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Improved Optical Zero Crossing detector : Strange Cap values

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eblc1388

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While searching the Net for zero crossing detector circuit, I came across the following circuit design idea that appeared in the EDN 12/14/2007 edition.

The link of the full article is here:

**broken link removed**

However, I have trouble understanding the value of the two capacitors being used. The schematic of Fig.2 is attached. The same capacitor values are used for another schematic(Fig.3) also. In my opinion, one capacitor should be for noise filtering and the other should be charge storage for a current source but their values are not appear to be appropriate.

Any comments?
 

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I have considered the possibility.

There is a similar comment on the cap values dated back to early January but there is no reply from the author or the two editors yet.
 
eblc1388 said:
I have considered the possibility.

There is a similar comment on the cap values dated back to early January but there is no reply from the author or the two editors yet.

It's so obviously incorrect, I wouldn't have bothered asking!.
 
I have a chance to simulate the above circuit using SwitcherCad3. The simulation result of the original design is not good.

The current pulse position is off by a large amount. I have no idea why the original design will work.

Can anyone advise if my simulation parameters are correct or not? The SwitcherCad asc file is also attached.
 

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On1aag,

Thanks for the information. The circuit works very nicely in the simulator. The startup time to stabilization is less than half a second and the zero crossing pulses occur at the right moment.

The attached image shows the AC main voltage and the current in R4, the 390 ohm resistor feeding the LEDs of the optoisolator TLP504A.
 

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No particular loading type in mind. Just try to understand how different circuit designs of zero crossing detection circuit perform in real life.
 
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