Assume that the load is a 100 Watt light bulb with a
typical filament resistance of approximately 1Ω. The
triac should be conducting 5 mA when the AC line voltage
reaches 5V. Translating this into a conduction
angle, the 5 mA hold current should have been
reached 2 degrees following the zero crossing (see
Equation 1).
EQUATION 1:
Angle = invsin(5V/(110V * 1.414))
Assuming a 60 Hz system, the minimum 5 mA hold
current should be achieved after 93 microseconds of
bias current (see Equation 2).
EQUATION 2:
Time = (1/60 hz) * (2°/360°)
So, we only need to hold the bias current of 3 mA for
approximately 100 μS to latch the triac on. If we average
the narrow pulse of current over ½ of a 60 Hz
cycle, it gives us an average current requirement for
the triac bias of less than 37.5 μA.