Resistor value
The value of the series gate resistor is a balancing act between limiting the peak current through the opto, and allowing enough gate current to turn on the triac.
From the Fairchild Application Note AN-3003:
The max surge current rating of the optoisolator, ITSM, is 1 A for the MOC series of optos.
The peak voltage for a 120 VAC line is 120 x 1.414 = 170 V, so R = 170 V / 1 A = 170 ohms minimum.
Round up to 180 ohms for a standard value.
The balance comes in here for the gate current and the line voltage to drive it. The minimum voltage needed to turn on the triac is determined by adding up the gate current through the resistor IGT, the triac gate voltage VGT, and the opto on-state output voltage VTM.
R x IGT + VGT + VTM = 180 ohms x 50 mA + 1.3 V + 3 V = 13 V.
The Littlefuse/Teccor Application Note AN1007 suggests dropping it to 100 ohms with a different opto:
"A common mistake in this circuit is to make the series gate resistor too large in value. A value of 180 ohms is shown in a typical application circuit by optocoupler manufacturers. The 180 ohms is based on limiting the current to 1 A peak at the peak of a 120 V line input for Fairchild and Toshiba optocoupler ITSM rating. This is good for protection of the optocoupler output triac, as well as the gate of the power triac on a 120 V line; however, it must be lowered if a 24 V line is being controlled, or if the RL (resistive load) is 200 W or less. This resistor limits current for worst case turnon at the peak line voltage, but it also sets turn-on point (conduction angle) in the sine wave, since triac gate current is determined by this resistor and produced from the sine wave voltage. The load resistance is also important, since it can also limit the amount of available triac gate current. A 100 ohms gate resistor would be a better choice in most 120 V applications with loads greater than 200 W and optocouplers from Quality Technologies or Vishay with optocoupler output triacs that can handle 1.7 APK (ITSM rating) for a few microseconds at the peak of the line. For loads less than 200 W, the resistor can be dropped to 22 ohms. Remember that if the gate resistor is too large in value, the triac will not turn on at all or not turn on fully, which can cause excessive power dissipation in the gate resistor, causing it to burn out."
On the SimpleIO boards, we went from 100 ohms to 180 ohms, just to follow the peak current limit more precisely.
The TriacOut4 board has 100 ohms from the original design. This is good for lower input voltages and turning on the triac. It is a little low for the peak current on the opto, but has been ok as a practical matter.
The TriacOut8 board has 180 ohms to help with the peak current. This may be too high for 24 Vac inputs, but the option above to use other optos was too expensive.
The resistor values can be changed to suit specific applications.
Fuente de información:saludos
