The STSS and MX500 use a huge, heavy mains transformer - only the MX5000 uses a switchmode mains supply, but it's still in a heavy cast case.
Ah, sorry, my bad. I was talking about the stuff in the circuitry itself to generate local supplies and the supply for the RF stage. My design is open to whatever one wants to use. While i have a rectifier and caps on the controller board, so that it can accept AC voltage, one could directly feed a DC in there instead. But since i have a bunch of 24V/2A transformers here from old, broken soldering stations, i also placed the rectifier and stuff there.
In any case, no need for a dual-output transformer like in the STSS for example. Always wondered why the schema on the net shows a 2x19V transformer, when the converter output for the RF stage goes to only 21V max. anyways....
The display on the MX5000 is nice, but in practice could be reduced to a LED that shows when power is below a certain threshold (i.e. heated up). Maybe a bicolour that did a gradual change of coluur over the power range.
Auto-reset on error is certainly useful.
Agreed, generally it's pretty useless. but i like that bling-factor somewhat. Plus it helps me a great deal with the developing stage. After all, this is the very first circuit and layout i did on this project.
However, i'm pretty sure that for now i keep at least the µC. Makes a lot of things much easier for me. Instead of putting a lot of analogue circuitry on the board, i can do all that with the µC instead.
The Metcals put some DC through the heater and use this to detect open-circuits, but an RF fault detection cct could probably do this as well as protection against shorts etc.
Yea, i have seen that in the schematic that floats around on the net. Some transistor to generate the bias, plus some comparator circuitry to process that. As written above, for now i prefer to get the detection done with the µC.
In the end, there is no real need to have extra circuitry for that, i think, since the µC should be able to handle all that. As far as the RF final is concerned, all that matters is to protect it against too much reflected power due to mismatch, and against too high a supply voltage from the feedback loop. The former detected by the µC, the latter simply a matter of having a low enough input voltage to begin with. My idea was to use 24DC or thereabouts for supply. That way the output of the buck converter for the RF can't get too high at all.
From the FET the RF is AC coupled into the filters. So any random short on the output would just mean a mismatch. No "real" short is happening there, as far as the RF stages supply is concerned.
I don't think the frequency is that critical as far as the heater is concerned- AIUI the choice of 13.56 is just to keep it in the ISM band.
Yes and no. True, it could be done with basically any frequency. However, the filters of the RF stage, plus the inductance and capacitance of the tip/handpiece are somewhat tuned. After all, the tip _is_ like an antenna to the RF output. It just happens that due to the Curie effect it will go into a mismatch once the temp. is reached. But then, i will see how it behaves once i use the right frequency.
As regards using a fan, this was purely to minimise size & weight, for cooling both the PSU and RF stage.
True. but since i plan to make all the stuff available, everyone can modify it to whatever he/she wants
I had another thought for a more 'extreme' approach to a lightweight solution : generate the 13.56MHz direct from a rectified, unsmoothed mains supply, and use an RF transformer to do the mains isolation.
Uh, not sure that this would be a good idea. For one, generating such high frequencies directly at these voltage levels is no simple feat. At least not if it is supposed to be efficient. The next problem is regulation. Once the tip reaches temperature, you will have a mismatch. That means that a most of the RF is reflected back into the supply. The purpose of the buck converter on that supply is to reduce the voltage the more power gets reflected. Otherwise you end up with several hundreds of volts reflected back.