Triac photocoupler with or without zero crossing?

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Hi

I'm a little bit confused of what two similar components
actually does (and how they are different).

I'm building a little circuit that just controls a 24VAC relay, and I found a old proven construction that does this that was using a MOC3042M. However my local distributor don't have this one in stock so I found one that seem to do the same thing EL3063.

But at the same time I found one that is missing the "zero crossing" but is a little bit cheaper.

So my question is what does that zero crossing do,
and when is it useful?
(feels like I missed something fundamental about triacs?)

Here are the symbols and data about the components:

EL3063 - ZERO CROSS TRIAC PHOTOCOUPLER
http://www.everlight.com/upload/thing/1238747322_8993.pdf

EL3053 - RANDOM-PHASE TRIAC PHOTOCOUPLER
http://www.everlight.com/upload/thing/1238748856_7907.pdf

Thanks
Johan

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That depends.
If you want to use the triac to dim a lightbulb, you need the random phase one. And some proper filtering to keep the HF-noise within acceptable limits.

If it's just to turn something on or off, the zero-cross is the one to use. Also suitable for heaters as those are slow in response and so you can live with 3 sine-cycles on and then 20 off, for 15% on-time.

Nard

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There is a *potential* problem using zero-cross devices to control inductive loads. The zero crossing detector circuit can *sometimes* be confused and may wind up triggering only on alternate half-cycles causing your load to be driven by half wave DC. If you know you are going to drive inductive loads, random turn on devices may be the better choice.

Tom Pappano
Tulsa, Oklahoma

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During a semiperiod we have two moments when the voltage pass through zero: at 0 and 180 dergrees.
If you turn ON an optotriac at 30 degrees for example, the one with 'zero cross' feature, will wait until 180 degrees is reached and then will turn ON the triac. The one without this feature, will turn ON straight away.
So for your relay, actually doesn't matter, since you just turn ON and OFF the relay, and a short delay induced by the one with built-in 'zero crossing' feature does not affect your circuitry as well.
George.

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Quote:
and a short delay induced by the one with built-in 'zero crossing' feature does not affect your circuitry as well.
George.

Except for the above-mentioned potential problem

Tom Pappano
Tulsa, Oklahoma

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But the ones with zero crossing detection are supposed to be meant for driving inductive loads in the first place, right?

They turn on at zero voltage and off at zero current.

And they are primarily meant for on/off control, not for control of individual half cycles, right?

A relay is inductive, I understand, but if the relay driving current is not that big, any triac should do it with suitable snubber. But a zero crossing triac may work without snubbers, generate less EMI and will control the relay faster than every 10ms. A relay is slow too, so maybe it does not matter if it is controlled every 10ms, at least not if the relay state is changed slower than every 1s.

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Zero-cross circuits are primarily a benefit when driving resistive loads which, particularly in the case of tungsten lamps, can have a substantial rapid current spike if turned on at points other than the zero crossing ("random"). This fast rise current spike causes radiated noise. Driving inductive loads is actually less problematic at turn on because the inductance slows the current rise so you have less noise at that point. Snubbers are needed with inductive loads because at the zero current turnoff, voltage will not be zero. The voltage across the thyristor will try to rise suddenly, and if the rate is too fast, the thyristor could re-trigger. The snubber is to control the rate of rise at turnoff to prevent dv/dt retriggering, and also there would be the side benefit of reduced radiated noise at turnoff. For purely resistive loads snubbers are not needed since turnoff happens at the zero voltage point.

Zero-cross style devices are indeed intended for full on/full off operation, and the random turn-on devices are for phase control of all loads, and they may also be better for on/off control of inductive loads.

Tom Pappano
Tulsa, Oklahoma

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tpappano wrote:
Zero-cross style devices are indeed intended for full on/full off operation, and the random turn-on devices are for phase control of all loads, and they may also be better for on/off control of inductive loads.

Jepael wrote:
A relay is inductive, I understand, but if the relay driving current is not that big, any triac should do it with suitable snubber. But a zero crossing triac may work without snubbers, generate less EMI and will control the relay faster than every 10ms. A relay is slow too, so maybe it does not matter if it is controlled every 10ms, at least not if the relay state is changed slower than every 1s.

Well it sound like the zero crossing is the safe choice, this application will usually just turn on/off the relay a couple of times per day. But most of the time just make sure that the relays are in the right state.

In the field of facility automation, you need to control different stuffs like heat pumps, boilers and normal water pumps. And with that type of things you usually just make sure that you are in the right state. :wink:

So basically I want to drive and hold a 24VAC for a long time, that 24VAC relay can in turn control loads with high voltage and currents. (Electricians are kind of sceptical to connect 230VAC stuff right into home made electronics :lol: )

/Johan

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Quote:
They turn on at zero voltage and off at zero current.

Which may be the cause of a nasty problem: take f.i. a transformer. Pretty inductive. Let's say : cos phi = 0.2. Which means that during normal operation on AC mains, there will be a considerable phase-shift between voltage over the transformer and current through the primary winding.
With the zero cross Triac, the triac turns on when the voltage is zero. So there is no way current can flow. Simple huh ? .... nah, not really: because now think back on that very same transformer during normal operation on AC mains: when the voltage is zero, the current is about on the maximum !

So what will happen at turn on and during the first second after that ? What will the plot of the current look like ? And why do transformers sometimes make that "boingngngk"-sound when turned on ?

If I would have all the answers I would post them here.
But too many moons have passed since the time I was supposed to know all this.

Nard

Edit: I found some answers. That turn-on phenomenon is called the in-rush current. It can be 20 to 100 times the normal current. More to study ....

http://en.wikipedia.org/wiki/Inrush_current
http://www.opamp-electronics.com/tutorials/inrush_current_2_09_12.htm

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Nard, you are absolutely right about transformers. It would be best to turn them on at peak voltage instead of zero crossing, because the magnetic field is zero at peak voltage, and turning a transformer on at zero voltage produces "DC offset" in the magnetic field that needs to be decayed to zero.

But as this is very unusual to drive transformers like this, normally transformers have thermistors or resistors and switches there to first limit the inrush current and let the magnetic field stabilize if not turned on near peak voltage.

Regarding the zero-current turn off, inductive loads have V=L*di/dt over them, so voltage over a inductive load is zero when there is no current flowing. But I get it, no load is purely inductive.