hi Dave,
Look at what a LTS sim shows for the power dissipation on those components, much lower than your estimated value.
E

 

Hmm I must have been doing something wrong.

I now know ac impedance is different to dc resistance now after a quick look. I always assumed it to be the same.

How do you get up that power display box for the part Eric?

 

hi.
Run sim 1st.
For Power, place cursor on the component of interest, press the Alt key [cursor becomes thermometer symbol], then press left mouse click.

To get Avg and RMS, place cursor on power plot label, press down Ctrl ley and left click.
E

 

Hmm I must have been doing something wrong.

I now know ac impedance is different to dc resistance now after a quick look. I always assumed it to be the same.

How do you get up that power display box for the part Eric?

I've been baffled by this in the past, and eventually figured it out, but it's been a while. Let's see if I remember this right...

I think what happens is that current only flows through the gate resistor while the voltage is rising in the process of triggering the TRIAC. Once the TRIAC fires, current passing through the other terminals keeps it active until the next zero current crossing (not all the way down to zero, but close.) While the TRIAC is active, the voltage across the gate resistor is very, very low.

Basically, the resistor never sees the full voltage, and therefore current and power dissipation are correspondingly lower as well.

Hopefully someone more experienced can clean up that mess of an explanation. I think I'm in the ballpark, but I'm sure it's not quite right.

 

hi 17,
Expanded the plots so that you can see more clearly.
Once the triac has 'fired on' , most of the Vsupply is applied across the Load.
E

 

hi 17,
Expanded the plots so that you can see more clearly.
Once the triac has 'fired on' , most of the Vsupply is applied across the Load.
E

Cool! Thanks for that.

That matches my expectations quite well - I just couldn't explain it clearly.

 

Ah I see! That makes perfect sense why it's just trigger signals and not permanent current at the gate!

Thank you both for the explanation. Makes perfect sense now! Also thanks for the tips on LTS Eric. I'm learning a lot from this project.

I'll keep you posted on how I get on. I've started making the bom of parts etc and will have the pcb sent away for hopefully tomorrow.