Some other considerations are:
-
The Zener Diode numbers needs more characters. A 1n524 is a large stud diode.
-
Optioisolators do not have a good current transfer ratios (CTR). The PIC817 ranges from 0.8 to 6. (6 is actually very good for this type of device)
-
the goal is to get the gate of T1 to switch between 12V (limited by D2, assumed to be 12V) and zero.
Lets start through your circuit. Assuming the PWM3 is 0 to 4.5 V (outputs never get to 5 V, especially when loaded.)
The IC1 A-K voltage is ~ 1.25V (from datasheet)
The Input current is:
(4.5V - 1.25V)/330 ohms = 9.8 ma
If we start by picking the lowest CTR the device could have ( 0.8 ) then the C-E current could be as much as 9.8 ma X 0.8 = 7.8 ma
Lets assume D2 = 12 V (would make sense to protect the gate from over voltage).
OFF: Gate should be at or near 0. The "dark" current of the IC1 if negligible so there is no issue turning off.
ON: if the IC1 was switched fully on the gate would be; 30V x 2200 ohms /( 4400 ohms) = 15V OK (t will actually be 12 due to the zener)
The current will be: 30V -12V = 18 V 18V / 2200 ohms = 8 ma. which is marginal if the CTR is as low as 0.8
Now, the above analysis is for "static" operation and some low frequency PWM operation. If you plan on PWM'ing the heater at any significant frequency this circuit will not be capable of switching the FET on fast enough.
Hope this helps.