Zero Crossing Detector

A voltage comparator circuit whose output changes concerning the input signal which is crossing the zero of the reference voltage level that’s why it is called a Zero Crossing Detector. These circuits are commonly known as switching circuits the output of these circuits lies between positive and negative saturation voltage levels. The most commonly used circuits are the Schmitt trigger, Astable, and Monostable Multivibrators.

What is a Zero Crossing Detector?

It is a circuit that compares the applied input voltage at one input terminal of an op-amp with a reference voltage = 0 at another input terminal of an op-amp. Because it uses reference voltage =0 that’s why it is called Zero Crossing Detector. The output of the Zero Crossing Detector is a square wave. So it is also known as the Square wave Generator.

Material Required For Making Zero Crossing Detector: For making a Zero Crossing Detector We have required two or three Resistors, a bridge rectifier, and an optocoupler.

Types of Zero Crossing Detector

There are Two Types of Zero Crossing Detectors :

• Inverting Zero Crossing Detector
• Non-Inverting Zero Crossing Detector

Inverting Zero Crossing Detector

In the inverting zero crossing detector the input voltage is applied at the negative terminal of the op-amp and reference voltage = 0 is applied at the positive node of the op-amp. In the inverting zero crossing detector output of the given circuit goes into negative saturation when the applied input voltage is positive and output goes to positive when input voltage is negative, the circuit diagram of inverting zero crossing detector is given below.

Circuit Diagram of Inverting Zero Crossing Detector

The figure given below represents the circuit diagram of Inverting Zero Crossing Detector. In this circuit we have used inverting op-amp. Here , the input voltage V1 is given in the inverting terminal of the op-amp and non-inverting terminal of op-amp is grounded. here we are also use Protection diode.

Protection diode: These are simple diode which is used to prevent the op-amp from the high input gain, because here we use direct input voltage.

CIRCUIT DIAGRAM :- INVERTING ZERO CROSSING DETECTOR

Working of Inverting Zero Crossing Detector

As per definition it detects the point where the input signal is Zero because it’s reference voltage is 0. Before start the working of Zero Crossing Detector we have to let the output voltage is +V sat.

Now see the circuit to understand the working.

As per circuit reference voltage is 0 which is applied at the non-inverting terminal of op-amp which is compared with Vin which is applied at the Inverting terminal of op-amp.

Now when positive half of the sinusoidal wave appears at the input and op-amp comparator compares its peak level with the reference voltage.

Vo = V ref – Vi

because the reference voltage is 0, so

Vo = 0 – Vi

as we have already mentioned that starting output voltage is (+V sat), so

Vo = 0 – (+V sat)

Vo = (-V sat).

Now when the negative half of the sinusoidal wave appears at the input and op-amp compares its peak level with the reference voltage.

So,

Vo = V ref – Vi

Vo = 0 – (- V sat )

We get

Vo = (+V sat )

Here we can clearly see that how a Inverting Zero Crossing Detector detects the changes in the level of applied input sinusoidal signal.

Input and Output Waveform

In the beginning we have already mentioned that Inverting Zero Crossing Detector is also known as a Square Wave generator. Same as the output of the Window Comparator is nothing but a Square Wave.

Now Let us see how a Zero Crossing Detector generate the Square Wave or Output waveform.

in the working part we have derived that Vo for the positive half of the sinusoidal signal is (-V sat). because of this we get negative output for the positive half of the applied signal. while Vo for the negative half of the applied input signal is (+V sat). because of this we get positive output for the negative half of the applied signal.

So after observation of output waveform of Inverting Zero Crossing Detector we can say that output is reflection of applied input sinusoidal signal.

INPUT & OUTPUT WAVEFORM

Non-Inverting Zero Crossing Detector

In non – inverting Zero Crossing Detector the input voltage Vin is applied at the positive node of the op-amp and the reference voltage that is zero is applied at the negative node of the op-amp. In the non inverting zero crossing detector output of the given circuit goes into positive saturation when the applied input voltage is positive and output goes to negative when input voltage is negative, the circuit diagram of non inverting zero crossing detector is given below.

Circuit Diagram of Non-Inverting Zero Crossing Detector

The figure given below represents the circuit diagram of Non-Inverting Zero Crossing Detector. In this circuit we have used inverting op-amp. Here , the input voltage Vin is given in the non- inverting terminal of the op-amp and inverting terminal of op-amp is grounded.

CIRCUIT DIAGRAM :- NON INVERTING ZERO CROSSING DETECTOR

Working of Non-Inverting Zero Crossing Detector

As per definition it detects the point where the input signal is ZERO because it’s reference voltage is 0. Before start the working of Non-Inverting Zero Crossing Detector we have to let the output voltage is +V sat.

Now see the circuit to understand the working.

As per circuit reference voltage is 0 which is applied at the inverting terminal of op-amp which is compared with Vin which is applied at the non – Inverting terminal of op-amp.

Now when positive half of the sinusoidal wave appears at the input and op-amp comparator compares its peak level with the reference voltage.

Vo = Vin – V ref

because the reference voltage is 0, so

Vo = Vin – 0

as we have already mentioned that starting output voltage is (+V sat), so

Vo = (+V sat) – 0

Vo = (+V sat).

Now when the negative half of the sinusoidal wave appears at the input and op-amp compares its peak level with the reference voltage.

So,

Vo = V ref – Vi

Vo = 0 – (+ V sat )

We get

Vo = (-V sat )

Here we can clearly see that how a NON – INVERTING Zero Crossing Detector detects the changes in the level of applied input sinusoidal signal.

Input and Output Waveform

In the beginning we have already mentioned that Zero Crossing Detector is also known as a Square Wave generator. Same as the output of the Window Comparator is nothing but a Square Wave.

Now Let us see how a Non-Inverting Zero Crossing Detector generate the Square Wave or Output waveform.

in the working part we have derived that Vo for the positive half of the sinusoidal signal is (+V sat). because of this we get Positive output for the positive half of the applied signal. while Vo for the negative half of the applied input signal is (-V sat). because of this we get negative output for the negative half of the applied signal.

INPUT & OUTPUT WAVE FORM

Applications of Zero Crossing Detector

Some of the Applications of Zero Crossing Detector

• It is mainly used for switching purpose in electronics circuits and in phase locked loop.
• It is used in frequency counters.
• It is used as phase meter for measuring the phase angles between two voltages applied at its terminals.

Advantages and Disadvantages of Zero Crossing Detector

Some of the Advantages and Disadvantages of Zero Crossing Detector are given Below

Advantages

• It is used to synchronize the switching of power devices.
• When we use zero crossing detector for circuit operation using AC voltage then it can detect where the waveform of AC signal crossing the zero voltage point.
• It is also ensure us in safe and efficient operation of circuit.

Disadvantages

• Major disadvantage of Zero crossing detector is that some time Vin take more time for crossing the 0 voltage that’s why can not switch quickly from one saturation voltage to another.

Comparison Between Comparator and Schmitt Trigger

PARAMETERS	COMPARATOR	SCHMITT TRIGGER
1. USE OF FEEDBACK	No feedback op-amp used in open loop configuration.	Uses positive feedback.
2. TRIGGER LEVELS	At 0V or at V ref .
3. OUTPUT VOLTAGE	+V sat or -V sat	+V sat or -V sat.
4. HYSTERESIS	0V

Conclusion

After all we can say that it is working like a COMPARATOR which is compare when the Vin is low or high than 0volt(Reference Voltage). it is very useful in the operation of circuit where the AC voltage is used, although some time it can not switch back quickly from one saturation to another. it is also very useful for switching purpose and in phase locked loop.

FAQs on Zero Crossing Detector

What is Zero Crossing Detector ?

Zero crossing detector is a simplest circuit configurations of op-amp switching circuits. in to the Zero crossing detector configuration , the input signal is applied to one of the input terminals while the other input terminal is connected to ground . this circuit has no need to feedback connection.

What is the use of Protection diode in Zero Crossing Detector ?

In the zero crossing detector protection diodes are used to prevent the op-amp from the high input gain, because here we use direct input voltage, as there is no feedback connection.

What is the major disadvantage of Zero Crossing Detector ?

In some applications it can not switch quickly from one saturation voltage to other saturation voltage that’s why it’s speed is slow and in presence of noise the output can fluctuate between two saturation voltages.