1. Zero Crossing Detector
M.S.P.V.L Polytechnic College,
Pavoorchatram

2. 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. 

3. Basic Zero cross detector

4. Top view of Zero cross detector circuit

5. 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.

6. Modified circuit of Zero cross detector

7. Simulation of Zero cross detector

8. 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.

9. 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.

10. 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.

11. Two effects 1. Variation in VCC.
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.

12. Output waveform

13. 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.

14. Comparator zero cross detector

15. 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.

16. Comparator timing Error

17. 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

18. The End
…… Thank You ……