# Zero Crossing Detector Circuit and Working

A zero-crossing detector or ZCD is one type of voltage comparator, used to detect a sine waveform transition from positive and negative, that coincides when the i/p crosses the zero voltage condition. In this article, we discuss the zero-crossing detector circuit with two different circuits, working principles, theory, and applications. The applications of the Zero Crossing Detector are phase meter and time marker generator.

## Zero-Crossing Detector Circuit

Zero crossing detector is a voltage comparator that changes the o/p between +Vsat & –Vsat when the i/p crosses zero reference voltage. In simple words, the comparator is a basic operational amplifier used to compare two voltages simultaneously and changes the o/p according to the comparison. In the same way, we can say ZCD is a comparator.

Zero crossing detector circuit is used to produce an o/p stage switch whenever the i/p crosses the reference i/p and it is connected to the GND terminal. The o/p of the comparator can drive various outputs such as an LED indicator, a relay, and a control gate.

### 741 IC-based Zero Crossing Detector

The zero-crossing detector circuit is a main application of the comparator circuit. It can also be named as the sine to square wave converter. For this, any one of the inverting/ noninverting comparators can be used as a zero crossing detector.

The only variation to be brought in is the Vref (reference voltage) with which the i/p voltage is to be compared, must be made reference voltage zero (Vref = 0V). An i/p sine wave is given as Vin. These are shown in the following inverting comparator circuit diagram and also i/p and o/p waveforms with a 0V reference voltage.

As shown in the below waveform, for a reference voltage (Vref), when the input sine wave permits through zero voltage and goes in the direction of positive. The o/p voltage is driven into negative saturation. In the same way, when the Vin permits through zero and goes in the direction of the negative, the Vout is driven to positive saturation. The diodes in the above circuit are called as clamp diodes. These diodes are used to guard the operational amplifier against damage due to an increase in Vin.

In some particular applications, the Vin may be a low-frequency waveform that causes an interruption in time for the Vin to cross the zero level. Furthermore, this causes a delay in the Vout to switch between the two saturation levels (upper and lower). At the same time, the i/p noises in the IC may cause the Vout to switch between the saturation levels. Thus zero crossings is identified for noise voltages in addition to the Vin. These problems can be detached by using a re-forming feedback circuit with positive feedback that causes the Vout to switch faster. So, removing the possibility of any false zero crossing due to noise voltages at the input of the op-amp.

The working of a zero crossing detector can be easily assumed if you know the working of a basic Op-Amp comparator. In this detector, we are setting one of the i/ps as zero that is Vref =OV. The o/p is determined into –Vsat when the i/p signal passes through 0 to +ve direction. Equally, when i/p signal passes through zero to –ve direction, the o/p switches to +Vsat.

### Applications of Zero Crossing Detector

Zero crossing detector circuits can be used to check the condition of an operational amplifier. And also used as a frequency counter and for switching purposes in power electronics circuits.

#### ZCD as Phasemeter

A ZCD can be used to measure the phase angle between two voltages. A sequence of pulses in the +ve and -ve cycles are acquired to measure the voltage between the time interval of the pulse of sine wave voltage and second sine wave. This interval of time is related to the phase difference between the two i/p sine wave voltages. The use of phase meter ranges from 0° to 360°.

#### ZCD as Time Marker Generator

For an i/p sine wave, the o/p of the zero-crossing detector being a square wave, further it will pass through an RC series circuit. This is shown in the following figure.