Author Topic: (Now with pictures!) I built the DIY metcal-compatible soldering station.  (Read 2856 times)

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Offline rfmerrill

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Pictures (edit: out of date, see thread for changes)! https://imgur.com/a/tWysv8m

Some of you may remember this project that got some attention a couple years ago: https://habr.com/en/post/451246/

I searched and while it's entirely possible I missed something, I couldn't find any evidence that someone other than the guy himself had actually built one. Therefore, I decided to try and be the first (maybe?) I can do up a more detailed thing if people want me to, but for now I'll just dump the major points.

Nothing but praise for the man who built it, and he isn't beholden to anyone to make his design easy to replicate, but it's certainly not a polished, easy-to-follow build. There are a few strange parts you can't buy in most of the world, some mistakes in the CAD, and the BOM is quite large and varied for such a small project. It comes across like it was designed for the specific set of random parts the guy had kicking around, which is totally fine and I won't blame him for it, but it did make replicating it from the opposite side of the world a challenge.

The first thing I'll say is if you live in North America, Japan, or somewhere else with a lower AC mains voltage, don't even bother trying to build his AC-DC converter circuit. He used a design that is intended to run off of 180VDC or higher from an active PFC, but he cheats and just uses rectified mains. If your rectified mains won't reach 180VDC, you're out of luck. I just left that entire part of the board unpopulated and used off the shelf supplies to provide the 30-40V and 10-11V inputs that the design needs. This had the bonus of eliminating like 35 of the 100+ unique BOM items.

Here's my BOM (lacking some components that went unused due to the above): https://docs.google.com/spreadsheets/d/1jAeOsUX6nM0jR9WvrQy5qf3qwqZx6GQ8kfE1Ulw6L-o/edit?usp=sharing

The second challenge is that I could not source the exact same cores and magnet wire as he did. Two of the inductors and the current transformer are built with "K16x8x6" cores that as far as I can tell were only made and are only found in the former eastern bloc (I substituted more standard T60-26 which are of similar size). It also seems like the cheap T130-6 iron powder cores he bought must have less permeability than the ones I bought. Finally, he uses magnet wire specified in millimeter diameter while I can only find magnet wire in AWG.

I used 22AWG magnet wire for the smaller coils and 16AWG for the larger. Note that since the holes in the PCB are made for the millimeter-sized ones, this is a tight fit (it almost doesn't fit at all with the enamel still on the wire) and you might be able to get away with the next size down. I tried just following his winding count but the resulting build cooked the main power transistor and I ended up pulling out all of the inductors and re-winding them with a borrowed LCR meter from work. However for the current transformer, you should just be able to follow his turn counts since it's only the turn ratio that matters.

Unless you can get the exact inductance measurements that are shown on the schematic, you will have to revise the sixth order (I think it's sixth order? pls correct me if I'm wrong) LC filter by changing the capacitance using the measured inductance of your coils. The LTSpice file for the simulations I ran is attached. There are basically only two things you care about in the frequency response from what I can see:
  • The operating frequency 13.56 MHz should not be too close to a resonant peak and should be boosted about 5-6 dB
  • All harmonics of that frequency should be attenuated at least 20 dB (27.12MHz is the first one, and unless you royally screwed up, each successive harmonic should be more attenuated)
I used 1% resistors and 10% capacitors for most of the circuit because there was not a tolerance spec on most of them--but if anyone wants to do a tolerance analysis and come up with a guide for a lower-cost BOM, that would be great!

Some guidelines for part purchasing that may not be obvious (and other stuff):
  • The variable buck converter that powers the RF section runs off of a 30V input in Sergey's design, but can go up to 40V. I used 36V. It outputs 22V at up to 4.5A so your input should be >4A capability
  • L1 (the aforementioned buck converter's main inductor) is specified as a Yageo part that, as far as I can tell, is not orderable and may in fact be discontinued. I did my best to try and match its specs but without knowing the actual requirements I could only go for 'at least as good' which due to the tight space it needs to fit in, left me buying a Wurth surface mount inductor that you can just barely bodge onto the thru-holes on the top side of the board
  • The "+10V" supply can go up to I think about 11.5V, but for reasons I haven't quite figured out the system will start to misbehave if you try to go closer to the 14V maximum of the MOSFET driver.
  • For U9, the part specified is just the one Sergey could most easily get. In that same family are other parts with different channel inversions, and since the board only uses one channel it doesn't matter which one you use
  • Metcal handpieces use an F connector. The connector on the handpiece lead is a slip-on clamp-down type and it may fit better on some F connectors than others.
  • Because of how close Q1 and Q2 are to other components, some heatsinks might be too wide (like the ones I bought).
  • Q1 and Q2's tabs must be insulated, they cannot be grounded
  • I did manage to find somewhere to buy the WinStar OLED that he mentions using, but it was expensive and shipped from Poland. Luckily, the 16x2 LCD display that Adafruit sells will connect right up with the same pinout. You can omit the middle four pins on the display header (D0-D3) as they are unused. Note that the microcontroller operates at 3.3V so other 16x2 displays that run on 5V may not work, but the adafruit one seems to work just fine
  • For an enclosure I got the BUD Industries CU-476, which has similar dimensions to the enclosure Sergey used (which is not easily available in the US). One major difference is that it has a center screw post on both sides which you will have to cut away to fit the display.
  • As far as I can tell there is no good reason for C4 to be thru-hole. The requirements are only modest with respect to voltage and stability. I bodged in the same X7R SMT cap used elsewhere in the design
  • The board fits tightly in the enclosure, but it has no mounting features. You will have to make sure to sand off any high spots on the edges or it won't fit. I just put rubber feet on some safe spots on the bottom of the board and put kapton on some parts that stick out. I haven't finished putting it in the enclosure but will probably add some makeshift way of keeping the board in place
  • The thru hole LED footprint (D13) will not fit a 'standard' full size LED. The holes are too small
Some other random things to look out for:
  • Going by the blog post, L8 is intended to be adjusted by hand (by manipulation) "for maximum efficiency". I assume this means you are trying to maximize the slew rate on Q2's gate, but I am not sure. It is also a bit difficult to manipulate due to being surrounded by other components. I would recommend stretching it out as far as it will go before installing it as I think compressing it will be easier than trying to spread it.
  • Some of the refdes are obscured, both on the schematic and on the PCB. Beware!
  • L7 and L8 ended up being closer to 10uH for me, I don't know if missing 9uH high or low is the safer option, but my setup seems to work.
  • The firmware still builds fine with the free version of embedded workbench. I can try building a hex file if people would rather not install that. A random ebay knock-off STLink programs it just fine
  • The blog post recommends bodging the crystal load caps directly across the crystal and running a ground wire across to the cap on the STM's ground pin. The pictures also show additional capacitors bodged in parallel there--I have no idea what value those are but my board works fine without them.
  • For the capacitor banks in the LC filter, you want to use as many caps in parallel as you can. I used C0G/NPO tight tolerance caps just to be safe. I only ended up changing one value: the first bank from 600pF to 496pF by removing two 100pF caps and replacing them with 47pF
  • The values for R6 and VR1 seem wrong unless I'm missing something. You would not be able to reach the nominal 22V as they are. I subbed 22k for R6 but using a larger value pot would probably be the better choice
  • In his photos he installed L12 raised up off of the board. I'm not sure why. I didn't do this and I do see some erroneous "tip failed" messages sometimes.

And improvements that could be made to this design:
  • The refdes are in no particular order. They should be renumbered
  • Porting to KiCAD would be nice. Diptrace is not very popular and kinda sucks.
  • There are probably some reductions to be made in the unique component count
  • A lot of the SMT components are 0805 for no good reason. Even just shrinking them to 0603 would free up a lot of board space
  • The AC/DC converter should be replaced with a more standard and universal design or just eliminated and the board shrunk and designed to run off an external DC supply
  • The design would probably be a lot easier on a four-layer board
  • display footprint doesn't fit 100 mil headers
  • Pin 1 mark missing on a lot of the headers (notably the programming header).

Let me know if you want more detail! I'll probably reply to this thread with pictures and videos and more stuff. Also, if I missed anything big or said something incorrect please correct me! This is definitely out of my comfort zone, I just did my best.

Housing front

Housing back

Inside

Board top

Board bottom

Power supply (cheap chinese amazon special in a random box. Yes that is a macbook power cord)
« Last Edit: November 02, 2020, 02:23:53 am by rfmerrill »
 
The following users thanked this post: thm_w, edavid, helius, iamdarkyoshi, james_s

Offline rfmerrill

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Re: I built the DIY metcal-compatible soldering station.
« Reply #1 on: October 04, 2020, 10:35:06 pm »
Some things I'm wondering about, if anyone can look and figure it out better than I did:
  • In Sergey's photos, the current transformer T2 has a single turn of regular insulated wire, rather than magnet wire. Is there a specific reason this was done?
  • What procedure would make the most sense for adjusting L8?
  • Did I come to the right conclusion that R6 + VR1 value is wrong, or is there something subtle about the circuit that I'm missing?
  • Why two inductors in series for the buck converter?
  • I'm not sure how to adjust VR1. I assume I want to bring the voltage at some node to 22V but not sure which one. It seems like the zener diode D2 is there as kind of an emergency stop to avoid that voltage going too high, but it does seem to limit it to just over 22V instead of a slightly higher voltage like I'd expect.
  • Which electrolytic caps should be low ESR? The literature for the DC-DC converter chip seems to indicate that you design the other parts of the circuit differently depending on whether you're using low-ESR caps or not, but not sure which
  • I'm using an external DC supply, what NTC inrush limiter would be appropriate? I have an 8D-20 in place right now but it's kinda big and does get noticibly warm in operation. I know they function by getting warm but I did not expect that much heat to get dissipated. I haven't shared which capacitors are in place just yet but I know that is important so I don't expect a direct answer. I was just bothered by the arcing with no NTC in place.
  • I assume L8 is supposed to resonate with the gate capacitance of the mosfet at close to the operating frequency. The math does seem to work out on this but the two diodes in parallel with the gate seem to have significantly more capacitance than the mosfet which would throw that off. Am I missing something here?
  • What would I need to change if L7 and L6 were significantly different than the specified 9uH? Some of the capacitors near them? I'm pretty good with linear circuits but this whole section of the board challenges my nonlinear knowledge.
 

Offline bmac_2780

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Re: I built the DIY metcal-compatible soldering station.
« Reply #2 on: October 04, 2020, 11:32:34 pm »
Hi RFMerrill

I have slowly been building this as well. I am in Australia (240V mains) and I have so far been working on the LLC mains part of the circuit. Its not something I have experienced before so I have been progressing gradually. After adding an extra 2 turns to the primary of T1 it is working. I had some email correspondence with Sergey. He was very helpful and explained quite a bit to me regarding getting the LLC converter running.

I have not yet powered the low voltage circuitry but like you, I did notice that VR1 and R6 seemed to be the wrong values to get 22V.

I was able to source the full BOM. Most parts purchased from Element14 (aka Newark, Farnell). The toroid cores, LCD and a couple of obsolete ICs were from Aliexpress and eBay - lets hope they are as advertised.

As you said, full credit and thanks to Sergey for this. I have a Thermaltronics iron and desoldering gun which travel with frequently. Really looking forward to having this power supply, which is much lighter and smaller than the Thermaltronics. I understand it is twice the power too. I am already happy with the performance of my iron, 80W will really turbocharge it!

Regards,
Bruce

 

Offline rfmerrill

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Re: I built the DIY metcal-compatible soldering station.
« Reply #3 on: October 04, 2020, 11:37:59 pm »
I have slowly been building this as well. I am in Australia (240V mains) and I have so far been working on the LLC mains part of the circuit. Its not something I have experienced before so I have been progressing gradually. After adding an extra 2 turns to the primary of T1 it is working. I had some email correspondence with Sergey. He was very helpful and explained quite a bit to me regarding getting the LLC converter running.
Very cool! I have emailed Sergey as well but have not received a reply yet, but it hasn't been very long.

Quote
I have not yet powered the low voltage circuitry but like you, I did notice that VR1 and R6 seemed to be the wrong values to get 22V.
I exported the bom csv from diptrace, but I have noticed a few components have misleading labeling, so it's possible Sergey actually used a 20k pot and the CAD is just wrong.

Quote
I was able to source the full BOM. Most parts purchased from Element14 (aka Newark, Farnell). The toroid cores, LCD and a couple of obsolete ICs were from Aliexpress and eBay - lets hope they are as advertised.
Which obsolete ICs? I just went with the recommended replacement for any that I found were not recommended or defunct. Seems to have worked fine. Excluding the LLC converter of course

Quote
As you said, full credit and thanks to Sergey for this. I have a Thermaltronics iron and desoldering gun which travel with frequently. Really looking forward to having this power supply, which is much lighter and smaller than the Thermaltronics. I understand it is twice the power too. I am already happy with the performance of my iron, 80W will really turbocharge it!
Yeah! I'm hoping to get some desk space back from my Metcal power supply :) An Australian friend is interested in revising this design and I might help him with that.
 

Offline rfmerrill

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Re: I built the DIY metcal-compatible soldering station.
« Reply #4 on: October 05, 2020, 07:47:22 pm »
Add thermal reliefs to the improvements list. It's nearly impossible to clear the thru-holes for the display mount if you need to redo the connection...
 

Offline rfmerrill

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Re: I built the DIY metcal-compatible soldering station.
« Reply #5 on: October 10, 2020, 01:16:31 am »
Does anyone know why OLEDs in that format (16x2 character) seem to have mostly vanished? Sparkfun and Adafruit both used to sell them but now they only sell LCDs.

I believe this uses a '6800' style parallel interface, but could the '8080' style be made to work because it only uses writes and not reads? I figure you could wire enable to write and tie read de-asserted.
« Last Edit: October 10, 2020, 01:56:05 am by rfmerrill »
 

Offline james_s

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Likely because OLEDs have had a lot of reliability problems and the 16x2 and similar character display formats don't really take much advantage of the unique characteristics OLED offers.

The thing that annoys me is that all of the QVGA and higher resolution 4:3 color OLED displays seem to have vanished.
 

Offline rfmerrill

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Likely because OLEDs have had a lot of reliability problems and the 16x2 and similar character display formats don't really take much advantage of the unique characteristics OLED offers.
Interesting. I've tried both the adafruit LCD and the winstar OLED with this guy and I definitely like the look of the OLED better... at least when it's new. Then again, metcal uses an LCD.
 

Offline james_s

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I love the look of OLED, they just never have been as reliable as LCD. I recall Agilent used them in one model of DMM and those had a very high failure rate. They work best in devices that are used occasionally vs things that are powered up continuously or for long periods. I don't know for sure why they are not more common though, I'm only speculating.
 

Online KE5FX

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I love the look of OLED, they just never have been as reliable as LCD. I recall Agilent used them in one model of DMM and those had a very high failure rate. They work best in devices that are used occasionally vs things that are powered up continuously or for long periods. I don't know for sure why they are not more common though, I'm only speculating.

As I understand  it, the bad thing about those Agilent DMMs wasn't that the displays failed after X hours of runtime, they failed after X hours period.  They were putting them in the U1253/U1273 handheld models, right?  Were the failures correlated with power-on time, or simply aging?
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #10 on: October 10, 2020, 06:06:45 pm »
Hmm. I think I goofed something. It looks like in his original design, mains earth coming in is connected to the chassis at the same point as the RF output jack and there's no other connection to DC ground, but in my build I have DC ground tied to mains earth at the input and then again to the RF output through the housing. Seems like the resulting ground loop might be bad. Might be easy to fix, but I'd need to figure out how to bring three wires in instead of two. Easiest interim solution might be to insulate the DC input from the housing and then just connect a separate grounding wire maybe?
 

Offline james_s

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #11 on: October 10, 2020, 06:15:11 pm »
As I understand  it, the bad thing about those Agilent DMMs wasn't that the displays failed after X hours of runtime, they failed after X hours period.  They were putting them in the U1253/U1273 handheld models, right?  Were the failures correlated with power-on time, or simply aging?

I don't know, I never had one of those meters, I just recall reading about it, I have had other experiences though. I've had a couple of those little 0.96" OLED displays deteriorate, I worked on a veterinary patient monitor recently that had a nice big dot matrix OLED panel which had a number of defective/dim/flickering pixels and later acquired another one that had a display in similar condition, usable but not great. My work laptop is a Macbook Pro with that stupid touchbar, when it was about a year old it developed an intermittent flickering white square on the right edge of the OLED strip. A friend of mine used to work for the BBC and they got some kind of fancy control panels that had OLED capped buttons, those quickly started to crap out and within a couple years they had all been replaced, I think with LED. I had a mobile phone back in the pre-smartphone era that had a small OLED display on the outside, when it was just a few weeks old it developed a bright vertical line through the display. The OLEDs looked great when new but I have not seen anywhere near the failure rate with LCD, in fact I don't think I've ever seen an LCD fail that wasn't physically broken.
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #12 on: October 11, 2020, 05:46:28 am »
Company I work for currently ships a product with an OLED display and as far as I know the only issue we have with it is 'burn-in' (really just change in brightness of the pixels with wear). But for all I know there are other issues.
 

Offline james_s

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #13 on: October 11, 2020, 05:55:59 am »
Quality may be improving, and it likely varies widely, it's no surprise that some of the dirt cheap random Chinese displays are crappy although even the random most crappy LCDs seem to last just fine, even if they look bland. I keep hoping OLED will really take off but so far it seems that "good enough" LCDs are the main enemy of superior looking OLED.
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #14 on: October 12, 2020, 05:09:30 pm »
If I wanted to update this design with a more standard forward converter type universal mains input, where would be a good place to start for that?
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #15 on: October 14, 2020, 05:41:52 am »
Figured out why the display was glitching. I eliminated the ground loop by isolating the DC jack and it actually made it worse, but then I realized



adding an extra ground link between where the coax solders on and the housing fixed it.
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #16 on: October 15, 2020, 03:55:47 am »
I want to reduce the capacitive coupling between those transistor tabs and the housing while keeping them thermally coupled. The easiest change is to switch to nylon fasteners, which I'm already doing.

I can just put an extra layer of the cheap coupling material or some Kapton on top, but are there good brands of thermal interface that lets me reduce parasitic capacitance? (either by just being thicker but still conducting heat, or by actually being more dielectric).

Also I think I will be replacing the 3.3V regulator with a higher current rating one as it's running the OLED display.
 

Offline Thomas

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Offline rfmerrill

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Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #19 on: October 17, 2020, 05:09:55 am »
OK, I might need help here.



For L6 and L7 I bought something that seemed close in AL, permeability and size to the weird Russian cores Sergey used. The only problem is I accidentally bought iron powder cores instead of ferrite and I'm pretty sure that's why L6 cooked itself. I swapped a new one in before I had any clue what was wrong, and it makes its way up to 90C in not very much time even with the iron in the stand.

Importantly, I'm pretty sure it's the core getting hot and not the winding.

So what should I use for those inductors instead? I'm looking at ferrite cores and they all have much higher AL and permeability than the core Sergey mentions in the blog article ("HF-ferrite 16x8x6mm ring cores with permeability of 50"). I think the max DC current it needs to take is 4A. Could I just buy an inductor instead of making my own? Or could I use a higher-permeability ferrite with way fewer turns? (like 3?)

Edit: A big part of why I'm unsure about this is that I don't know how much effect a change in that inductance would impact the circuit, or how to compensate for it, and hitting 9 uH on the dot is hard with high-permeability cores.
« Last Edit: October 17, 2020, 05:36:30 pm by rfmerrill »
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #20 on: October 19, 2020, 12:52:02 am »
I patched the firmware to support an easier-to-buy display: https://github.com/SergeyMax/SolderingStation/pull/1/

The BuyDisplay model only comes in yellow, and is not as large, but I think I still prefer it to the LCD. It's a lot cheaper and easier to find than the WinStar OLED, although you do have to wait a while for it to arrive from China.
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #21 on: October 22, 2020, 09:31:16 pm »
I spent a while trying to figure out this circuit here which provides negative feedback to the DC-DC converter

At first I thought it was a simple current sense circuit, but then I noticed that the RC filter is a lowpass at around 2 kHz, and the diodes are not arranged in a rectifier configuration.

After poking at it a bit with some friends, we realized this is actually a ring mixer that mixes the output current with the output voltage to measure the phase angle between voltage and current! Which makes a ton of sense actually: We want to reduce the output power as the iron tip reaches the curie point. As it does so, the inductance of the iron tip will drop, causing the phase angle between voltage and current to move away from 45 degrees.

In other words, it's effectively a power factor meter!
 

Offline james_s

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #22 on: October 23, 2020, 03:01:29 am »
That's neat, I looked at that a number of times and my brain said "bridge rectifier", only after your comment I see that it indeed is not.
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #23 on: October 23, 2020, 03:14:38 am »
Going by what I could see from SPICE simulations, it looks like the voltage across C75 is roughly (Vrms*Irms/31 A) * cos θ, which some of you may recognize as P/31A (real power). Doesn't quite gibe with what I measured on the real circuit though (I was seeing 1.3V with the iron in the stand and negative voltage with the iron under load). I'm sure there's something my simulation is missing.
 

Online KE5FX

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #24 on: October 23, 2020, 03:47:52 am »
Looks similar to a Bruene coupler, sans the low side of the capacitive voltage divider. 
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #25 on: October 23, 2020, 06:28:36 am »
Going by what I could see from SPICE simulations, it looks like the voltage across C75 is roughly (Vrms*Irms/31 A) * cos θ, which some of you may recognize as P/31A (real power). Doesn't quite gibe with what I measured on the real circuit though (I was seeing 1.3V with the iron in the stand and negative voltage with the iron under load). I'm sure there's something my simulation is missing.

Of course, I got the sign backwards, it's actually negative that XD
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #26 on: October 23, 2020, 06:46:06 pm »
I replaced L6, L7 and T2 with parts that seem to work a lot better

For L6 and L7 this EPCOS VHF choke works pretty well although it is a little tricky to bodge into place: https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B82111B0000C022/4245895

For T2 I wound a new transformer with the same turns ratio on an ft-63a-61 core from Amidon. The specs on the core don't look ideal (complex permeability goes bumpy a little bit above the operating freq) but it seems to work and it's probably a lot better than the uxcell iron powder core.
« Last Edit: October 23, 2020, 06:49:01 pm by rfmerrill »
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #27 on: October 23, 2020, 07:02:10 pm »
I should probably mention that I'm normally a software engineer and this is my first time doing a project with this much analog and really any RF in it  :scared:
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #28 on: October 25, 2020, 07:59:10 am »
Looks similar to a Bruene coupler, sans the low side of the capacitive voltage divider.
Thanks for the lead on this! It looks like it is indeed similar.

I've done hours of more head-scratching and simulation work and the circuit is much more nonlinear than I expected
Click for big


So coming back to his schematic:


Here's how it works:

Current flows in the same direction on the schematic on primary and secondary side (same polarity voltage = opposite polarity current!)

The node between D16 and D17 is clamped to approximately +/-2V by the diode ring.

Because of the above, we can think of C47 as being from the output voltage to essentially ground, as the voltage on the other side of C47 is much greater than 2V in magnitude. Therefore, the current through C47 is approximately the same as it would be were it connected to ground--in quadrature with the output voltage and equal to approx Vout / 1173 ohm (either RMS or peak since it's a sinusoid)

When primary current is flowing left to right--i.e. towards the soldering iron tip--D16 and D17 are reverse-biased, and therefore no current flows through R40

When primary current is flowing right to left, the current induced in the secondary flows through D16 and D17, and the additional current injected by C47 has nowhere to go but into R40--until the voltage at that node gets high enough that either D21 or D22 is forward biased. Because the current injected by C47 is so high, this results in R40 effectively seeing a square wave current which you can see in the LTSpice simulation above.

So if we want to create a simplified model of how R40 is being driven, we can think of it like this:
  • Not driven when primary current is flowing left to right
  • Driven at +2V when primary current is flowing right to left and current is flowing down through C47
  • Driven at -2V when primary current is flowing right to left and current is flowing up through C47

Because the current through C47 is 90 degrees out of phase with the output voltage we can then reach the conclusion that if the output current and voltage are in-phase, periods 2 and 3 are of equal time and thus the voltage at C1 is zero. Of course, that voltage is also being pulled towards 0.8V via its connection to the feedback loop of the DC-DC, but we'll ignore that for now.

If the output voltage starts to lead the output current, then period 2 will lengthen and period 3 will shorten, and the voltage on C47 will be pulled higher. Likewise if the current leads the voltage, the inverse will happen and the voltage on C47 will be pulled lower.

So in essence this is not as dependent on the magnitude of output current and voltage, and much more on the relative phase, which makes sense.
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #29 on: October 27, 2020, 02:42:06 am »
So the above got me curious to see what the actual current waveforms look like on the iron in operation. I took some scope shots:

Iron in stand


Iron heating up


Iron at temperature


Channel 1 (yellow) is the voltage at across the output jack. Channel 2 (blue) is the voltage across an 0.1 ohm current shunt right before the output jack.

The current waveform not being a clean sinusoid kinda threw me, but it makes sense if we're operating around the saturation of the heating element. Still, I'm curious why the current is leading the voltage while power is being delivered to the tip. Capacitance? Intentional effect of the length of the handpiece lead? Something about inductive heating I don't quite understand?
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #30 on: October 28, 2020, 04:32:28 pm »
Talking about the tip detect circuit located right near the output jack, I was a bit confused how it works but now I see that basically that it's basically measuring the DC resistance of the hot side of the RF circuit. With no iron tip connected it gets pulled up to 3.3 and otherwise gets shorted to ground.



I have smoked R37 a few times and I'm not 100% sure but I think this was just a result of accidentally shorting the 22V supply to the iron (probably through my panavise). One thing I'm confused about is why he used such a huge 22 microhenry inductor, especially since I have a lot of trouble finding axial inductors with a self-resonant frequency above 13.56 MHz (although having a self-resonant frequency close to that might be beneficial since it would have a much higher impedance). Seems to me R37 and R41 could easily both increase in value, and L12 seems like it could be replaced by a ferrite bead rather than a plain inductor?
 

Offline TimNJ

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #31 on: October 28, 2020, 09:07:21 pm »
OK, I might need help here.



For L6 and L7 I bought something that seemed close in AL, permeability and size to the weird Russian cores Sergey used. The only problem is I accidentally bought iron powder cores instead of ferrite and I'm pretty sure that's why L6 cooked itself. I swapped a new one in before I had any clue what was wrong, and it makes its way up to 90C in not very much time even with the iron in the stand.

Importantly, I'm pretty sure it's the core getting hot and not the winding.

So what should I use for those inductors instead? I'm looking at ferrite cores and they all have much higher AL and permeability than the core Sergey mentions in the blog article ("HF-ferrite 16x8x6mm ring cores with permeability of 50"). I think the max DC current it needs to take is 4A. Could I just buy an inductor instead of making my own? Or could I use a higher-permeability ferrite with way fewer turns? (like 3?)

Edit: A big part of why I'm unsure about this is that I don't know how much effect a change in that inductance would impact the circuit, or how to compensate for it, and hitting 9 uH on the dot is hard with high-permeability cores.

"HF-ferrite 16x8x6mm ring cores with permeability of 50". A typical ferrite material (that is, one made from MnZn) does not have a permeability of 50u. The minimum permeability for a MnZn material is about 1,000u, and more typical values are 3,000 to 5,000u.

If the core he used was 50u, this implies some sort of powder core, whether that's iron powder, sendust, high-flux etc.

You're probably looking for something like Micrometals T60-18, although I have not looked at the circuit to see exactly what's needed.

You can buy here: https://www.cwsbytemark.com/index.php?main_page=index&cPath=206_217&zenid=s7qsh136tlkgqjm4iuslnf8fk4
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #32 on: October 29, 2020, 03:46:04 am »
"HF-ferrite 16x8x6mm ring cores with permeability of 50". A typical ferrite material (that is, one made from MnZn) does not have a permeability of 50u. The minimum permeability for a MnZn material is about 1,000u, and more typical values are 3,000 to 5,000u.
Yeah this gibes with what I could figure out on my own and why I was confused initially.

Quote
If the core he used was 50u, this implies some sort of powder core, whether that's iron powder, sendust, high-flux etc.
I honestly believe he used some kind of cold war era soviet ferrite made with much older tech. I googled some of the terms he used and get mostly russian results with some people talking about old russian radios. I went and looked just now and it does look like NiZn ferrite materials have permeability in that kind of range. Fair-Rite's 67 material is around 40 for example, and does say recommended freq up to 100 MHz

Quote
You're probably looking for something like Micrometals T60-18, although I have not looked at the circuit to see exactly what's needed.
It's a drain choke for something kinda-sorta like a class E amp but not really I guess? It needs to withstand DC current of around 4-5 amps, and around 100+V peak-to-peak at 13.56 MHz, and be around 9 uH

Quote
You can buy here: https://www.cwsbytemark.com/index.php?main_page=index&cPath=206_217&zenid=s7qsh136tlkgqjm4iuslnf8fk4
Thanks!
« Last Edit: October 29, 2020, 03:51:00 am by rfmerrill »
 

Offline james_s

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #33 on: October 30, 2020, 06:25:59 pm »
Seems like an RF guru could probably suggest some suitable cores for the task. Maybe try on a ham radio forum? Someone who builds/modifies shortwave transmitters and linear amplifiers is likely to have relevant experience, if anyone does that anymore.
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #34 on: October 31, 2020, 06:59:06 pm »
Seems like an RF guru could probably suggest some suitable cores for the task. Maybe try on a ham radio forum? Someone who builds/modifies shortwave transmitters and linear amplifiers is likely to have relevant experience, if anyone does that anymore.
Was my thought, yeah. I joined a ham radio chat but there didn't seem to be much discussion of radio circuit details.

So I've somehow ended up stuck in a state where Q2 is cooking and R37 goes out with a bang any time I disconnect the handpiece. The only major changes I can think of are that I switched to a different core for the current transformer and the RF jack has a shorter lead to the PCB... Q2 is getting hot enough to melt the plastic insulator bushing, even the ones I just bought that are polyphenylene sulfide!

One time I did seem to blow Q2 by pulling the cartridge, which I ascribed to the TVS not doing its job. I'm now wondering why there's no TVS on the output of the filter. I also tried a few different inductors for L12 because I realized that jellybean axial inductors are probably past self-resonance at 13.56 MHz, but that might have accidentally made it worse!

This is all so frustrating because I can't even scope these signals before the components blow up or cook themselves. I'm glad I bought extras, but those STP19NF20s aren't cheap!
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #35 on: October 31, 2020, 11:38:31 pm »
OK, I think I might have a good idea of what is going on. Here's what I've observed:
  • Thermocouple on Q2 heatsink shows it slowly rise to about 40-60C before staying there, unless you give it a heavy load
  • Everything seems totally fine until you pull the cartridge out of the handpiece
  • once you do that, R37 will enthusiastically fail, and Q2 heatsink temperature starts to climb rapidly.
  • I have not directly confirmed, but it seems like D19 (or something near it) is also getting hot during this time
  • Importantly, this doesn't happen if you start the system with the tip disconnected.

So this made me scratch my head because the microcontroller does properly display "TIP FAILED" immediately after you pull it out, so it should not be driving the signal reference into the amplifier. I believe what is happening is that the inductive components I'm using are significantly lower loss/higher saturation current than what Sergey used, and this is allowing the amplifier to start to self-oscillate once D19 opens up.

I did however find a surprisingly simple workaround for the short term: Connecting the hot side of C63 to the EN pin on U2 will shut down the DC-DC converter when the tip is disconnected. This is so simple and obvious that I'm starting to question if there's a really bad downside to it that caused the original designer not to do it.
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #36 on: November 05, 2020, 01:43:25 am »
As I suspected, the drain voltage is coupling throught Q2 to its gate, causing self-resonanceoscillation. I'm now scratching my head wondering how this could have worked for Sergey, maybe the saturation and hysteresis loss in the inductors he used dampens it? Or maybe I just have it tuned wrong.
« Last Edit: November 05, 2020, 06:13:51 am by rfmerrill »
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #37 on: November 13, 2020, 05:51:40 am »
One thing I was scratching my head about, I think I've solved.



I was wondering what the point of D15 was, as it's not possible for current to flow the other direction anyway (as it would instead flow through the body diode of Q2 well before exceeding the clamping voltage of the TVS).

I did a quick google and confirmed one of my hypotheses: This is a neat trick you can use to reduce the effect of parasitic capacitance from a Zener or TVS diode. It seems like there are a few different factors that accomplish this:
  • The parasitic capacitances of the two diodes form a capacitive divider and an overall lower capacitance
  • The DC voltage of the middle node will quickly settle on a value such that the rectifier diode stays reverse biased (the positive peaks of either side line up, and since the middle node has a smaller AC voltage on it, the top node will then stay <= the middle node).
  • With a DC bias on the TVS diode, its parasitic capacitance decreases
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #38 on: November 21, 2020, 06:58:43 am »
Side note: Am I breaking any etiquette by making update replies to this? I don't want to annoy anyone.

My weird Russian toroids finally arrived, only to find out that they're the wrong weird Russian toroids :(

Thanks to a friend I met in a Telegram chat who is familiar with RF, speaks Russian and lives in eastern Europe, I have learned more about the toroids he used for the current transformer and the two drain chokes: In the original Russian version of Sergey's blog post, he in fact mentions the cores are M50BH, but for some reason the translation replaces that with just their permittivity.
 

Online KE5FX

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #39 on: November 22, 2020, 02:46:30 am »
Side note: Am I breaking any etiquette by making update replies to this? I don't want to annoy anyone.

Not at all, it's good to see where the proverbial bodies were buried at Metcal.  There's a lot of interesting stuff in there.
 

Offline rfmerrill

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Re: (Now with pictures!) I built the DIY metcal-compatible soldering station.
« Reply #40 on: November 23, 2020, 06:53:06 pm »
Well, to be fair, I'm only reverse engineering sergey's design, which was based on him reverse-engineering Metcal's design XD
 


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