Zero-crossing detection based on operational amplifier

The zero-crossing detection circuit is a common application, where an operational amplifier is used as a comparator. It is often used to track changes in sine waveforms, such as the zero-crossing voltage from positive to negative or from negative to positive. It can also be used as a square wave generator. There are many applications for zero-crossing detection circuits, such as marker signal generators, phase meters and frequency meters. The zero-crossing detection circuit can be designed in many ways, such as using transistors, using operational amplifiers or optocoupler ICs. In this article, we will use an operational amplifier to build a zero-crossing detection circuit. As mentioned above, the operational amplifier here is used as a comparator.

The ideal waveform of the zero-crossing detection circuit is as follows

It can be seen from the above figure that when the sine waveform crosses zero, the operational amplifier will change from positive to negative or from negative to positive. This is how the zero-crossing detector detects the zero-crossing of the waveform. As you can see, the output waveform is a square wave, so the zero-crossing detector is also called a square wave generating circuit.

5f44bceb5967a-thumb.png

Required components

Operational amplifier (LM741)

Transformer (230V to 12V)

9V power supply

Resistance (10kΩ x 3)

Breadboard

wire

Oscilloscope

Circuit diagram

The 230V power supply is supplied to a 12-0-12V transformer, and its phase output is connected to the second pin of the operational amplifier, and the neutral wire is short-circuited with the ground terminal of the battery. The solicitation pin of the battery is connected to the No. 7 pin of the operational amplifier (Vcc).

Principle of zero-crossing detection circuit

In the zero-crossing detection circuit, the non-inverting pin of the operational amplifier is connected to the ground as a reference voltage, and a sine wave input (Vin) is input to the inverting pin of the operational amplifier, as the circuit diagram says. Then the input voltage is compared with the reference voltage. Most operational amplifier ICs can be used here, here we use LM741.

Now, let's consider the positive semi-axis of the sine wave. We know that when the voltage at the non-inverting pin is lower than the inverting pin, the output of the operational amplifier is low or in a reverse saturation state. Therefore, we will see a negative voltage waveform.

Looking at the negative half axis of the sine wave again, the voltage of the non-inverting pin (reference voltage) is greater than the inverting pin (input voltage), so the output of the operational amplifier is high or positively saturated. Therefore, we will see a positive voltage waveform as shown in the figure below.

02.png

Zero-crossing detection circuit using optocoupler

We mentioned above that there are many ways to design a zero-crossing detection circuit. In the following circuit, we use an optocoupler to implement the same zero-crossing detection circuit. By observing the output voltage, you can find that whenever the input AC wave crosses zero, the output waveform is high.