Solid state relay application, would this work?

[XaviPacheco]

According to the attachment, if I place the AC input of a full wave rectifier as the load, would this circuit switch effectively? The MOC3061 has zero crossing. According to the second circuit, someone told me:

Zero crossing will work for the first 1/4 cycle, but then
a) There will be no current flow to the "zero-volt switch" IC for it to know when the zero-cross happens, because "2200uF" is charged to 170V & all bridge diodes are reverse biased.

b) Even if a firing pulse is given to triac @ zero-cross, it will not latch-on, because the available current is below "hold-on" current for Triac
 
What do you think of?

[JS]

That seems reasonable, how long does the trigger stays on after the zero crossing? i guess if you are leaving it on for long enough it will work, at least for half cycle or even all the time if your intention is to let it running. I don't know the output of the zero crossing detection module.

Couldn't you replace the triacs for SCR and do the rectification there, you replace 2 diodes for controlled ones and leave the other two with standard diodes. Your optocoupler could be a few things but there are dedicated gate drivers which could do.

JS

[XaviPacheco]

The opto signal will be continuously applied by a microprocessor. My worry is whether the triac would be always on, I mean, if the holding current is kept.

[JS]

Holding current won't be kept, the question is if the trigger pulse from the zero crossing detection module is kept from when the signal crosses zero while the input is high till the input is low again or just shoots a short pulse after each zero crossing while the input is high.

I'll look the DS and come back.

JS

[JS]

Haven't find my last question on the DS, I leave the link.
https://www.jameco.com/Jameco/Products/ProdDS/133874.pdf

Check fig 9. that's similar to what I was talking about, but you need to complete the bridge with diodes for it to work as a AC-DC trigger and the question I made before still doesn't have an answer. The answer could be pretty simply found with a prototype and checking, maybe just directly from the opto triac to a small rectifier and cap, you might blow one as it's not designed for this. Why do you want to use an opto triac with zero crossing detection. I guess the inrush would be much greater if not using it.

What you could do to ensure holding is to provide an alternative path for the holding current to flow, a CC in the load could do that, you could use a small bridge rectifier and a DC CC load to guarantee the holding current, not very efficient but would do the job.

I would probably go for SCRs and have more control over the rectification with a smarter µC than just turning on and off and relying on the zero crossing detection for the first half cycle to charge the cap slower.

JS

[XaviPacheco]

Your solution with the SCRs is okay. I'm just trying to study this case with the optotriac and zero crossing. I'm just interested in the topic, and wanted to clear my mind with regards to it.

What do you think of placing a resistor between AC1 and AC2 of the bridge to ensure the holding current?

[JS]

Your solution with the SCRs is okay. I'm just trying to study this case with the optotriac and zero crossing. I'm just interested in the topic, and wanted to clear my mind with regards to it.

What do you think of placing a resistor between AC1 and AC2 of the bridge to ensure the holding current?

Indeed, I'm also puzzled by this now, I would hook up a rig tight now and answer it once and for all but there's somebody waiting me for dinner. The thing is missing information from the datasheet I looked at, looking at more datasheets might help as well.

I wouldn't go with resistors, you need to ensure the holding current at low voltages so the dissipation at high voltages will be much more than required and wasting much more energy than required. If I would go for a passive approach I would consider some reactance, I would excercice what happens with a cap there, then a series RC and if they don't give the desired effects a more complex load, but then would be too complex. I guess a cap would generally work but might be some problematic condition than a series resistor could help with.

JS

[XaviPacheco]

I attach an APP note which may contain useful information (I've just read the title). I will read it now.

[David Hess]

The TRIAC and optocoupler do not "see" any voltage until the instantaneous AC voltage forward biases the bridge rectifier so as far as they are concerned, the real zero crossing is when the instantaneous voltage exceeds the voltage across the charged capacitor plus the voltage drop across the diodes.  So it should work fine.

[JS]

The TRIAC and optocoupler do not "see" any voltage until the instantaneous AC voltage forward biases the bridge rectifier so as far as they are concerned, the real zero crossing is when the instantaneous voltage exceeds the voltage across the charged capacitor plus the voltage drop across the diodes.  So it should work fine.

I think the thing triggers when it finds a zero, not when it goes outof it. The question and missing information from the DS is if once it detects a zero it keeps triggering till it hooks, till the next zero, till the input goes low, or what,

JS

[David Hess]

I think the thing triggers when it finds a zero, not when it goes outof it. The question and missing information from the DS is if once it detects a zero it keeps triggering till it hooks, till the next zero, till the input goes low, or what,

Zero crossing thyristor drivers are not that complicated and the datasheets cover it.  For the MOC3061:

Zero Crossing Characteristics - Inhibit Voltage (MT1-MT2 voltage above which device will not trigger): 12 volts typical and 20 volts maximum

So while the voltage is below 12 volts, the output is active assuming the LED is being driven.  The beginning of Motorola application note AN916 also describes them as working this way.

[XaviPacheco]

Update about this:

I've tried the circuit in real life, and the capacitor charges extremely slow, as the triac is no effectively switching, i.e, the voltage across the bridge is really low.