I have a project where I need to make a microcontroller trigger an old, non-controlled, camera flash (think of discharging a capacitor through a xenon lamp). First idea was use a transistor-based optocoupler, but there are high voltages involved (200 V) and it is not so easy to find optoisolators with high collector-emitter breakdown voltages where I live.

My plan is use the MOC3021, a phototriac-based optoisolator, as it seems to work well: once the voltage on flash's capacitor goes bellow a certain value, the xenon lamp cuts, current goes down the holding current and the TRIAC opens. I have some experience with transistor-based optoisolators (4N25) and TRIACs, but not with photo-triacs. I don't know how exactly the internal LED will trigger the TRIAC yet, for example.

Question: Do I must generate a pulse train to trigger a photo triac ?


Once you put the minimum current through the LED (10mA or whatever the datasheet tells you for the grade of product you have) the triac turns on. If the current through the triac exceeds the holding current during the time the LED is on, the triac will stay on until the current drops below the holding current. Otherwise it will not stay on. The holding current varies with temperature and unit-to-unit considerably. Presumably you intend to inhibit charging of the cap to ensure the current drops far enough.

In the case of mains switching a (possibly somewhat) inductive load it is sometimes useful to feed a pulse train to the optocoupler to save power, but that's not your proposed situation. Also, optocouplers used in a phase control application will have a pulse train at double the mains frequency because they have to trigger on for each half cycle- again not your situation.

Note: It is "triac" not "TRIAC" as triac is not an acronym.

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    \$\begingroup\$ TRIAC is an acronym for Triode for Alternating Current and like DIAC (Diode for Alternating Current) should be capitalized. \$\endgroup\$ Dec 9 '17 at 6:24
  • \$\begingroup\$ @JackCreasey No, it should not be, and the Wikipedia entry should be corrected. General Electric (inventors of the triac) referred to it as such in their bible (GE SCR Manual) and subsequent makers have generally observed the convention. Should we use TRANSISTOR because it was formed from TRANsconductance +varISTOR? \$\endgroup\$ Dec 9 '17 at 6:49
  • \$\begingroup\$ You'll have to alter much more than the Wiki.style manual ....it's in dozens of Manual of Style's. However I do agree you see both capitalized and non-capitalized. \$\endgroup\$ Dec 9 '17 at 16:49
  • \$\begingroup\$ It's interesting to actually look at the roots of the TRIAC, back in GE SCR Manual (1972): ia600209.us.archive.org/14/items/GESCRManual1964/… They capitalize SCR, LASCR, and THYRISTOR (but use Thyristor and thyristor). They use Thyristor mostly, but introduce Triac (and triac) in the text. ....I'm not sure this introduces any convention, but is what you'd expect in what is simply a series of collected papers. \$\endgroup\$ Dec 9 '17 at 17:23
  • \$\begingroup\$ @JackCreasey Someone scanned it, nice. I still have my original but the cover has fallen off it so I can't see the year. '72 likely. \$\endgroup\$ Dec 9 '17 at 17:33

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