# Zero-Crossing Detector

Discussion in 'General Electronics Chat' started by cheesemunger, Feb 15, 2009.

1. ### cheesemunger Thread Starter Active Member

Jan 25, 2009
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I've been using http://www.andrewkilpatrick.org/blog/?page_id=445 and it says to convert an AC voltage to DC and then use a 4n25. But someone else says I can just use a H11AA1 straight away which would make sense. If that is true then that is gd but I don't have access to that , I have access to ILD620, is that fine?

2. ### Ron H AAC Fanatic!

Apr 14, 2005
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ILD620 should work. Limit the peak input current to ±40mA or less.

3. ### cheesemunger Thread Starter Active Member

Jan 25, 2009
53
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How do you suggest I do that?

4. ### Ron H AAC Fanatic!

Apr 14, 2005
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With a series resistor. Choose the value using Ohm's law: R=V/I.

5. ### cheesemunger Thread Starter Active Member

Jan 25, 2009
53
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so voltage is 240v and current is 60MA which is 0.06A. 240/0.06=4k
so.... I could use a 4.7K 1/2W resistor? Or do I need 2 resistors in parallel to dissipate heat better?

Apr 5, 2008
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7. ### cheesemunger Thread Starter Active Member

Jan 25, 2009
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Do I really have to you a capacitor? Can't I just use resistors? If I have to use a capacitor, does it matter what value?

Apr 5, 2008
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Hello,

On the page I gave you is stated how to calculate the value.

Greetings,
Bertus

9. ### cheesemunger Thread Starter Active Member

Jan 25, 2009
53
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Wait, I'm not sure where I got my 0.06A from? LOL Where should I be getting it from?

Apr 5, 2008
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Hello,

Ron H has said to limit the current for the ILD620 to 40 mA.
I think the 60 mA was from the original schemaitc.

Greetings,
Bertus

11. ### cheesemunger Thread Starter Active Member

Jan 25, 2009
53
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On the website I posted to begin with he used 2x 47K 1/2W resistors in parallel, why can I not just use that?

Apr 5, 2008
15,648
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Hello,

It depends on how many current you want to have on the led.
In the origenal with 2X 47K there will be ca. 10 mA on the led.

Greetings,
Bertus

13. ### cheesemunger Thread Starter Active Member

Jan 25, 2009
53
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all its got to do it provide a high signal to a microprocessor (arduino) so 10mA should be fine.

Apr 5, 2008
15,648
2,347
Hello,

It is the current in the led, not the transistor.

Greetings,
Bertus

15. ### eblc1388 Senior Member

Nov 28, 2008
1,542
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Using a capacitor to provide voltage drop is common in many applications but not in this case where the aim is to detect the zero crossing of the AC voltage.

Adding a capacitor in the detection path will inevitable shiftes the zero crossing point from the true zero crossing and makes the detection invalid.

16. ### cheesemunger Thread Starter Active Member

Jan 25, 2009
53
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eblc1388: Thats what I though

If you look later on in his post at his final schematic, he removes the led and just connects it to an input pin on a microcontroller.

17. ### eblc1388 Senior Member

Nov 28, 2008
1,542
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Which post?

Where can I see the final schematic? Can you provide a link?

There has been a recent thread about feeding AC voltage into a PIC pin via high value resistor to detect zero crossing. Doing so has other problems too.

Jan 25, 2009
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19. ### eblc1388 Senior Member

Nov 28, 2008
1,542
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I opened the pdf but still can't see what you meant. Sorry.

Which input pin on the MCU are you referring to? Pin#22?

20. ### cheesemunger Thread Starter Active Member

Jan 25, 2009
53
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look at Pin 21