how do get Square wave for zero cross detection when i used transformer?

[manikandan123]

Dear friends!.

I want to make transformer based power supply. I used 12-0-12 transformer. I want to get 24V from both end. after that 24V to 5V. in the mean time I am also want square wave for zero cross detection. I put 230VAC on primary winding. we already know that there is a phase shift between primary and secondary of transformer. So how do i get square wave in phase with primary AC ? is it possible to get from secondary output? otherwise how do get square wave from primary with common ground? give suggestion .

awaiting for your responses.

 

[barry]

Unless you know ahead of time the amount of phase shift from primary to secondary, you're out of luck using the secondary to generate your square wave. You could use your primary 230V, and use a voltage divider to drop it to a non-lethal level and apply it to an op-amp zero crossing detector.

 

[walkura]

You have opto's with 2 antiparallel leds in it, a few 100 kiloOhm's of resistor to the primairy and You have a zero crossing.
(ofcourse You will have to provide a collector or emitter resistor on the secundairy side to Your 5 Volt rail)

I hope this will help You.

 

[klystron]

You could use your primary 230V, and use a voltage divider to drop it to a non-lethal level and apply it to an op-amp zero crossing detector.

No a good idea, if you swap L and neutral by accident around your ground is at 220VAC !!!! Always use galvanic isolation

 

[manikandan123]

hi Barry !.
Thanks for response. if I use OP amp for zero cross detection,input to the op amp from primary, is it isolated one? may I take Power and ground from secondary for that Op amp?

---------- Post added at 13:26 ---------- Previous post was at 13:23 ----------

You have opto's with 2 antiparallel leds in it, a few 100 kiloOhm's of resistor to the primairy and You have a zero crossing.
(ofcourse You will have to provide a collector or emitter resistor on the secundairy side to Your 5 Volt rail)

I hope this will help You.

may i use MOC3021 6 pin opto for that Opto? can you show any schematics in net? is it better isolation?

---------- Post added at 13:28 ---------- Previous post was at 13:26 ----------

No a good idea, if you swap L and neutral by accident around your ground is at 220VAC !!!! Always use galvanic isolation

thanks for your response. may be the isolation problem raised in that point.we consider it sir. thank you..

 

[barry]

You have opto's with 2 antiparallel leds in it, a few 100 kiloOhm's of resistor to the primairy and You have a zero crossing.
(ofcourse You will have to provide a collector or emitter resistor on the secundairy side to Your 5 Volt rail)

I hope this will help You.

The only problem with this method is that it's not going to give you zero crossing-the LEDS in the opto dont' turn on until there's about 2volts across them, thus you will have about 4 volts of deadband as the input crosses through zero volts. If this is acceptable, then it's a good solution.

 

[manikandan123]

Thanks all for your responses. I want Exact zero crossing from Primary. I couldn't take from secondary.. What the solutions.. is it possible to get it from Optocoupler? Somebody using 817a. 4n35 opto... Can you suggest me some links?

 

[crutschow]

The unloaded phase-shift of a transformer is quite small. What phase accuracy do you need?

 

[manikandan123]

The unloaded phase-shift of a transformer is quite small. What phase accuracy do you need?

Thanks for ur response. i Don't know about phase shift.. for my project,I need that Square wave detection. At each zero cross point of AC wave , i need square wave .

 

[crutschow]

Thanks for ur response. i Don't know about phase shift.. for my project,I need that Square wave detection. At each zero cross point of AC wave , i need square wave .

In your first post you are concerned about the phase-shift of the transformer. Now you say you don't know about phase shift. So which is it?

To do a proper engineering design you need to specify your requirements, otherwise how do you know when the design is right? :?: You state you need a square wave at each zero crossing of the AC waveform. But no design can give you the exact crossing point, there's always some error.

So you need to define (or decide based upon what the square wave is doing) what accuracy you need. Then we can help you with the design. Otherwise we're just blowing smoke.

 

[vimalkhanna]

It is advisable to use a 100Kohm res. + 2nos 4007 back to back and another 100Kohm returned to neutral across the 230VAC for zero crossing detection.
To isolate from mains use an optocoupler in series with the 4007s and derive isolated output from the collector.The zero crossings will be exact square wave clean ,no phase shift ,good isolation,float terminals from whatever the supply ,5V ,12V ,15V or 24V to be used by the operational subsequent circuits.

 

[manikandan123]

In your first post you are concerned about the phase-shift of the transformer. Now you say you don't know about phase shift. So which is it?

To do a proper engineering design you need to specify your requirements, otherwise how do you know when the design is right? :?: You state you need a square wave at each zero crossing of the AC waveform. But no design can give you the exact crossing point, there's always some error.

So you need to define (or decide based upon what the square wave is doing) what accuracy you need. Then we can help you with the design. Otherwise we're just blowing smoke.

I agree with you. Initially I told that I need square wave for zero cross detection. and we already know that there is a phase shift between primary and secondary of transformer. So how do i get square wave in phase with primary AC ? ... What i am mean in last reply. I don't know about the phase shift value? the square wave should be in phase with AC. I want this one..Did you got it.. and also that I am also designed square wave zero cross detection for transformer less power supply.I got Exact zero cross point. In case of transformer based power supply, how do I maintain the ISOLATION?.. So only I asked about some Opto configuration.

---------- Post added at 09:35 ---------- Previous post was at 09:25 ----------

It is advisable to use a 100Kohm res. + 2nos 4007 back to back and another 100Kohm returned to neutral across the 230VAC for zero crossing detection.
To isolate from mains use an optocoupler in series with the 4007s and derive isolated output from the collector.The zero crossings will be exact square wave clean ,no phase shift ,good isolation,float terminals from whatever the supply ,5V ,12V ,15V or 24V to be used by the operational subsequent circuits.

Thanks. Please check image. Is it correct ? at what place I got square wave? anyway I check it... Thanks...

---------- Post added at 09:38 ---------- Previous post was at 09:35 ----------

 

[vimalkhanna]

This is exactly what I had in mind but nonisolated across the diodes. ..However ,what is done is place an optocoupler led in series with the 4007 and the collecytor/emitter output is duly isolated ,in exact phase ,zero crossings corrected ..In case you need to be more accurate, magnify using a gain stage and clip/clamp again so that accuracies are within 0.25 * ...1/4 degree. ...ok?