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  1. #1
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    Seeking for zerocrossing switching without using microcontroller

    Hello There,


    Hope this post find you well.
    May be or may not you would love it, because this issue has reached huge number of problems.
    Someone could say "school project for infants".


    I was trying to build a SOLID STATE RELAY with zero crossing switching.
    Similar to this product, http://demo5.zmpress.net/solid-state...ys-xssr-2402p2

    In my case I was using MOC3021http://www.farnell.com/datasheets/97984.pdf and BTA100-800A.
    dv/dt of BTA100 is 500V/us.
    My application is 380V 40A load switching.
    Lets say for inductive and capacitive load.

    Main issue is "someone said me not to use any microcontroller to drive the TRIAC gate".
    So, random switch type SSR has been chosen,
    Click image for larger version. 

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    It might be non-zero crossing type SSR. 5V dc as been used that does not make sense for gate switching or its keeping the gate ON always.


    How about using a TIMER as follows?
    Click image for larger version. 

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    Can we control zero crossing with this idea?
    Hasan

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    Re: Seeking for zerocrossing switching without using microcontroller

    Quote Originally Posted by Hasan2017 View Post
    Lets say for inductive and capacitive load
    Therefore it's a reactive load. It may cause AC voltage crossings to be out of alignment with AC current crossings. Then you're not certain which you are sensing.
    Or it may cause continued current flow through the triac, causing it to stay On after it's supposed to turn off.

    - - - Updated - - -

    To detect zero current crossings, it does not necessarily work properly if you sense voltage across a capacitor or inductor.
    Instead sense voltage across an ohmic resistance (low ohm value, low enough to create a usable reading).



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    Re: Seeking for zerocrossing switching without using microcontroller

    If you want zero crossing switch, you'll use a ZC opto triac like MOC304x. MOC 302x isn't.



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    Re: Seeking for zerocrossing switching without using microcontroller

    Hi,

    zero cross switch and reactive load... some additional informations:

    Zero cross switch means:
    * It will switch ON at (or close at) zero cross of mains voltage
    * this is the worst case for inductive loads --> this may cause saturation in core --> this may cause high inrush current
    * a triac, thyristor, SCR always switches OFF at zero current, this is not necessarily zero cross of mains voltage.

    Klaus
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    Re: Seeking for zerocrossing switching without using microcontroller

    * this is the worst case for inductive loads --> this may cause saturation in core --> this may cause high inrush current
    Right, I overlooked the welding transformer application. In this case, you better use a circuit that starts at voltage maximum.



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    Re: Seeking for zerocrossing switching without using microcontroller

    Quote Originally Posted by BradtheRad View Post
    Therefore it's a reactive load. It may cause AC voltage crossings to be out of alignment with AC current crossings. Then you're not certain which you are sensing.
    Or it may cause continued current flow through the triac, causing it to stay On after it's supposed to turn off.

    - - - Updated - - -

    To detect zero current crossings, it does not necessarily work properly if you sense voltage across a capacitor or inductor.
    Instead sense voltage across an ohmic resistance (low ohm value, low enough to create a usable reading).
    Thank you very much to join in this thread.
    You said well, for the inductive load PF is less then 1. Phase between V & I makes problem during zero crossing. Manufacturers said, not to use a "zero crossing type". Random type fits well.


    Any way, if you look this circuit, a monostable 555 timer is used to make pulse train with certain frequency. MOC3021 has isolation between primary and secondary. That means this circuit is not usefull for zero crossing detection, but zero crossing can be done here.

    Now if I fix the ON time and OFF time of 5 v pulse with proper duty cycle, cant it be possible to see in scopes ? Can we compare pulse and full AC 1 cycle in load? May be not!

    MOC3021 pin 4 gate pulse will decide when the TRIAC will turned ON. There is " holding time" and voltage ON state. Not sure what it tells about zero current and zero voltage switching.
    Sometime TRIAC may not turned ON. Gate current and dv/dt facts here.

    Most of the people use microcontrollers to make particular pulse width and firing angle for zerocrossing.

    - - - Updated - - -

    My case it capacitive load.

    - - - Updated - - -

    Sure, may be its dedicated to zero crossing.
    Hasan



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    Re: Seeking for zerocrossing switching without using microcontroller

    Quote Originally Posted by KlausST View Post
    Hi,

    zero cross switch and reactive load... some additional informations:

    Zero cross switch means:
    * It will switch ON at (or close at) zero cross of mains voltage
    * this is the worst case for inductive loads --> this may cause saturation in core --> this may cause high inrush current
    * a triac, thyristor, SCR always switches OFF at zero current, this is not necessarily zero cross of mains voltage.

    Klaus

    Again you came with nice feedback.
    Thanks.

    Now looking at the datasheet of MOC3083
    https://www.mouser.com/datasheet/2/1...81M-356014.pdf, it is dedicated to achive "zero crossing".
    I have tested and zerocrossing works, my output was a heater with 220VAC/2A.

    Click image for larger version. 

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    If I use this circuit for 40A, 380 VAC reactive load then what would be the Watt Limit for resistors and capacitor ?
    I was using 2W for 360R and 630VDC for film capacitor in case of 220VAC/2A.
    Hasan



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    Re: Seeking for zerocrossing switching without using microcontroller

    Hi,

    If I use this circuit for 40A, 380 VAC reactive load then what would be the Watt Limit for resistors and capacitor ?
    I'm not sure whether you ask about this:
    P(in Watt) = V x A x cos(phi)
    V and I need to be RMS values.

    Klaus
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    Re: Seeking for zerocrossing switching without using microcontroller

    Quote Originally Posted by KlausST View Post
    Hi,


    I'm not sure whether you ask about this:
    P(in Watt) = V x A x cos(phi)
    V and I need to be RMS values.

    Klaus
    Could you kindly see here, https://www.st.com/content/ccc/resou...CD00004096.pdf
    Hasan



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    Re: Seeking for zerocrossing switching without using microcontroller

    Hi,

    You talked about a reactive load... and power and resistor and capacitor. Thus I assumed you talk about load resistor and load capacitor.
    But reviewing the document I now guess you want to know about snubber R and snubber C.

    The snubber power dissipation (at the resistor) depends
    * triac OFF: the applied voltage, the series impedance of R and C --> the resulting current --> P = I x I X R
    * triac switching: the stored energy in the load, that now has to be dissipated in the snubber R. Since at switching OFF the current is close to zero I don´t expect much stored energy in the load inductance.
    I expect in your case the power dissipation is higher when the triac is OFF.
    For sure when the TRIAC switches ON at non zero voltage, then one can expect a big peak power dissipation (high dV/dt), but the average should not be higher than when the triac is continously OFF.

    Klaus
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  11. #11
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    Re: Seeking for zerocrossing switching without using microcontroller

    Quote Originally Posted by KlausST View Post
    Hi,

    You talked about a reactive load... and power and resistor and capacitor. Thus I assumed you talk about load resistor and load capacitor.
    But reviewing the document I now guess you want to know about snubber R and snubber C.

    The snubber power dissipation (at the resistor) depends
    * triac OFF: the applied voltage, the series impedance of R and C --> the resulting current --> P = I x I X R
    * triac switching: the stored energy in the load, that now has to be dissipated in the snubber R. Since at switching OFF the current is close to zero I don´t expect much stored energy in the load inductance.
    I expect in your case the power dissipation is higher when the triac is OFF.
    For sure when the TRIAC switches ON at non zero voltage, then one can expect a big peak power dissipation (high dV/dt), but the average should not be higher than when the triac is continously OFF.

    Klaus
    Datasheet tells us to calculating snubber circuit. Damping, Transisent, Resonance all are considerable.
    But for 380Vpk, 40 A, PF=-0.9 capacitive load, what would be wattage of resistor 360R,39R,340 R?
    Maximum 3W?
    My pcb need more space ?
    Hasan



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