# help me with calculations

Discussion in 'The Projects Forum' started by Dimpi, Jan 3, 2014.

1. ### Dimpi Thread Starter Member

Sep 6, 2013
96
0
that may be a very simple ques what m goin to ask
..
m making a zero cross detector circuit using optocoupler and there is a attached pdf of it..

-but please anyone confirm me when we deal with ac...calculations are always done using peak value,peak to peak value or rms value??????

-how the wattage of resistor be decided??is it somewhat we should consider according to power dissipation???pls ny1 elaborate.

-if the average current of led in the optocoupler is mentioned as 50ma,,so is a current of 3ma or 4ma sufficient to make the LED on??

-one more thing the diode m using is for reverse blocking purpose to protect the LED so what value of diode should we take??

• ###### Design6.pdf
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Last edited: Jan 3, 2014
2. ### panic mode Senior Member

Oct 10, 2011
1,320
304
there is no such thing as always use "X". for computing loads, power etc, one uses RMS, for computing breakdown voltage one uses peak etc.

in this case I would:
1. lookup datasheet such as
http://www.avagotech.com/docs/AV02-0265EN

2. use that datasheet to determine CTR (50% is doable but it is terrible, I would choose something closer to 400%).

3. determine value of R47, say 1k

4. compute current through optos output side when it is on: Iout=5V/R47 = 5mA

5. use that and CTR to determine input current of opto: Iin=Iout/CTR =5mA/0.50=10mA

6. get value of R46: R46=230V/Iin=230V/0.010A=23k

7. get power of R46: P=V*Iin=230V*0.010A=2.3W, so we choose next bigger common rating such as 5W. you see the
problem now? if your opto was with better CTR, current through R46 would be smaller.

in case that current was 1mA, dissipated power is only 230mW, even 1/4W resistor would be acceptable (not to mention less power consumption and heating of your project). of course in this case R46 resistance would be higher too R46 = 230V/1mA=230k. see the difference?

btw, i made this simple so few things are ignored but even so this will get you in the ballpark.

you can use any diode as a protection as long as it can handle same current that is flowing through opto. reverse voltage is not important in this case. you can use 1N914, 1N4148, 1N4001 etc. or choose opto with AC input

3. ### Dimpi Thread Starter Member

Sep 6, 2013
96
0
TNK U SIR FOR QUICK RPLY..but 23k with 2 watt or more is not available...

sir,howw is it if we take r47 as 10k..iout would be 0.5mA.n den Iin would be 1 mA..so that R46 becomes 220k...if i calculate the power disspated thru R46 becomeswould be 220mW(that is pretty much low)..

but i m confused at one point seeing the datasheet of opto m using,it is mentioned that CtR=50% at If=5mA..what does this means.????is it like we have to provide 5 mA or above that in order to make led on????

Last edited: Jan 3, 2014
4. ### Dimpi Thread Starter Member

Sep 6, 2013
96
0
how to decide the diode ratings???
considering the peak value 400V of ac mains..m selecting the diode whose PVR is 600V(correct me if m wrong)

but wat about the current rating..what are the basics that i should know in order to decide the current rating of diode...please see the circuit attached in pdf and guide me according to that...that what must be the amperage rating of diode that i should select and the reson for that??

• ###### Design6.pdf
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Last edited: Jan 3, 2014
5. ### t06afre AAC Fanatic!

May 11, 2009
5,939
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Dimpi! That circuit is not suited. Then you want to control something connected to the mains. For this you should get an optocoupler with Phototriac Output. Those may have a zero cross turn on or not depending on type. This I have already mentioned to you in some other thread. Those optocoupler are quite common. So is it any reason why you do not want to follow that path?

6. ### Dimpi Thread Starter Member

Sep 6, 2013
96
0
Sir why this circuit is nt suited??please elaborate.

i wil explain it to u. this circuit m making is for for zero cross detection(or i can say zero cross detector)...the zero cross pulse which m getting at the output i will apply it to the input pin(interrupt pin) of microcontroller.
the phototriac output opto u are talking about i have used it at the output side whose output would trrigger the triac..(this way dimming application is acheivd)
what u find wrong in this circuit??please guide sir??

7. ### ronv AAC Fanatic!

Nov 12, 2008
3,287
1,252
If I understand correctly you want a zero cross detector. If so this idea won't work. CTR of 50% means so get 3 ma out of the transistor side of the opto you need 6 ma thru the diode. So if you think about it that takes a lot of voltage into the 100K resistor - certainly not zero.

May 11, 2009
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9. ### inwo Well-Known Member

Nov 7, 2013
2,433
315
Seems like I posted this before.

Uses 2 30k half watt.

Do you want to detect all zero crossings with bridge circuit or just half?

• ###### zero-cross.jpg
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