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Need explanation about zero crossing detector

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dummy55555

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Hi, I see that there are so many different design of zero crossing detector. I found that this design use most less component. Can anyone explain the operation of this zero crossing detector for me.. Including what is the role for every components. Really want to understand how this circuit work..
Thank you.
Your help is much appreciated...
 
hi,
These are the basic components that are used for the ZCD in that circuit
 
Hi, I see that there are so many different design of zero crossing detector. I found that this design use most less component. Can anyone explain the operation of this zero crossing detector for me.. Including what is the role for every components. Really want to understand how this circuit work..
Thank you.
Your help is much appreciated...

Hi,

Your only concern is with the bridge rectifier and the transistor and associated resistors. The bridge rectifier takes the AC input and converts it to a pulsing purely positive wave. This pulsing turns the transistor on and off. Because even a small pulse will turn the transistor on, the transistor only turns off during the short time near zero degrees in the AC waveform. This means at the collector you see a short pulse and that tells you that the input AC wave is near zero degrees. Zero crossing means it has switched from positive to negative or negative to positive.

Many times the rest of the circuit (not shown) will only use the falling edge of the pulse at the transistor collector because most circuits want to know when the AC input has gone just a tiny bit past zero degrees rather than exactly at zero degrees.
 
i am surprised no one mentions ditching the extra diode by moving the sense point to the INPUT of the bridge!
 
i am surprised no one mentions ditching the extra diode by moving the sense point to the INPUT of the bridge!

hi D,
If you did that you would get only half cycle operation of the ZCD.?

Thats if I follow what you are saying, could you post a circuit.?

E.
 
i am surprised no one mentions ditching the extra diode by moving the sense point to the INPUT of the bridge!

Hi,

Like Eric said this is probably not possible because then we would not get the same signal out of the transistor. The output of the transistor with the circuit as is gives us pulses at 120Hz. With the base connected to the 60Hz input we'd only see a 60Hz output, and that would not work as well for detecting the zero cross. It's true we might be able to use the falling and rising edges to get the 120Hz, but the sync would be slightly different too because the falling edge occurs BEFORE the zero crossing rather than slightly after it. That makes a big difference in triac firing circuits because it it triggers too soon we end up with the output being the whole half cycle rather than a fractional portion of it.
A quick simulation would show the difference at the collector of the transistor.

One of the things not to leave out however is some resistive loading of the bridge rectifier to ground. That's to make sure the output goes all the way to ground near the zero crossing. This circuit already has some resistance to ground (the two 4.7k resistors in series) so that should be just fine.
 
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