Download Zero Crossing Detector

Zero Crossing Detector
M.S.P.V.L Polytechnic College,
Pavoorchatram
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Introduction

A zero crossing detector literally detects the transition of a
signal waveform from positive and negative, ideally providing
a narrow pulse that coincides exactly with the zero voltage
condition.
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Basic Zero cross detector
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Top view of Zero cross detector circuit
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Circuit details

This circuit has been around (almost) forever, and it does
work reasonably well.

Although it has almost zero phase inaccuracy, that is
largely because the pulse is so broad that any inaccuracy is
completely swamped.

The comparator function is handled by transistor Q1 - very
basic, but adequate for the job.
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Modified circuit of Zero cross detector
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Simulation of Zero cross detector
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Simulation of Zero cross detector

The zero crossing detector is a device for detecting the point
where the voltage crosses zero in either direction.

The first section is a bridge rectifier, which provides full wave
rectified output.

This is applied to the base of the transistor through a base
resistor, R2.
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Cont.,

The capacitor charges to maximum of the bridge rectified
output through the diode,D5.

This charge is available to the transistor as VCC .

The capacitance value is kept large in order to minimize ripple
and get perfect dc.
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Cont.,

Once the transistor is ON and IB increases according to
the input wave, the transistor moves slowly towards
saturation where the output reduces to the saturation
voltage of the transistor which is nearly equal to zero.
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Two effects

Now the diode is reverse biased and hence does not
provide a discharging path for the capacitor, which in turn
has two effects.
1. Variation in VCC.
2. It will provide base current to the transistor in
the region where both diode and transistor are OFF.
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Output waveform
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Comparator zero cross detector

The comparator used for a high speed zero crossing
detector, a PWM converter or conventional ADC is
critical.

Low propagation delay and extremely fast operation are
not only desirable, they are essential.
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Comparator zero cross detector
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Comparators

Comparators may be the most underrated and under
utilised monolithic linear component.

Comparators are frequently perceived as devices that
crudely express analog signals in digital form - a 1-bit A/D
converter.
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Comparator timing Error
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Cause the Error

The comparator can mess with our signal, causing the
transition to be displaced in time, thereby causing an
error.

The significance of the error depends entirely on our
expectations - there is no point trying to get an error of
less than 10ns for a dimmer
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The End
…… Thank You ……
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