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Zero crossing detector

akalii

New Member
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
ZERO-CROSSING.JPG
 

schmitt trigger

Well-Known Member
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
 

AnalogKid

Well-Known Member
Most Helpful Member
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
 

Visitor

Active Member
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.
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.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
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.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
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.
Which is exactly what you want - the pulse is generated when the LED is OFF, so you want it ON as quick as possible.
 

KMoffett

Well-Known Member
"ANOUXR said:
I want to use it for dimming a halogen lamp (220v ac)"
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.
Dimming Halogen bulbs
 

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