A Zero Crossing Detector is a circuit that detects the transition of a signal waveform from positive to negative when it crosses zero voltage. It is essentially a voltage comparator whose output changes when the input signal crosses the zero of the reference voltage level. The zero crossing detectors can be designed using various components such as op-amps, transistors, or optocoupler ICs. They are ideal for power control circuit applications such as time marker generators, phase meters, and frequency counters.

Working Principle: When the input increases from zero to positive value, the differential voltage is positive and the op-amp will be in positive saturation, +V_sat. When the input decreases from zero to a negative value, the differential voltage is negative and the op-amp will be in negative saturation, -Vsat. Thus the output changes from +V_sat to -Vsat when the input makes a transition near to zero.Zero-crossing detector (ZCD) also known as sine to square wave converter is a comparator circuit with zero reference voltage. A comparator circuit compares the two inputs applied to it and generates an output that indicates which of the inputs is greater. 

ZCDs are designed as open-loop circuits with no feedback. The open-loop gain of op-amp is very high and therefore the output of ZCD is always either positive saturation or negative saturation. This circuit triggers when the input signal crosses the zero-voltage level. It is commonly used in phase-locked loops, motor control, and power electronics where it is essential to synchronize the switching of power devices with the zero-crossing point of the AC waveform. This helps in minimizing switching losses and harmonic distortion.

There are two types of zero-crossing detectors:

• Non-inverting zero crossing detector
• Inverting zero crossing detector

Non-inverting zero crossing detector: In a non-inverting ZCD, the input is given to the non-inverting terminal, and the inverting terminal is grounded. 

Non-Inverting Zero Crossing Detector Circuit

Working Principle: When the input increases from zero to positive value, the differential voltage is positive and the op-amp will be in positive saturation, +V_sat. When the input decreases from zero to a negative value, the differential voltage is negative and the op-amp will be in negative saturation, -Vsat. Thus the output changes from +V_sat to -Vsat when the input makes a transition near to zero.

Non-Inverting Zero-Crossing Detector Output Waveform

The input-output waveform for a non-inverting zero crossing detector is as shown above. The output voltage is +V_sat for V_in > 0 and – V_sat for V_in < 0. Thus, in a non-inverting zero crossing detector, the input is not inverted at the output.

Inverting Zero Crossing Detector: In an inverting ZCD, the input is given to the inverting terminal and the non-inverting terminal is grounded.

Inverting Zero-Crossing Detector Circuit

Working Principle: When the input increases from zero to positive value, the differential voltage is negative and the op-amp will be in negative saturation, -Vsat. When the input decreases from zero to a negative value, the differential voltage is positive and the op-amp will be in positive saturation, +Vsat. Thus the output changes from +Vsat to -Vsat when the input makes a transition near to zero.

Inverting Zero-Crossing Detector Output Waveform

The input-output waveform for an inverting zero crossing detector is as shown above. The output voltage is -Vsat for Vin > 0 and +Vsat for Vin < 0. Thus, in an inverting zero crossing detector, the input is inverted at the output.