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From: Robert Baer on 12 Dec 2009 18:38
Spehro Pefhany wrote:
> On Fri, 11 Dec 2009 13:22:54 -0800, Robert Baer
> <robertbaer(a)localnet.com> wrote:
>
>> Vishay makes some nice 6-pin (4-terminal) optocouplers frotriac
>> control; their VO3052 is rather inexpensive and their IL420 costs bout $
>> 2 more for the same control.
>> The difference is that the cheapie is non-zero crossing; a quick look
>> on a scope did not show zero-crossing glitches of the triac itself (used
>> datasheet app circuit).
>> Perhaps the difference is microseconds or less.
>> Which type should be used,and exactly why?
>> Thanks.
>
> You'll need to use the non-zero-crossing ("random") type for phase
> control.. for static switching applictations there will often be some
> advantage in reducing EMI with the zero-crossing type, certainly when
> the load is nominally resistive.
>
> The zero crossing type inhibits triggering of the triac until the line
> voltage is less than a certain value, so turn-on could be delayed as
> much as (approximately) 1/2 cycle of the mains (8.3msec(a)60Hz,
> 10ms(a)50Hz) Turn-off, as always, occurs when the current drops below
> the holding current (a fairly low current of the same order as the
> gate trigger current).
>
Thanks; the loads are inductive (relays) with one possible exception.
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