I am trying to understand the parameter Vgt (max) for a triac (or any semiconductor with a gate threshold parameter), mainly the max. part of it. For example, on page 2 of the following datasheet:
VGT (VD = 12V RL = 60 Ω) MAX. 1.3 V
Could anyone please help me understand how to interpret max. here? Does it mean gate threshold can be 1.3V at max. and that we need at least 1.3V to trigger it and allow the current flow?
It's not an absolute max rating so, it can be assumed the supplier is saying that to guarantee switching you must exceed 1.3 volts. However, you must protect this part by limiting current to no more than 1.2 amps (see AMR table for \$I_{GTM}\$).
It means that at the moment of trigger, when you are forcing just enough current into the gate to guarantee triggering of the thyristor , the gate voltage will be no higher than 1.3V (at 25°C).
It may be higher at lower temperature, and it may be higher if you force more current into the gate.
So, suppose you have a +5V supply that yields minimum 4.5V after a transistor switch drop and tolerance, you can calculate the required gate current-limiting resistor from Rgt = (4.5-1.3V)/Igt
simulate this circuit – Schematic created using CircuitLab
In the circuit above, I am showing the gate driven with negative current since the standard triac is not guaranteed to turn on in Quadrant-IV.
Note that at low temperatures, the triac will generally be less sensitive than at 25°C and the gate voltage will also be higher, so to guarantee triggering at low temperatures you need to either put more current in at 25°C than would otherwise be necessary (the usual method) or make the gate current temperature sensitive with a PTC or something like that (rare).
