Calculate Resistor Power Rating

Are the following four steps correct in determining the power rating of the 38Ω resistor from the above pic?


1) find max current draw of gate from datasheet of triac



2) Calculate voltage drop across 38Ω resistor
v=IR
=(max gate current from triac)x(Resistance at pin 6 of optocoupler)
=0.05 X 38
=1.9V
3) calculate power rating (wattage=currentXvolts)
=0.05 X 1.9
=0.095W
4) Derate max power rating by 2 to yield minimum power rating of 38 ohm resistor
=.095 X 2
=0.2W power rating of resistor

Does it really matter what current the gates pulls?

What you really need to know is the maximum voltage on pin 4 of the IC and hence across the 1k resistor?

I presume it will be pulse width modulated so to be safe side count it as a steady VCC and discount the PWM.

Calculate the current through the resistor and hence power. A 0.25W will be more than enough.

The maximum Triac gate-current is the maximum you can apply, not what it will take. Since the gate looks like a forward biased diode, the theoretical peak gate current will be about ((24V *1.4)-0.7) / 38Ω = .87A at the peak in the input sinewave (this may be reduced some by the optocoupler output impedance).

But that current will only occur for a short period of time until the Triac turns on (perhaps several tens of microseconds), at which point the voltage drops to the forward conduction voltage of the Triac of perhaps a couple volts. So the average power dissipation in the 38Ω resistor will quite small and the lowest power available resistor should work fine.

Assuming 33V, 0.87A, and 15µs (pulse width used to measure t[SUB]gt[/SUB]) for the triac to switch on, that's ≈0.5 mJ per gate pulse. t[SUB]gt[/SUB] is usually on the order of just a few microseconds though so this is a conservative estimate.

At 120 times per second for phase control that amounts to about 60mW so I'm not sure I'd use something as small as an 0603. You might be below its continuous power rating but its rating for repeated 15µs pulses will be lower. Panasonic rates their 0603's at 75V but I usually see 50V ratings and sometimes even 25V ratings for 0603's of dubious origin. I'd stick to something rated a little higher.