Zero Cross Detector (ZCD) Circuit

Some of the circuits require zero cross reference mark of applied AC waveform as an input for their operation. The circuit will produce output based on these zero marks. The output is either a short positive or negative pulse when AC wave crosses zero line. For example in thyristor phase angle control circuit, zero cross reference mark of AC supply waveform is required to trigger thyristor at a specific angle. The phase difference measurement circuit will take zero cross mark inputs of two different AC waveforms and calculates phase difference between these two waveforms. Refer the waveforms given in following figure to understand above two examples better.

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The circuit presented here produces one short duration positive pulse and also one short duration negative pulse whenever applied input AC waveform crosses zero line– that means it produces 2 pulses (positive and negative) per AC waveform cycle. In this experiment standard 230 V @ 50 Hz AC supply is taken as input. It is first step down by transformer and then given to circuit input. Any AC input with peak to peak voltage lower than 12 V can be directly applied to circuit input. The outputs are

1. Short positive pulses (<1 ms) of 100 Hz

2. Short negative pulses (<1 ms) of 100 Hz

These outputs can be used as zero crossing marks in different applications like thyristor phase angle control, phases difference measurement etc. They can be used as input in different circuits to generate output with reference to this zero cross mark.

Circuit connections:

As shown in figure transformer T1 step downs 230 VAC in to 6 VAC and this is given to Diode Bridge. This rectified output is directly fed to base of Q1 through diode D1, resistors R1 & R2. Same rectified output is filtered through C1 to produce pulsating DC output. This unregulated 9 VDC is given as biasing voltage for both transistors Q1 & Q2. RED LED indicates supply is ON. Both transistors are connected in switch configuration.

Circuit operation:

Let us understand circuit operation with the help of waveforms.

· As shown in figure the first wave form is full rectified wave produce by diode bridge (position ‘A’ in schematic)

· It is voltage divided by R1 and R2 and fed to the base of Q1. Q1 is configured in switch configuration so whenever this voltage falls below 0.7 V it is switched off. So its output goes high.

· This will produce one short positive pulse at ‘B’ as shown in figure as second waveform

· This positive pulses are fed to base of Q2 which is again connected in switch configuration

· It will produce one negative pulse at ‘C’ of same width of positive pulse. This is shown as third waveform

The output of the given circuit built on bred board is observed on DSO. Here is the snap of output