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Is a snubber circuit required with zero-cross optocoupler

tekbird

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I am a newbie to power electronics but can understand the basics. My very specific question is, whether a snubber circuit is needed for a triac that is driven by an optocoupler that is based on zero-crossing, like MOC3043?
 

barry

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I think it depends on the application. If you've got an inductive load, where current and voltage are 90 degrees out of phase, you might still need a snubber.
 

tekbird

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I think it depends on the application. If you've got an inductive load, where current and voltage are 90 degrees out of phase, you might still need a snubber.
Hi Barry, thank you for response. So, if I understood you correctly, since it is zero cross voltage, current might still not be zero which can cause undesirable effect. in this case, what type of capacitor is ideal. should I go for capacitors at some rated AC voltage of like 275VAC or a smaller one will do as we only deal with current?
 

Easy peasy

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A snubber is a good idea if the load is unknown, these drivers switch the triac on soon after zero crossing, so the load current will start from zero too ( although will go up quickly for some loads ) when the opto led drive is removed the triac will turn off as the load current goes through zero - as long as the re-applied dv/dt does not trigger it back on - some inductive or motor loads can provide a back emf which can trigger the triac back on - the snubber is to prevent this ... ( the dv/dt at turn off being too high )
 

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I experienced a case where a MOC3040 plus triac combination was periodically self triggered even with pure resistive load by power supply dV/dt. A 220 ohm/47 nF snubber and filter was added to the MOC3040 circuit.
 

I_think

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I had same type circuit working and the triac blew off (open). I am still wondering why. I use it with ceiling fan. Attaching the schematic.
 

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I_think

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I get that. But I am wondering what was that thing or possibly point that I missed while designing that got the triac damaged.
 

I_think

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nature of load and size of triac ...
Inductive load of 60W 230VAC( celling fan) . BTA16

1596342045780.png

Looking at the specs I am still not able to understand that.

Triac specifications are surely over rated. Please correct me is I am wrong.
 

Easy peasy

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presumably the ceiling fan has capacitors....? => high peak currents in the triac ( unless you fit a series choke )
--- Updated ---

also your peak gate current is undefined ....
 

I_think

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OK, I guess.... Let me see if I can find out surge because of capacitor. But then the capacitor too gets switched during Zerocrossing.
 

d123

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Hi,

If it helps, for DC:
Inrush current magnitude = Capacitive load x (dV/dT)
 

Easy peasy

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try looking at the currents in a 0.1 ohm resistor with a scope triggered in normal mode ( isolation Tx on the scope ) this will give you a far better idea of what you are dealing with and if the zero xing is really happening or not.
 

I_think

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Zero Crossing is sure happening or then its not firing the gate. Because I am using MOC3063. Please check the schematic pdf attached. But if you still feel there could be other reason for not firing at zerocrossing please let me know.
Logically I am eliminating the chance of delayed firing because its getting fired at Zero.

Measurement on scope is going to take a while as I burned my oscilloscope a week ago.


Edit Addition: Please got through the application note.

Please got through the application note.

They have recommended snubber with explanation.Quote from Application note " Almost pure inductive loads with saturable ferromagnetic cores may display excessive inrush currents of 30 to 40 times the operating current for several cycles when switched “on” at the zero crossing point. "

" For example, incandescent lamp filaments when energized at the zero crossing may draw ten to twenty times the steady state current that is drawn when the filament is hot. A motor when started pulls a “locked rotor” current of, perhaps, six times its running current. "
 
Last edited:

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