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I'm using this triac driver to control a triac. In its datasheet it says 'provide random phase control of high current triacs [...]'. What does that mean? It doesn't have zero-crossing detection?

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    \$\begingroup\$ "random" in this context seems to mean "arbitrary" \$\endgroup\$
    – JustJeff
    Sep 1 '11 at 22:59
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Yes, it means it doesn't have zero-crossing detection. The latter is useful for purely resistive loads, where the current is in phase with the voltage. For reactive loads, inductive or capacitive, this doesn't offer an advantage.
"Random" just means the triac switches on at the time you signal it to switch on, whatever the voltage phase at that moment. So its actual meaning is that it can switch on at any time.

A note I made earlier on zero-crossing switching:
You may have noticed that incandescent bulbs always fail when they're switched on. That's because the mains phase can be near its maximum when switching on. Combined with the low resistance of a cold bulb this results in a high current peak, which may burn the filament. When you switch on a zero crossing you avoid these peaks.

Random switching is needed if you want to build a triac-controlled dimmer, where you want control of the phase angle where you ignite the triac over the full 180° of a half cycle.

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    \$\begingroup\$ Random means "can be switched on at arbitrary time", not "switches on at some unpredictable time". Much like "random access memory" means "can be addressed in arbitrary sequence", not "accesses things in whatever order the memory chip feels like". \$\endgroup\$
    – supercat
    Aug 14 '11 at 15:49
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"Random phase" is just a marketing term for, as you suggest, "hasn't got the sophistication of zero crossing detection".

A random-phase driver will provide a trigger signal whenever the input trigger conditions are met. Mains voltage can be at any point in its cycle and will trigger immediately. With a TRIAC the device will remain turned on until the next zero crossing point is reached - about 0 to 180 electrical degrees later.

Random phase has the advantage that controlled voltage can at least notionally be varied more finely that zero crossing triggering allows. Zero crossing only allows whole half cycles so on periods are in multiples of 10 ms (50 Hz mains) or 8.33 ms (60 Hz mains). In most cases this is not an especially important advantage.

Zero crossing triggering minimizes RFI, notionally stops inductive transients and will place less loads on e.g. lamp filaments. With highly reactive loads (inductive or capacitive) current and voltage are not in phase so zero crossing switching still leaves problems to be solved.

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  • \$\begingroup\$ @Stevenh -(1) Yes- Thanks - I seem to have had trouble dividing a second by 100 and 120 :-). (2) ...s... is fine spelling where real English is spoken - I'm happy to leave it that way. Not all the werld spiks US Engrish. Whatever :-) (2) Siemen :-). (Yes, that's arguable and has been argued long and fruitlessly elsewhere :-) ). \$\endgroup\$
    – Russell McMahon
    Aug 13 '11 at 12:50
  • \$\begingroup\$ The "S"/"s" for units is not debatable. SI says units are lowercase except eponymous units, i.e. units derived from a person's name. So "S" means Siemens, derived from the man's name, "s" for second should always be lowercase, like the base rule says. As for the "s"/"z", my Firefox spelling checker seems to be US English, I should change it to International English, I guess. \$\endgroup\$
    – stevenvh
    Aug 13 '11 at 13:09
  • \$\begingroup\$ @Stevenh - S/s I was not debating - it was a typo on my part - I would never purposefully use S for "second" although a number of such no doubt creep through unawares. // Siemens / Siemen I was :-). Pure pedantry on my part, and people will trot out standards based chapter and verse to support both points of view and hotly argue it by the hour. I lean towards no 's' on proper name named units (Volt, Ampere, Ohm, Joule, Siemen, Farad, Newton, ...) , other's chapter and verse to the contrary notwithstanding :-). Incidentally, lumen is often incorrectly rendered lumens and sometimes even Lumens. \$\endgroup\$
    – Russell McMahon
    Aug 13 '11 at 13:54
  • \$\begingroup\$ Oh, I see. But in this case the final "s" is part of the guy's name, Werner von Siemens, actually. And so is the unit: 1 Siemens. I wouldn't dare to add a plural-"s" to units. \$\endgroup\$
    – stevenvh
    Aug 13 '11 at 14:12

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