...The module takes up to 40vdc input...

I cannot count on it. please read this part:

"The product is actually pretty good an loosely follows the suggested circuit for the regulator data sheet. The main problem is the use of a 35V cap on the input. With any capacitor, you never, ever exceed the rated voltage. They can and will fail dead short and blow up.
In fact, we normally never use anywhere near the max rated voltage stamped on a capacitor. So with it being 35V rated, MAX recommend input is roughly 30V making the output a safer 28.5 voltts since both caps are rated 35 volts and the input must be about 1.5 volts higher than the output. The best and easiest thing to do is just change the rating to match the actual components used.
Ideally they should have used 50 volt rated caps. Also note that per the suggested circuit in the data sheet, you should have a larger value 680uF VS the current 220uF.
[...]
Again, it's not a bad circuit and it works, just don't run it up to the max voltage rating. Hopefully they will change the rating but this review is for those who already bought or are looking and need that high of an input rating."

I mean I'm a bit worry about it although my module has a 50v Cap at input. look:

if you think there is no problem to connect it, then I'm going to connect it to the 30vDC.

He means thermostat, not thermocouple.

The maximum allowed voltage input to the buck module is 40V. Here is what is proposed:

View attachment 89859

I would need to know the heater Wattage (or resistance), and the actual current into the buck module to do this more accurately.

Note that with my guess of a 24V, 24Ω, 24W, 1A heater, if you feed it full-wave rectified, unfiltered 24Vrms, the actual power is only 20.4W. This makes sense, because of the two diode drops in the fwb. See the light blue trace. I let LTSpice integrate that waveform to find the 20.4W.

For the buck regulator, I guessed the input current.

In fact it's a soldering iron's tip. look at this:

The heater resistance is 9Ω practically.

I have yet to see any mention of heater current and Transformer Va rating?

These tips draw 2-2.5A to work. What do you mean "Transformer Va rating"?

 

Where is it written that the soldering tip heater must be run off the f.w. rectified DC? Why not just run it directly off the transformer secondary AC?
Is there a secondary power switch in addition to the built-in thermostat? If you had just posted a complete proposed circuit or block diagram, we wouldn't have to be playing this stupid twenty questions game...

That leaves only the issue of reducing the peak input voltage to the buck module when the buck module is lightly loaded. I wouldn't be squeamish about letting the peak input voltage get to 35V...

What is the actual output voltage from the buck module?

What is the min. current that it delivers to its output load?

What is the max. current that it delivers to its output load?

 

Where is it written that the soldering tip heater must be run off the f.w. rectified DC? Why not just run it directly off the transformer secondary AC?

But I want to turn it off/on by a MOSFET.

Is there a secondary power switch in addition to the built-in thermostat? If you had just posted a complete proposed circuit or block diagram, we wouldn't have to be playing this stupid twenty questions game...

I doubt if there would be any switch. the schematic:

Please ignore the parts that is marked with multiplication sign. I just added one of this LCD modules to the circuit. sorry if it bothered you.

That leaves only the issue of reducing the peak input voltage to the buck module when the buck module is lightly loaded. I wouldn't be squeamish about letting the peak input voltage get to 35V...

I ran the buck module and it's ok. no problem so far (also without any load. I mean just measured the output with my DMM).

What is the actual output voltage from the buck module?

5VDC

What is the min. current that it delivers to its output load?
What is the max. current that it delivers to its output load?

you can assume min=0.5A and max=0.6A
I think this assumption is suitable.

 

Maybe this will help you with future calculations for power supplies

 

Min. power out of the buck module is 5*0.5 = 2.5W
Max. power out of the buck module is 5*0.6 = 3.0W

Figure the efficiency of the buck module at 80%. Assume the input voltage is about 30V.

Min. input current to the buck module is (2.5/0.8)/30 = 104mA.
Max. input current to the buck module is (3/0.8)/30 = 125mA.

Here it is again. Note what happens to V(unreg) when the heater turns on. Note what I did to the filter capacitor...

 

Thanks Mike
Ok, I have connected my circuit to the buck module and it's working very well. perfect. now my alone anxiety is the thermostat. I'm worry that maybe 33v peak would damage it. can't I clamp it to 24v?

 

Thanks Mike
Ok, I have connected my circuit to the buck module and it's working very well. perfect. now my alone anxiety is the thermostat. I'm worry that maybe 33v peak would damage it. can't I clamp it to 24v?

Not to worry. Think about it, the soldering iron likely normally works on 24Vac (which has a peak value of 24*1.414V= 34V).

 

It will see the mean level so it should not harm a device such as that.
Also the transformer does not appear to be near 70va which is what you appear to need?
Max.