is a commonly used term in electronics, mathematics, sound, and image processing. In alternating current, the zero-crossing is the instantaneous point at which there is no voltage present. In a sine wave or other simple waveform, this normally occurs twice during each cycle.

A zero crossing detector (ZCD) can be built using a 741 operational amplifier IC. One input must be set to zero for the reference voltage, while a sine wave voltage is applied to the other input. When the input sine wave passes through zero in a negative direction, the output voltage is driven into positive saturation. Similarly, as the input passes zero in a positive direction, the output is driven into negative saturation. This arrangement is also known as a sine to square wave converter.

ZCDs are useful tools for reducing or eliminating electrical noise. Noise produced during switching is proportional to the amplitude of the AC voltage at the switching point; therefore, switching should take place at the voltage’s zero crossing to minimise this noise. Effective zero crossing detection can facilitate this function.