Hi Richard,
1) This PSU and the Metcal one both regulate based on Peak voltage at a certain point in the filter, It seems to me it should also work regulating for constant RF current or perhaps for constant reflected power. Does anyone see a problem with either of those?
According to the original patent (which is now expired), it is indeed a regulation for a constant RF current. The original patent:
http://www.google.com/patents/US4626767In my circuit i use the same point in the RF path for the feedback loop, however, i end up at the dc/dc converter at a different point compared to the schematics of the MX-500P you can find here:
http://www.eserviceinfo.com/downloadsm/33981/Metcal_MX-500P-11.htmlIn that circuit they use a zener + trimmer, on the input side of the regulator, which goes to the feedback pin together with the signal picked up before L7 (which in my circuit would be L5, the last coil before the output). In my circuit i feed that signal through an op-amp and series resistor into the feedback pin of the regulator.
2) Power requirements - I've seen 100V Peak mentioned during heating, so 100 Watts for a few seconds assuming the tip looks vaguely 50 Ohms below its Curie temperature. The SWR vs time graphs are very nice but would it be possible to plot some Power vs time for typical usage, or perhaps post the raw data so I could have a go at estimating it.
I think you got the numbers a bit wrong. At 100V pp into a 50 ohms load, it should be around 25 watts. In reality i get a bit higher even, up to 120V pp, but that also depends on the maximum voltage that the dc/dc converter can supply, of course, and thus can be adjusted.
3) What does the tip really look like in impedance? [...snip...] (or do the tips have a small capacitor to resonate out the L?).
Good question, and i simply don't know what the real tip cartridge characteristics are. I dont have the equipment to test that.
However, you can take a look at the relevant patents to get an idea of their workings. For the tip cartridges themselves, specifically the heater method, check these:
http://www.google.com/patents/US4745264http://www.google.com/patents/US4769519http://www.google.com/patents/US4877944As for the handpiece, the patent is
http://www.google.com/patents/US4839501According to that patent there seems to be a capacitor in series to the tip cartridge.
In any case, continuing to pump lots of power into the tip even after the temp is reached will cause it to heat up more, up to a point the cartridge starts glowing where the coil assembly is inside. So it really is important to regulate the RF power into the cartridge once it is heated up and the mismatch happens.
My current thinking is to full wave rectify the mains supply to get a 350V dc bus and run a small non-isolated switcher off of that to power the control circuitry. I plan to generate the RF directly from the high voltage dc and provide the output isolation and much of the matching via a suitable RF transformer. I'm currently toying with the idea of using a pair of line output valves for the high voltage RF source - designed for switching, suitable for the peak voltages expected, tolerant of overloads on timescales of ten seconds or so (tip heatup time), generally bombproof. What's holding me back at the moment is not really knowing what the load is like, and being a little unsure of the best parameter to regulate for.
While it would probably look nice with tubes, i'm not sure that it is really practical. Tubes need heaters, which by themselves consume quite some power. You might end up using more energy there compared to the actual soldering cartridge. Also keep in mind that other countries have different line voltages. If you intend that others may build your circuit as well, i think that directly using the line voltage is a rather risky thing to do. But then, this is of course completely up to you
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/xwink.gif.pagespeed.ic.cldandycH0.png)
About usage data, attached is a zip file with the raw data log i made a while ago, using a STTC-126 tip. The .gpd contains the data, one sample every 100 miliseconds, the .plt is the gnuplot commands i used to create an actual plot. Vsup is the voltage out of the dc/dc converter, in volts. Vfwd and Vref are more or less raw ADC readings. SWR is what i calculated as SWR out of the Vf/Vr, and DELTA is just what i internally use in the firmware to detect load changes, it is a combination of various parameters.
The file start with a normal heatup with a cold tip. It starts out with a VSWR of 1.00, then slowly rises. When it reaches temp it makes a dip to almost 1.00 again and then quickly rises, which is also the point where the regulation kicks is and reduces the supply voltage. The next event is when the unit goes to sleep mode with reduced power output. it then wakes up with boosted output for a while, then switching back to normal power output. Then follow a few quick solder joints, a pause, another set of quick solder joints, then again sleep, wakeup, and that's it. Hope it is helpful for you.
Greetings,
Chris