This topic often comes up, so I'll put down all the math for the various combinations of supply volage and make the assumption you are using the 'standard' Sean Bowf SSR, which is the same as my SSROZ PCB.


Gate Resistor value

With reference to SimpleIO.com and some additional math and editing by me
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.

120VAC. The peak voltage for a 120 VAC line is 120 x 1.414 = 170 V, so R = 170 V / 1 A = 170 ohms minimum. At 120VAC, Round up to 180 ohms for a standard value.

240VAC.
The peak voltage for a 240 VAC line is 240 x 1.414 = 338 V, so R = 338 V / 1 A = 338 ohms minimum. At 240VAC, I rounded to 330 ohms for a standard value.

24VAC. The peak voltage for a 24 VAC line is 24 x 1.414 = 33 V, so R = 34 V / 1 A = 33 ohms minimum. At 24VAC, I rounded to 33 ohms for a standard value.

12VAC. The peak voltage for a 12 VAC line is 12 x 1.414 = 17 V, so R = 17 V / 1 A = 17 ohms minimum. At 12VAC, I rounded to 22 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.

120 VAC operation. R x IGT + VGT + VTM = 180 ohms x 50 mA + 1.3 V + 3 V = 13 V.

Therefore, the supply voltate for a 120VAC circuit must reach 13VAC before the TRIAC will trigger. This is well below the minimum voltate required to get the lights to illuminate.

12 VAC operation. R x IGT + VGT + VTM = 22 ohms x 50 mA + 1.3 V + 3 V = 5.4 V.

Therefore, the supply voltate for a 12 VAC circuit must reach 5.4 VAC before the TRIAC will trigger. This is close to the minimum voltate required to get the lights to illuminate. You would just need to take not of the setting in Vixen for dimming range @ 12VAC is likely to be different to 120VAC operation.

The next question is "I have used 180 ohm and have 24 Volt lights and they work, why bother changing the gate resistor value?" Well, the answer is - you can, and it will work. The only impact is that the lights wont turn on until the AC waveform gets to 13VAC, on its way to 33VAC peak (24Volt RMS). This also explains why using 180ohms @ 12VAC will be marginal at best, not functional at worst.

The last question is what size (rating) should the resistor be. For all cases quoted above, the power disappation of the gate resistor will be less than 10mW, so a 1/4 watt (250mW) resistor will be fine.

John Wilson,
(B Elec Engr, M Telecom Engr)