This is a follow-up to a previous question, regarding the VO2223A opto-coupler/Phototriac. My understanding of the chip is that the IR LED triggers a phototriac, which in turn drives the gate of a power triac. In which case... what's pin 5 for? It's listed as a "triac gate", but there's no obvious reason that I can see for it (unless it's in a different configuration to the one that I'm assuming).

And if I don't need to use it, do I need to strap it to ground, to another input, or to leave it floating? Unfortunately the specimen circuit diagrams in the datasheet (which are usually my first point of reference for pin assignments) aren't particularly helpful -- they don't include pin numbers, and appear to omit 5 entirely -- and I've never been able to find online any circuit example that uses the chip.


Often, gate connections on photootriacs (and phototransistors) are used to:

  1. Turn on the device separately from the LED (a remote trigger function)
  2. Hold the device on once the stimulus goes away (a latch function)

I would leave the gate floating, with an option for a tie-down resistor in your layout. Most of the time, you need to supply current to the gate in order to activate the device - tying the gate high will most likely turn on the triac all the time, which is something you don't want.

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