Zero crossing detector

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akalii

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Hi,
I want to use a zero crossing detector on my projet, i need help about the value of R1,R2 so i cant dammage the bridge (2W04G) and the optocoupleur(4n25).Im new to electronics so i have some issues on reading datasheet.
Thanks for your time
 
1) Go to the optocoupler's data sheet and read the LED's maximum continuous current.
2) Calculate the peak sinewave voltage Vpk = Vrms * 1.414
3) Use Ohm's law to calculate R from the above values. Use the next highest available value.
4) Calculate the resistors power rating. P= V^2/R Give yourself some margin and choose the highest resistor rating.
5) You did a safe thing in dividing R into two components R1, R2. Both which are R/2
 
The peak value of the input sinewave sets the peak current through the LED. This is the most important thing to control for long-term reliability. Read the datasheet, run some numbers, and tell us your conclusions and how you got them.

Note: the higher the LED peak current, the narrower the output pulse centered about the true zero-crossing. BUT - also, the higher the operating current, the higher the power dissipation in the two resistors. It is a trade-off.

ak
 
AnalogKid said:
Note: the higher the LED peak current, the narrower the output pulse centered about the true zero-crossing. BUT - also, the higher the operating current, the higher the power dissipation in the two resistors. It is a trade-off.
Click to expand...

I'm curious why a higher peak current would result in a narrower pulse. Higher current means lower value resistors, which should allow the LED to turn on earlier in the sine wave cycle.
 
I'd also add a moderately high value resistor across the LED itself to discharge any residual voltage, eg. something that takes around 5% of the total LED drive current.

That should give a cleaner and more reliable zero pulse.

Note that if you need to synchronise to the AC polarity, you can use two optos (or a dual one) with the LEDs connected in opposite directions, eliminating the bridge rec.
 
Visitor said:
I'm curious why a higher peak current would result in a narrower pulse. Higher current means lower value resistors, which should allow the LED to turn on earlier in the sine wave cycle.
Click to expand...

Which is exactly what you want - the pulse is generated when the LED is OFF, so you want it ON as quick as possible.
 
I want to use it for dimming a halogen lamp (220v ac)"
Click to expand...
From my post to your thread on another forum:

I don't think anyone has said it, but dimming a halogen lamp is not a good idea. Halogens need to operate a full power to produce the heat necessary to maintain the halogen cycle. Lower power will darken the glass and shorten the lamps life.
**broken link removed**
 
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