I am trying to do a dimmer and below is my schematic. Before doing an actual PCB, I did a proto on a veroboard and the circuit works perfectly. Now when I did a PCB and got it back from the manufacturer, I have noticed that MOC and BTA is heating up too much quickly. The only difference between my proto board and the finished PCB is that, on my proto board I have used the through hole resistors where as in the finished PCB they are SMD. R14, R15 and R7 is 1 watt SMD. In the PCB, I had footprint of SMD versions of 4N35 and MOC, but as the vendor did not have the SMD versions, they soldered the through hole versions on the ptoto PCBs. Also I used 4 1N4007 to create the bridge rectifier on the veroboard, where as on the PCB, I have used a rectifier IC. Everything else are just same. But in the proto PCB, the MOC and BTA is heating up even with a 60W bulb :( Any clue?
When you use Quadrant IV, it requires much more Ig which with 1k series requires more line voltage and thus switches with more transient loss. A 60W appears as a 600W load when cold.
To reduce Pd in Triac
Thus I suggest you verify ZCS is ok then swap pins on U5-4,6. You can remove the snubber for this snubberless Triac part. Ref
This fix should also reduce heat in MOC, but to improve heat rise further...
To reduce heat in MOC
Ir=(3.3-1.3)/(20+25)= 44mA which is excessive for 25 Ohm 3.3V drivers. (57mW is 57% of 100mW spec @ Tj=125'C) Therefore design for 20mA. Rs=(3.3-1.15)/0.02A -25ohm driver= 82 Ohm.
I assume you are using a uC with 3.3V logic equiv to 25 Ohm RdsOn nom.
- This design is far from optimized using 1W resistors when it can be done with 1/4W easily
- Worst case Tolerance calculations are essential for production to factor all variables.
- 3 Quadrant Snubberless Triacs need to be used in preferred quadrants I,III especially when driving motors due to BEMF.
Pulse mode trigger is far more efficient than linear analog mode for triggering due to lower losses from ratio of Igk^2/Rgk yet Rgk is never given in datasheet.
Simple method uses DIAC with cap discharge, otherwise SMD pulse transformer works best for precision triggers at low phase angle. Then you can discharge higher pulse currents yet short T so low energy.
The minimum power dissipation on the BTA12 is about 650mW with your 60W load, so I'd expect it to get quite warm. Assuming you used a TO-220 I'd expect that it would hit close to 100degC junction temperature. Depending on whether you used a heatsink or how much airflow you have around it that could mean tab temperatures in the 40-50degC range.
I'd suggest you do need a heatsink for any application at these power levels (60W+).
The ZC detector is another real question. The 4N35 DOES NOT have 100%+ CTR at very low currents.
I'd expect your ZC detector to provide a signal at about 23V (best case) and at worst case around 70-100V input. This is far from the zero crossing.
Lastly, you don't say which variant of the BTA12 you use. If you are using a type with 50mA I(gt) then this will push much more dissipation onto the MOC3021. You need to reduce R16 to about 100-120 Ohms to ensure you get more gate current for the BTA12. Your snubber is not doing anything at all, you need to reduce R7 (start here).
The other answer already covered the need to alter your R17 value, but I think the majority of the heat generated in the MOC3021 is likely from the gate drive problem rather than the LED dissipation.