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### Topic: Question: Zero crossing triac driver (Read 1 time)previous topic - next topic

#### Voidugu

##### Jun 22, 2013, 11:49 am
Let's say i have a triac connected on a zero crossing triac driver. The sine wave of the triac is at 90 degrees. I trigger the triac driver with a signal from 95 degrees to 100 degrees and then remove the trigger signal. Will the triac driver activate the triac at 181 degrees? Or, will i need to keep the trigger signal going to the triac driver active until 181 degrees for the triac driver to activate the triac?

Thank you very much
Regards
Void

#### runaway_pancake

#1
##### Jun 22, 2013, 03:13 pmLast Edit: Jun 22, 2013, 03:28 pm by Runaway Pancake Reason: 1
It needs to be there when line crosses zero (at 0deg, 180deg).

> > > Slightly before and for a time afterward.  The triac has to conduct a certain amount of current in order to stay on absent gate current.
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

#### Voidugu

#2
##### Jun 22, 2013, 09:47 pm
Therefore i can assume that the second example i gave is actually what needs to take place ("i need to keep the trigger signal going to the triac driver active until 181 degrees for the triac driver to activate the triac") and not the first one. Am i correct on that?

#### runaway_pancake

#3
##### Jun 23, 2013, 12:34 am
Typically, I think, a zero-crossing triac driver is for simple on-off stuff.

So, maybe you're asking:
if the input got pulsed, say at line peak (90deg or 270deg), would the driver side sit "armed", as a latch, primed, ready for zero-cross, where it would turn the triac on?  Is that it?

I'm not sure the form of the zero-crossing circuitry (internal).  I haven't used one where half-cycles was an issue.
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

#### retrolefty

#4
##### Jun 23, 2013, 02:44 amLast Edit: Jun 23, 2013, 02:48 am by retrolefty Reason: 1

Typically, I think, a zero-crossing triac driver is for simple on-off stuff.

So, maybe you're asking:
if the input got pulsed, say at line peak (90deg or 270deg), would the driver side sit "armed", as a latch, primed, ready for zero-cross, where it would turn the triac on?  Is that it?

The triac can be triggered ON at any part of the sine wave except a actual zero crossing +/- a little, but a triac (or scr) can only be turned OFF at zero crossing AND an absent of a trigger signal. Most solid state AC relays allow internal turn ON only at zero crossing by design not because it can't be done. Those SSRs that have 'random' or 'proportional' turn on capabilities can switch on anytime in the AC cycle and are used for PWM applications rather then the slower on/off type control that most standard AC SSRs use.

Lefty

I'm not sure the form of the zero-crossing circuitry (internal).  I haven't used one where half-cycles was an issue.

#### runaway_pancake

#5
##### Jun 23, 2013, 03:38 am
retro,
I am aware.
The OP is asking about zero-crossing triac drivers, though his objective isn't transparent (no offence, void).
As far as SSRs go, I don't know about "most", but "most" available from surplus concerns are zero-cross.

If I wanted to get tricky and trigger only one alternation or something, I'd go full out with a random-type triac driver, that's what they're for.
If all I care about is turning a lamp on/off, it's not important if I miss an alternation before it goes on or if it stays on an alternation after I disable the driver's (DC) input.

Anyway, should a zero-crossing driver get an input tick (indeterminate) in the middle of one alternation, I don't know to a certitude whether the output latches and consequently triggers the triac that it's responsible for, at zero-cross, in the following alternation.
I think that there would be a pulse-width (noise filtering) minimum for that just the same.

In any event, given a zero-cross triac driver, I would plan to have a solid input enable in anticipation of that zero-cross and hold it there till I required it to go off.

I haven't seen any zero-crossing triac drivers used in conjunction with a zero-cross detector, as in a random trigger, AC phase control situation, as the timing reference isn't required.
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

#### Voidugu

#6
##### Jun 24, 2013, 09:58 am
Pancace that:

"So, maybe you're asking:
if the input got pulsed, say at line peak (90deg or 270deg), would the driver side sit "armed", as a latch, primed, ready for zero-cross, where it would turn the triac on?  Is that it?"

is exactly what i was asking about

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