Problem when firing a TRIAC at zero-crossing

Thread Starter

Muhammad Irfan 6

Joined Dec 5, 2017
6
Schematic.jpg Case 1.jpg Case 2.jpg Hey guys im working on a BTA41-800 triac . i just want to turn on my triac when 2nd zero-cross imean my load is a power transformer so i dont want my transformer turn on at peak value of AC 220V input. i just want my transformer turn on through Triac at zero-level crossing to reduce the inrush current.
Okey my problem is that im detecting the zero-crossing using op-amp and X-OR gate.
Case 1 : In start up my circuit will work fine when input (12VPP AC ) to op-amp starts from negative cycle to positive .

Case 2 : In start up if the (12VPP AC ) waveform starts from positive imean from peak at that time my circuit will not work fine. i will generate an extra high (+5v) signal at the output of x-OR gate which is what idont need.

my requirement is that i want to make a logic which can skip the first output pulse of X-OR and take the second pulse (2nd zero-crossing) as a final output.

Regards: Engr. Irfan
 

Motanache

Joined Mar 2, 2015
523
Okey my problem is that im detecting the zero-crossing using op-amp and X-OR gate.


That's why I use MOC3041 and you have galvanic isolation with the control circuit.

Until now I only used this optoriac alone. Give it the command before passing through 0V, but it only starts when the sinusoid passes through 0V.
 

Motanache

Joined Mar 2, 2015
523
just want to turn on my triac when 2nd zero-cross imean my load is a power transformer so i dont want my transformer turn on at peak value of AC 220V input.
- I did not understand why is usefull to switch at a second pass through 0V
- I do not think it's a problem when the triac starts at maximum value of sinusoid.
(E.g. the triac is firing at the phase 90 degrees) because the primary of the transformer has a fairly large impedance and will not allow the current to grow very fast through it.

If you want what you said solutions are:
- microcontroller and remove the rest of the pieces (it can make the detection of 0V cross and counting second one)
- or the schematic presented by you plus a counter or bistable
 
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Motanache

Joined Mar 2, 2015
523


If we consider the impedance of the primary transformer L and the resistance of the primary coil R, then your time base(tau) does not depend on the voltage.

If you fire triac at a 90 degree angle you might even have a lower current then normal
(not the same thing happens to the SMPS that charge a imput capacitor. There are big currents)
 
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Thread Starter

Muhammad Irfan 6

Joined Dec 5, 2017
6
Hey guy i hope you are all fine .. im very thanks for your help .
Basically im using a power transformer 50A transformer and i want to make a soft starter. so i make a decision to try a different and reliable technique, which is relay base technique .
Normally my transformer primary coil charges up though a current limiting resistor and after one or 2 seconds my relay makes the contract for inputting the full power to transformer. I hope by using this idea my transformer will not takes high inrush current. Now i want to calculate that current liming resistor.
 

crutschow

Joined Mar 14, 2008
26,969
(E.g. the triac is firing at the phase 90 degrees) because the primary of the transformer has a fairly large impedance and will not allow the current to grow very fast through it.
That depends upon the transformer load.
With a low impedance load, such as a rectifier capacitor filter, firing at the peak of the sinewave will generate a much larger inrush current than firing at the zero crossing.
 

anishkgt

Joined Mar 21, 2017
547
Just sharing my experience with one such project. i used an arduino to accomplish this. H11AA1 used a zero cross detector, meaning it would give a pulse for every zero cross detected. Then use arduino's interrupt to detect it.


Hope this helps.
 
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