(Now with pictures!) I built Sergey's 80W metcal-compatible soldering station. - Page 1
Forum section: electronics / projects

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 04, 2020, 07:41:14 pm
content: Update 2021-03-01:New picturesaddedfurther downUpdated bom 2021-02-24:https://docs.google.com/spreadsheets/d/1ThZLzR7Nggm0Z4HKQb2HZf72yWK6yGwCSyFWkue1RE0/edit?usp=sharingPictures (edit: out of date, see thread for changes)!https://imgur.com/a/tWysv8mSome of you may remember this project that got some attention a couple years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/1ThZLzR7Nggm0Z4HKQb2HZf72yWK6yGwCSyFWkue1RE0/edit?usp=sharingThe 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.Update 2021-03-01: The T60-26 cores worked okay until they fried. Don't trust Amazon listings--they said they're ferrite but they were iron powder! The cores that Sergey originally used were K16x8x6 М50ВН (careful! That's Cyrillic, not Latin! "M50VN" can sometimes find them) which are readily available in most of the former east bloc but not easy to get in the US. Thankfully I have a Ukranian friend who arranged to ship me some. I'm currently planning on trying to find a Fair-Rite/Amidon or ferroxcube substitute.Note that Amidon is a reseller: Their Iron Powder cores are mostly made by Micrometals with the same part number, while their ferrite cores are made by Fair-Rite, but with different part numbersI 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.Update 2021-03-01: 20AWG also fits for the two drain chokes. I used it for L6 on my second unit because that one carries more current. On my first unit I ended up using TDK/EPCOS solenoid VHF chokes, which worked alright but I think the open magnetic circuit caused all the glitchiness I experiencedUnless 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 dBNote: as simulated with an ideal source and load! Not measured with a VNAAll 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 capabilityL1 (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 boardThe "+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 itdoesn't matter which one you useYes it does! Q1 gate must be LOW when the system is in a disable stateMetcal 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.2021-02-03 -- recommend connecting a nearby ground to Q2's screw to shunt the capacitive coupled noise that will otherwise return through the housing and output jackI 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 fine2021-02-03 -- I managed to make a significantly cheaper, though not as cool display from BuyDisplay/EastRising work. It requires FW changes which I created a PR for. Note that you can't just run a display off of the 3.3V regulator in the design, as the board layout just isn't made for it. I ended up adding a second one just for the display.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 designThe 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 placeThe thru hole LED footprint (D13) will not fit a 'standard' full size LED. The holes are too smallSome 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.2021-02-03 -- once I actually had a way to measure efficiency I found that it makes a difference, but not a game-breaking one--75% vs 80% eff or something like thatSome 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.2021-02-03 -- much higher is almost certainly ok.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(d'oh, there's one in the repo). A random ebay knock-off STLink programs it just fineThe 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 47pFThe 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.2021-02-03 -- I think that 10k works, the SWR feedback circuit affects the node as wellIn 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.2021-02-03 -- was almost certainly unrelatedAnd improvements that could be made to this design:The refdes are in no particular order. They should be renumberedPorting 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 countA lot of the SMT components are 0805 for no good reason. Even just shrinking them to 0603 would free up a lot of board spaceThe 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 supplyThe design would probably be a lot easier on a four-layer boarddisplay footprint doesn't fit 100 mil headersPin 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.2021-03-01 -- These pictures are out of date, see the top of this post for newer ones. I ditched the cheap power supply and used a decent quality 36V 4A brick. When using an external power supply, you do not want the housing to be tied to DC- at the input, as that causes a ground loop (which is not present in Sergey's design). In my current setup I have a separate mains earth wire running to the housing.Housing frontHousing backInsideBoard topBoard bottomPower supply (cheap chinese amazon special in a random box. Yes that is a macbook power cord)

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 04, 2020, 10:35:06 pm
content: 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 whichI'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.

author: bmac_2780
author_rank: Newbie
timestamp: October 04, 2020, 11:32:34 pm
content: Hi RFMerrillI 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

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 04, 2020, 11:37:59 pm
content: Quote from: bmac_2780 on October 04, 2020, 11:32:34 pmVery cool! I have emailed Sergey as well but have not received a reply yet, but it hasn't been very long.QuoteI 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.QuoteWhich 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 courseQuoteYeah! I'm hoping to get some desk space back from my Metcal power supplyAn Australian friend is interested in revising this design and I might help him with that.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 05, 2020, 07:47:22 pm
content: 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...

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 10, 2020, 01:16:31 am
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: October 10, 2020, 04:33:50 am
content: 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.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 10, 2020, 05:44:24 am
content: Quote from: james_s on October 10, 2020, 04:33:50 amInteresting. 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.

author: james_s
author_rank: Super Contributor
timestamp: October 10, 2020, 06:27:17 am
content: 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.

author: KE5FX
author_rank: Super Contributor
timestamp: October 10, 2020, 07:19:11 am
content: Quote from: james_s on October 10, 2020, 06:27:17 amAs 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 hoursperiod. They were putting them in the U1253/U1273 handheld models, right? Were the failures correlated with power-on time, or simply aging?

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 10, 2020, 06:06:45 pm
content: 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?

author: james_s
author_rank: Super Contributor
timestamp: October 10, 2020, 06:15:11 pm
content: Quote from: KE5FX on October 10, 2020, 07:19:11 amI 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.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 11, 2020, 05:46:28 am
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: October 11, 2020, 05:55:59 am
content: 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.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 12, 2020, 05:09:30 pm
content: 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?

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 14, 2020, 05:41:52 am
content: 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 realizedadding an extra ground link between where the coax solders on and the housing fixed it.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 15, 2020, 03:55:47 am
content: 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.

author: Thomas
author_rank: Regular Contributor
timestamp: October 15, 2020, 10:38:02 am
content: How about aluminium oxide discs?https://www.elfadistrelec.no/en/aluminium-oxide-disc-to-220-fischer-elektronik-aos-220/p/17559973?q=&pos=1&origPos=1&origPageSize=50&track=true1.5mm thick, that should help.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 15, 2020, 03:06:58 pm
content: Quote from: Thomas on October 15, 2020, 10:38:02 amHmm... has a pretty high dielectric constant tho.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 17, 2020, 05:09:55 am
content: 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 havemuchhigher 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.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 19, 2020, 12:52:02 am
content: 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.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 22, 2020, 09:31:16 pm
content: I spent a while trying to figure out this circuit here which provides negative feedback to the DC-DC converterAt 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 aring mixerthat 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!

author: james_s
author_rank: Super Contributor
timestamp: October 23, 2020, 03:01:29 am
content: 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.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 23, 2020, 03:14:38 am
content: 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.

author: KE5FX
author_rank: Super Contributor
timestamp: October 23, 2020, 03:47:52 am
content: Looks similar to aBruene coupler, sans the low side of the capacitive voltage divider.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 23, 2020, 06:28:36 am
content: Quote from: rfmerrill on October 23, 2020, 03:14:38 amOf course, I got the sign backwards, it's actually negative that XD

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 23, 2020, 06:46:06 pm
content: I replaced L6, L7 and T2 with parts that seem to work a lot betterFor 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/B82111B0000C022/4245895For 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.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 23, 2020, 07:02:10 pm
content: 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

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 25, 2020, 07:59:10 am
content: Quote from: KE5FX on October 23, 2020, 03:47:52 amThanks 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 expectedClick for bigSo 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 R40When 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 rightDriven at +2V when primary current is flowing right to left and current is flowing down through C47Driven at -2V when primary current is flowing right to left and current is flowing up through C47Because the current through C47 is 90 degrees out of phase with the output voltage we can then reach the conclusion thatif 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.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 27, 2020, 02:42:06 am
content: So the above got me curious to see what the actual current waveforms look like on the iron in operation. I took somescope shots:Iron in standIron heating upIron at temperatureChannel 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 isleadingthe 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?

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 28, 2020, 04:32:28 pm
content: 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?

author: TimNJ
author_rank: Super Contributor
timestamp: October 28, 2020, 09:07:21 pm
content: Quote from: rfmerrill on October 17, 2020, 05:09:55 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.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

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 29, 2020, 03:46:04 am
content: Quote from: TimNJ on October 28, 2020, 09:07:21 pmYeah this gibes with what I could figure out on my own and why I was confused initially.QuoteI 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 MHzQuoteIt'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 uHQuoteThanks!

author: james_s
author_rank: Super Contributor
timestamp: October 30, 2020, 06:25:59 pm
content: 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.

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 31, 2020, 06:59:06 pm
content: Quote from: james_s on October 30, 2020, 06:25:59 pmWas 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!

author: rfmerrill
author_rank: Regular Contributor
timestamp: October 31, 2020, 11:38:31 pm
content: 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 loadEverything seems totally fine until you pull the cartridge out of the handpieceonce 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 timeImportantly, thisdoesn'thappen if youstartthe 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.

author: rfmerrill
author_rank: Regular Contributor
timestamp: November 05, 2020, 01:43:25 am
content: 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.

author: rfmerrill
author_rank: Regular Contributor
timestamp: November 13, 2020, 05:51:40 am
content: 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 capacitanceThe 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

author: rfmerrill
author_rank: Regular Contributor
timestamp: November 21, 2020, 06:58:43 am
content: 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 toroidsThanks 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 areM50BH, but for some reason the translation replaces that with just their permittivity.

author: KE5FX
author_rank: Super Contributor
timestamp: November 22, 2020, 02:46:30 am
content: Quote from: rfmerrill on November 21, 2020, 06:58:43 amNot at all, it's good to see where the proverbial bodies were buried at Metcal. There's a lot of interesting stuff in there.

author: rfmerrill
author_rank: Regular Contributor
timestamp: November 23, 2020, 06:53:06 pm
content: Well, to be fair, I'm only reverse engineering sergey's design, which was based on him reverse-engineering Metcal's design XD

author: rfmerrill
author_rank: Regular Contributor
timestamp: November 27, 2020, 01:29:36 am
content: Just did some more experimenting and measurements, and I've concluded that the number of turns specified on the three iron powder toroids is low enough that you can't calculate the inductance using the AL valueSince I have micrometals T130-6 instead of the cheap aliexpress ones that Sergey used, I've had to change the turn count and I've determined it's fairly consistent:L9 - 3 turns, ~240 nHL10 - 5 turns, ~440 nHL11 - 6 turns, ~540 nHL9 is the only one that you can't get reasonably close to the specified values. To compensate for that, one 100pF cap is replaced with 47pF in the first parallel array, resulting in a very similar response to Sergey's values at the two critical frequencies (13.56MHz and its first harmonic 27.12 MHz).

author: rfmerrill
author_rank: Regular Contributor
timestamp: December 04, 2020, 07:04:19 pm
content: Thanks to my Ukranian friend, I found this component library that contains everything you need to run Sergey's LTSPICE models:http://bordodynov.ltwiki.org/With those simulations and some modeling calculations I have done, I am pretty confident that the drain choke inductance can be increased up to double with very little change in the behavior of the circuit:(click for big)Basically the coupling capacitor impedance needs to be much less than the load impedance and the choke impedance much greater. You only really need to calculate impedance at the fundamental because the condition gets easier at each harmonic. If that is satisfied:The current through the choke can be treated as approximately DC, its AC fluctuation is small compared to its DC valueThe voltage across the coupling capacitor is also essentially DC.The peak to peak value of the current through the coupling capacitor is approximately proportional to the current through the choke for a given load. The load can pull some additional current backwards through the FET if it's reactive enough, but this is small compared to the contribution from the supply.Since all of the power that is dissipated or used in the circuit comes from the supply, the current through the choke times the supply voltage must equal the average total power (mostly in the load and the FET).I'm not 100% to completely understanding this but I'm psyched that I've made this jump.

author: Free_WiFi
author_rank: Regular Contributor
timestamp: December 07, 2020, 07:01:30 pm
content: Look at here :

author: rfmerrill
author_rank: Regular Contributor
timestamp: December 08, 2020, 08:39:51 pm
content: Quote from: Free_WiFi on December 07, 2020, 07:01:30 pmYeah that could totally be a problem, but it seems to work for the most part anyhow. Are you saying that is what is causing the self-oscillation?If it were up to me I would have not placed all of that so close together.

author: rfmerrill
author_rank: Regular Contributor
timestamp: December 08, 2020, 08:54:34 pm
content: From Sergey's LTSpice models (specifically rf4.asc) I finally found a simple simulated load circuit that works.(click)The resistor can be a 50 ohm RF dummy load off of a coax if you want. The inductor can go higher, it is just there to provide a DC short to fool the tip detect circuit.Now if anyone knowswhyan inductive soldering handpiece has an impedance that looks like a series RC circuit (which gibes with the scope traces I took) that would be interesting to know.Update 2021-03-01 -- this was answered in the mamalala thread a while back: There's an LC matching network in the handpiece assembly.

author: rfmerrill
author_rank: Regular Contributor
timestamp: December 13, 2020, 09:15:39 am
content: Quote from: Free_WiFi on December 07, 2020, 07:01:30 pmYou know, I just realized you might be on to something because the VHF chokes I substituted are solenoid inductors and thus have an open magnetic circuit, so it's likely there IS coupling going on there, which Sergey would not have experienced because he was using toroidal core inductors with a closed magnetic circuit.

author: richard.cs
author_rank: Super Contributor
timestamp: December 13, 2020, 02:31:19 pm
content: Have you come across this thread?https://www.eevblog.com/forum/projects/diy-metcal-13-56-mhz-rf-supply/It's a different design to do the same thing, but quite a few people on eevblog (including myself) made one. My write-up is here:http://randomfunprojects.co.uk/metcal.html

author: rfmerrill
author_rank: Regular Contributor
timestamp: December 13, 2020, 05:04:59 pm
content: Quote from: richard.cs on December 13, 2020, 02:31:19 pmHuh... kinda wish I knew about it before I got so far into building this one! XDIt looks like sergey may have referenced that design a little uncredited, or maybe not.Does it reach 80W?

author: richard.cs
author_rank: Super Contributor
timestamp: December 13, 2020, 06:30:59 pm
content: Quote from: rfmerrill on December 13, 2020, 05:04:59 pmFrom memory I had something like 100W out of it into a 50 Ohm dummy load when I was building it. In any case it works well with all the tips I have tried it with, and is not noticeably slower to heat than the genuine Metcals I have used at work.

author: rfmerrill
author_rank: Regular Contributor
timestamp: December 13, 2020, 07:38:09 pm
content: Quote from: richard.cs on December 13, 2020, 06:30:59 pmWell crud. This whole project seems pointless now

author: richard.cs
author_rank: Super Contributor
timestamp: December 13, 2020, 11:52:26 pm
content: Quote from: rfmerrill on December 13, 2020, 07:38:09 pmHmmm, not so fast, I have been looking back through the thread. In Reply #376 I said "At this point I have 62V peak RF into 50 Ohms (just shy of 40 Watts)". It may well be more in the "boost mode". In any case I have found there to be enough power for the tips I use but it looks like I mis-remembered the power.

author: rfmerrill
author_rank: Regular Contributor
timestamp: December 15, 2020, 09:19:39 pm
content: Quote from: richard.cs on December 13, 2020, 11:52:26 pmThe non-expensive commercial products all seem to be in the neighborhood of 40W, with some of the higher end ones being 80W. Assuming that the power bar graph on my unit is at all reasonable, it appears that you rarely go over 40W for actual soldering tasks, but it does easily exceed that when cleaning the iron tip on a wet sponge or warming it up from cold.

author: rfmerrill
author_rank: Regular Contributor
timestamp: December 16, 2020, 09:22:56 am
content: By happenstance I was soldering an SMA connector to a big copper heatsink today, and with my biggest hoof tip flat against the surface of the heatsink it only barely hit 45 Watts.

author: Free_WiFi
author_rank: Regular Contributor
timestamp: February 16, 2021, 03:28:02 pm
content: Hello.First of all Sergay has already told in russian language that this project is crapware made just for fun and i completely agree with him since i perfectly understood russian language.By the way to tweak this DIY you can replace (i think you must) the main RF transistor with something like :STP17NF25or an smd mosfet like : FDMS8622Another thing you need to understand is that's you must tune the rf mosfet impendance with your chain of L's, at least try to place in parallel or in series 1 melf resistor with the first torus,the resistance according to the frequency must be the same.(Take a look at TDR to find your R valuse for 13.56Mhz)Another try you can run is just to decouple the internel resistance betwen the drain and your 1st inductor,by doing this yor RF mosfetshould get cooler,since without good impendance the mosfet is not running completely open during the cycles.Good luck.

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 17, 2021, 07:46:23 am
content: Not quite sure what you mean, but thank youI actually have been using the first unit I built as my main soldering station for a while now. I tweaked the potentiometer in the buck converter a bit and it slowed the voltage change and reduced the power but now it doesn't glitch out anymore. I'm building a second unit with actual M50BH K16x8x6 toroids and maybe that will work a bit better (I'm using solenoid inductors right now and they're probably spewing noise into other parts of teh circuit).It actually stays pretty cool at this point. The Mamalala design dissipates a lot more heat (or at least dissipates more heat in one component).Quote from: Free_WiFi on February 16, 2021, 03:28:02 pmIs this just for the higher voltage rating? What's the reason for preferring the other transistor? It looks like the specs are worse besides those (although the turn-on time is faster... which I don't understand since the input capacitance is higher)QuoteBy "the first torus" do you mean the drain choke of the first mosfet or the first inductor in the filter? What must the resistance be the same as?QuoteHow do you do this?Thanks!

author: Free_WiFi
author_rank: Regular Contributor
timestamp: February 17, 2021, 01:11:00 pm
content: Hello.I can't help you ragarding your power bucket converter,i just can give you some suggestions regarding the rf part.By the way,try to play with the impendance (ESR) over your mosfet and the 1st torus connected with your Drain.There is a lot of combinations to play around .... you can try to shorcut an R with torus,you can connect the R in series over Drain and your choke to find the golden meta for the impendace etc ...I suggest you to go on Stack EE and mabe read there something or make your post regarding your questions about this stuff.PS:The best way to make this high. freq. power supply more stable and powerful would be to take 2 of FDMS8622 and apply them both in parallel on each other on opposite sides of your pcb,but to do this you shoud be willing to re-make your pcb

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 18, 2021, 03:58:01 am
content: Thanks for the suggestions. I'll try to understand it

author: t0m
author_rank: Regular Contributor
timestamp: February 22, 2021, 01:13:23 pm
content: As a fellow software guy, I'm impressed that you got this design working. It has some exotic features. Are the boards 2oz copper?Sergeymax did say that the constant current RF design wouldn't scale to 80W, but I don't think he mentioned why he thought that.

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 23, 2021, 01:20:51 am
content: Quote from: t0m on February 22, 2021, 01:13:23 pmThanks!I do have a degree in EE, technically, but barely any professional experience with analog and none with RF.QuoteI just got whatever OSH park's default is, which appears to be 1oz.QuoteIf you missed it I think I figured out perhaps one reason: Trying to achieve 80W with constant current would potentially overheat the iron tip. You'd be doubling the power applied to the tip when it's at the curie point as well as when it's cold. I do also notice that the mamalala design is slightly less efficient and the waste power seems to be more concentrated in one component (the single power transistor).

author: t0m
author_rank: Regular Contributor
timestamp: February 23, 2021, 04:36:29 am
content: Well, if it is truly an advance over the previous technology, I would expect it to be protected by its own patent.

author: t0m
author_rank: Regular Contributor
timestamp: February 24, 2021, 09:41:48 am
content: The closest thing I could find to K16x8x6 (https://russian-electronics.com/products/miscellaneous/:m2000nm-36-k16x8x6) is Fair Rite 5977004901 (https://www.fair-rite.com/product/toroids-5977004901/) aka Amidon FT-63A-77 (http://www.amidoncorp.com/ft-63a-77/). Although I can see why you would have picked the higher frequency material.

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 24, 2021, 05:55:35 pm
content: Quote from: t0m on February 24, 2021, 09:41:48 amThe biggest thing that was making me wring my hands about ferrite material is that I wasn't sure how the circuit behavior changes if that inductance is different. I now understand that I can freely make it quite a bit higher with no ill effect, but at the time I was trying to hit 9 uH as close as I could, and an AL that's too high makes that impossible. The remaining difficulty is that manufacturers like Fair-Rite and similar who sell in my market seem to make "thinner" toroids than the Russian ones (picture attached), meaning that I don't know if I can get a ferrite with similar performance that physically fits in the same space. For the current transformer the specs of the core are not quite as critical--it seemed to work fine with a kind of random ferrite and even with an iron powder core.Just checking because I'm not sure about it myself: If you look upthe 77 material datasheet, the permeability is shown plummeting down to nearly nothing at 5 MHz, so I don't think it will effectively choke a 13.56MHz signal will it? I think I went with the FT-63A-61 as the best match for the K16x8x6 M50VN (M50ВН in Cyrillic, not M50BH--took me a long time to realize why I wasn't getting search results).

author: t0m
author_rank: Regular Contributor
timestamp: February 24, 2021, 06:28:50 pm
content: Quote from: rfmerrill on February 24, 2021, 05:55:35 pmLooking again, it appears that K16x8x6 is just the form factor. If the material is designated by M50VN then Fair Rite 61 would definitely be the better match. (I can't immediately see where M50VN comes from, but I'll take your word for it.)

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 24, 2021, 07:24:40 pm
content: Quote from: t0m on February 24, 2021, 06:28:50 pmThe original Russian blog post says M50VN but the translation everyone looks at (the one on habr.com) just says "with permeability about 50"

author: t0m
author_rank: Regular Contributor
timestamp: February 24, 2021, 10:31:19 pm
content: Couple of quick questions.1. It looks like you substituted a LM2596 module for something -- presumably that is supplying 10V to the FET driver?2. What does "anchor" signify in the BOM?Thanks!

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 24, 2021, 10:46:30 pm
content: Quote from: t0m on February 24, 2021, 10:31:19 pmDelighted to answerQuoteBecause Sergey's AC-DC circuit only works with 230V input, I had to provide an alternate power source. I'm using an external 36V DC supply via a barrel jack, and then the LM2596 module is used to derive the "+10V" supply (which I run at around 11V), which powers the RF preamp as well as the 5V and 3.3V regulators.QuoteLocal store where I bought some of the parts.QuoteYou are quite welcome

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 24, 2021, 11:17:05 pm
content: I made an updated BOM snapshot to reflect some of the part changes I made along the way, and correct some errors or missing info:https://docs.google.com/spreadsheets/d/1ThZLzR7Nggm0Z4HKQb2HZf72yWK6yGwCSyFWkue1RE0/edit?usp=sharingI still have the K16x8x6 toroids in the BOM as I have not found a replacement I'm comfortable suggesting just yet.

author: quadtech
author_rank: Regular Contributor
timestamp: February 25, 2021, 01:25:59 pm
content: М50ВН :M - product is made of ferrite50 - initial magnetic permeabilityВН - High Frequency nickel-zinc for weak magnetic fieldsInitial magnetic permeability: 50 (+10, -10) , F up to 70 MHz.After a long search, I found a PDF mapping Epcos materials to Soviet onesas per which М50ВН maps to material K1http://forum.cxem.net/applications/core/interface/file/attachment.php?id=467310As per the cross ref below, K1 is equivalent to Fair-Rite 61, Ferroxcube 4C65https://www.neosid.com.au/ref-b.htmlHence it appears this one from Digikey may be a close match to М50ВН K16x8x6 -Ferroxcube TX14/9/5-4C65 14 x 9 x 5 from Digikey might be close.Disclaimer : I am just figuring out this RF stuff...In Segey's pic of the MX-5200 board, light violet color cores (probably 4C65 etc) are used for the current transformer.Judging by the width of the T130 cores, it appears the outer dia is around 0.5" (12.5mm).https://hsto.org/webt/cq/hs/gy/cqhsgyb-ytpsducjzwtlgrb8rbe.jpeg

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 25, 2021, 07:45:21 pm
content: Quote from: quadtech on February 25, 2021, 01:25:59 pmAmazing, thank you!I might be wrong about this, but I think for the current transformer there are much looser requirements on the material:Saturation is less of a concern because of the lower magnetizing current -- it sees only the output current of <2A RMS through a single turn winding, versus the 4+A DC through 15 turns on one of the chokesThe impedance presented at the primary need only be low enough to be negligible, and an exact value is not necessary

author: t0m
author_rank: Regular Contributor
timestamp: February 26, 2021, 03:01:20 am
content: That's an awesome bit of research, Prasad!Based on photos of the original MX-5200 PCB that I have seen Amidon FT-63A-61 also looks about the right size, although I can't get a handle on the internal diameter.I just have a couple of comments on the updated BOM. Just for the sake of reducing the parts count, I suggest replacing D20 with another MBR140SFT1G. For L12 I suggest a cheaper part with similar specs (78F220J-RC0). A part in the same range might also work for L4 (78F2R2K-RC). I also picked out higher power resistors for R12,R8, and R9 although that might be overkill.I'm working on a cut-down PCB assuming 30-40 VDC input. It will retain the $0.99 fleabay LM2596 module

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 26, 2021, 04:57:29 am
content: Quote from: t0m on February 26, 2021, 03:01:20 amYeah, can't see why not. I wonder why Sergey used different diodes.QuoteShould work. The self-resonant frequency is below our operating point, but only very slightly so it should be high enough impedance there.QuoteI actually never placed L4, the DC-DC is going in where it would.QuoteMax current through those three parallel resistors is under 5A if that helps.QuoteCoolMaybe I'll build it.

author: t0m
author_rank: Regular Contributor
timestamp: February 26, 2021, 07:40:42 am
content: Quote from: rfmerrill on February 26, 2021, 04:57:29 am

author: quadtech
author_rank: Regular Contributor
timestamp: February 26, 2021, 07:53:56 am
content: Tom, that would be great, to have a small board with the CPU on it.The Mamala board dimensions are nice, as it would fit well on a 4"x4" or 100mmx100mm heatsink.So even if you do 100mmx90mm , that would make it easy to mount the Mosfet drivers.R8/R9/R12 - assuming a 1.5A current through each of them, it is just under a quarter watt.CRM1206-FX-R100ELF at 1/2W would be plenty of safety margin.FWIW , the Amidon FT-63A-61 appears to be identical to the Fair Rite 5961004901, which is easier to order, from say Digikey.As per Sergey's comment , consider using MAX17600/01 which allow paralleling thetwo outputs, similar to the Mamala design.Since all the parts have the same pinout, you can still use MAX17602if you don't populate the extra pair of 2.2ohm resistors for the other output.Assuming you use an external 30V supply, here is a simplified schematic, after getting rid of thecomponents for the board mains switching supply.

author: t0m
author_rank: Regular Contributor
timestamp: February 26, 2021, 08:34:34 am
content: My schematic looks similar. The only thing that needs to be added are connections for the DC input, and the buck module that pumps out 10V. I have cut the board down to 96 x 64 mm. That is small enough to fit on the freeware version of Eagle, so I might convert it if I get horribly fed up with DipTrace. Ideally I would get it to fit in a 1-gang exterior electrical box, which you can get here for about $3.A second output would certainly be nice. The firmware might need to be updated?

author: quadtech
author_rank: Regular Contributor
timestamp: February 26, 2021, 09:38:44 am
content: That's really compact - if you could make 100mmx50 , you can fit twoof them in usual 100mmx100mm China PCB fab size.Eagle freeware would be awesome - I could help as well, as I am more comfortable with Eagle.I guess the 2nd output would need firmware changes, unless it is switched so only one is active at any time.

author: t0m
author_rank: Regular Contributor
timestamp: February 26, 2021, 05:52:58 pm
content: As an aside, I recently bought a SP200 power supply (SP-PW1-10) on fleabay for $40. I'll start a new thread with pictures of the insides. It'll be interesting to see the differences. I suspect it is slightly easier to design a 470kHZ system than a 13MHz one.

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 26, 2021, 06:40:46 pm
content: If someone can port this design to KiCAD that would be really cool... so far we have one design in Eagle and one in Diptrace, but none in KiCADOne thing I think could really be improved is better decoupling between the RF and digital sections, and in general better grounding (like you have a thin trace between two ground pads on the bottom layer that are both just attached to a ground flood on the top layer... wtf.For a particular example, on the bottom layer around the uC, the microcontroller's one ground pin is located on one side, connected to the top layer via a trace that snakes up to the 5V regulator and cap, and also a thinner trace that goes down to the display pcb header. You can see in these scope measurements why I think that's a problem:(click for gallery)Path to from GND pin to the top layer ground fill. Signals on the opposite side of the chip are referenced to a bottom layer GND fill that is only distantly connected. Note particularly the BOOT pin (pin 1)Probing between "ground" and "ground"resulting scope traceI noticed this issue when I was trying to figure out why the display power indication was jumping all over the place despite measuring a constant power output.

author: t0m
author_rank: Regular Contributor
timestamp: February 26, 2021, 11:26:48 pm
content: Ugh. It looks like 2 main frequencies in the noise. Presumably one of them is 13.56MHz?I'll see what I can do with the layout. I haven't used KICAD, only gEDA and Eagle.

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 27, 2021, 02:11:33 am
content: Bit of a false alarm: noise doesn't show up nearly as high when I use a low impedance probe (coax probe) so I think it might have been coupled noise from the RF circuitry into the probe.Got some better 3.3V measurements and I'm fairly certain it was just noise on AVCC from the OLED display. I think I might just stick down a second 3.3V regulator rather than running the display off the same one as the uC.

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 27, 2021, 04:15:37 am
content: Yeah that was the ticket. Display is a lot smoother now. I put the LD2985BM33R back in place and used the MaxLinear SPX part to just power the display, and now the power meter output is very stable.So yeah, I think that's the way to do it if you're going to use a 3.3V display. If anyone is spinning the board maybe put a stuffing option for a larger 3.3V regulator with better routing to avoid that noise?Also to t0m and Prasad and anyone else who may spin a board based on this design: The display header has a 5V supply but 3.3V IO. This is technically not supported by most displays, but you get away with it with the adafruit display. If you want to use a 5V display adding a level shifter might make it more reliable. If you want to use the BuyDisplay/EastRising OLED that I'm using, you might want to change the pinout otherwise it will require a lot of cross-wiring.

author: t0m
author_rank: Regular Contributor
timestamp: February 27, 2021, 05:25:55 am
content: I'm not clear which is the MaxLinear part, is that on your BOM?The display is certainly part of the design that needs sorting out. Can you post a link to the OLED you used? Can we get rid of the 5V regulator altogether?

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 27, 2021, 06:17:56 am
content: Quote from: t0m on February 27, 2021, 05:25:55 amIt was. I just removed it as it didn't work how I intended. The part was SPX3819M5-L-3-3/TRQuoteThis is the product:https://www.buydisplay.com/i2c-16x2-oled-serial-character-display-module-screen-yellow-on-blackThe choices in the dropdown I believe just change the stuffing of some setting resistors on the display, which you can fix yourself (and they may goof and ship you the wrong one). 6800 4-bit is the correct interface.It's not as sexy as the Winstar module that Sergey used, but it's cheaper (at least if you're in the US--winstar module goes for like $80 on ebay and the other choices are shipping from poland).Requires modified firmware that repurposes the LED pin to drive the RESET of the display, instructions are in my pull request here:https://github.com/SergeyMax/SolderingStation/pull/1If youcanget the winstar module (WEH001602AGPP5N00001) for a price that you're ok with, you could easily build the board to support both with just different firmware for each. Beware that the display on the one I linked above is upside down relative to the Winstar and Adafruit modules (in terms of where the io header is) and they're very mechanically different (overall size is the same but display cutout and hole positioning are different).QuoteThe 5V regulator only directly drives the display if you are using a 5V display. Otherwise if you can derive 3.3V directly from the 10V rail, that is also an option. The 3.3V regulator sergey used does nominally support 10.5V input but it will dissipate 4x the power of course (and it might only barely be able to drive enough current to power both the display and the uC).I chose to keep the 5V reg as it's a much bigger package and can easily tank the waste power compared to the 3.3 regulator. If you're going to spin a new board, though, you could substitute a bigger 3.3V regulator that can dissipate all the power you need.

author: t0m
author_rank: Regular Contributor
timestamp: February 27, 2021, 08:20:00 am
content: Thanks. I'll try to spin the new board so that different configurations are possible, but I'm starting to think the Adafruit display is not a bad option.

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 27, 2021, 09:02:49 am
content: Quote from: t0m on February 27, 2021, 08:20:00 amI suspect that the Adafruit 16x2 display actually contains the exact same LCD that Metcal puts on their current MX-500 line.It's similar size and the colors and general look are very reminiscent.Cursory testing indicated you might be able to get away with 3.3V IO but a level shifter isn't too expensive and is good insurance.I like the OLED because it is much easier to read but ultimately it's not like you spend a lot of time looking at the display, and the OLED displays are pricey.

author: t0m
author_rank: Regular Contributor
timestamp: February 27, 2021, 09:13:19 am
content: The display is an optional extra in my book.I'm also thinking it might not be the worst idea to put the microcontroller on a separate PCB and make it actually, rather than theoretically, optional. If the signals to and from the microcontroller need to be shielded you could use copper tape, or wire braid, or even coax.With stacked PCBs in mind, how high do the tallest components stick up off the board? I'm guessing the T130-6's need headroom of 4cm or so?

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 27, 2021, 09:26:20 am
content: Quote from: t0m on February 27, 2021, 09:13:19 amFair enough. It does help to have some kind of indication of whether it thinks there's a tip present or not.QuoteCurrently the microcontroller divides its clock in half to produce the 13.56MHz. You would need to provide an alternate generator of that and make sure that you respond to the tip disconnect event by immediately holding Q1 gate low (turned off) and waiting at least 100 ms before turning it back on. I do not know if instead disabling the DC-DC works just as well.QuoteAbout that yeah. One other consideration with this board is the one inductor on the bottom that sticks out way further than any of the other bottom-layer components.

author: t0m
author_rank: Regular Contributor
timestamp: February 27, 2021, 07:44:21 pm
content: I figure that without the microcontroller you need a flip flop to divide the clock by 2, some logic for the tip detect, and something like a bicolor LED for signalling. There should be plenty of space on the bottom board for all the low voltage stuff, I can prototype it without changing the RF circuit, and people can remix it to put their favourite microcontroller on or whatever. Twisted pair should be OK for the connections between boards.The bulky surface mount inductor will probably have to move to the top of the board. But if necessary it's easy enough to use a deeper box e.g. Red Dot IDH3-1 instead of IH3-1.

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 27, 2021, 08:32:21 pm
content: Quote from: t0m on February 27, 2021, 07:44:21 pmYou might as well just steal the 13.56M oscillator from the mamalala design. You'd need something to generate a good clock from the crystal. I think Sergey just went with the divide-by-2 because he couldn't source a 13.56MHz crystal in Russia.You could take the SN74LVC1GX04 and the crystal and associated passives from that design, drive the "in" pins of the MAX1760x with its output, and then connect the tip detect logic to the "ENA" pins--if the enable pins are grounded, the output will be low regardless of whether you have the inverting or non-inverting version.

author: t0m
author_rank: Regular Contributor
timestamp: February 28, 2021, 07:45:10 am
content: I have some suggestions for further changes to your BOM, based on what I can see from the pictures of your build* You have a LM2596 module (as discussed) and NTC 8D-20 thermistor on the board that are not in the BOM.* AFAICT C13,C27,C37,D11,D12,D14,D9, and L4 are omitted from your build.* It looks as though you have omitted D7 and R18, so you probably intended to omit R10 as well.* I'm not sure if you went through with your idea of a ferrite core for L8, but it's not in the BOM.Some further comments:* I think R17 should be omitted.* I'm not entirely sure that you need C36 (but it probably can't hurt).

author: rfmerrill
author_rank: Regular Contributor
timestamp: February 28, 2021, 08:46:01 pm
content: Quote from: t0m on February 28, 2021, 07:45:10 amthat's true but I restricted the BOM document to just things which are placed in actual footprints on the board. I'm not including the housing, switches, jack etc either.QuoteI left components on the primary side partially filled in because I started buying them before I realized I would not be able to use the AC-DC circuit. I can probably just remove them or make a column for "not fitted". I figure if someonedoeswant to build that part, having a suggested list might help, but maybe it would be good to separate it since I never actually built or tested that.Quotenope I used an air-core inductor, it works fine once I got myself the proper coil form and learned how to measure it properly in order to tweak it.QuoteI kept it since it discharges C6 once power is removed.QuoteI don't trust the output caps on the cheap DC-DC module, and it's a pretty cheap and small capacitor to add. Linear regulators are great but not perfect, and any noise on the input will get passed through to the output, even if significantly attenuated. LM7805 datasheet does recommend a cap on the input if it is located "far" from the power supply filter. It's not clear from the schematic but C37 is located much much further from the 7805, right on the rectifier by the transformer.

author: t0m
author_rank: Regular Contributor
timestamp: February 28, 2021, 11:56:58 pm
content: Thanks again for the detailed responses -- very helpful!

author: t0m
author_rank: Regular Contributor
timestamp: March 01, 2021, 09:58:26 pm
content: Here is the modified schematic I am working from.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 02, 2021, 03:19:38 am
content: As an update, I havean albumof pictures that show what the first unit I built looks like today.Here's the power supply I'm using. Isolated 36V 4A LED supply--Amazon's choice! Spliced into the load end of the IEC cable to get an earth ground to pass alongHere are the connections on the back. I only had metal barrel jacks, so I used some plastic washers and a rubber grommet to isolate DC negative from ground. The screw holding Q1 to the housing doubles as a grounding lug (afaik the ground connection can be anywhere, but the area by the transistors is particularly noisy).Inside you can see some minor changes I've made.And here it is right at home

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 02, 2021, 06:28:15 am
content: For anyone who is revising this design or iterating on it: I might recommend also making the feedback path to the DC-DC more like the one in the mamalala design (using a buffer/amp and the potentiometer in series with the RF feedback instead of the regular feedback) as it's a bit more obvious how it's supposed to work. Mamalala's design in general uses a lot of op-amps to buffer signals which I think simplifies getting the design to work.

author: quadtech
author_rank: Regular Contributor
timestamp: March 02, 2021, 01:36:20 pm
content: Quote from: t0m on March 01, 2021, 09:58:26 pmNice...I did something similar, pruning Sergey's PCB to remove the Switch mode supply components,and add a couple of connectors for the 32V and one for the 10V (using an external LM2596 or other DC-DC buck module).The diptrace file is attached - rename it from .doc to .dip extensionThe intent was to learn Diptrace, so did not really check if I ripped out anything which is really needed.

author: quadtech
author_rank: Regular Contributor
timestamp: March 02, 2021, 02:20:01 pm
content: Tom, if you want to use an external processor, only 4 pins of the STM are really used :PA0 , PA1 : current detection (I think) (shown in blue, yellow in the attached image)PA9 : Tip detect input to the cpu (shown in Red in image)PA10: 13.56MHz freq output from the cpu (shown in Violet in the image)Pins 7, 8 , 9, 10 of the LCD Header are unused - could be put to use for this.

author: t0m
author_rank: Regular Contributor
timestamp: March 02, 2021, 06:05:57 pm
content: Quote from: quadtech on March 02, 2021, 02:20:01 pmPA0 measures the current, PA1 measures the voltage, and together they are multiplied to calculate powerI've decided to use a cheapo 1604 LCD display, which is overkill for communication purposes, but fits nicely into a GCFI wallplate.

author: t0m
author_rank: Regular Contributor
timestamp: March 02, 2021, 06:41:40 pm
content: Quote from: rfmerrill on March 02, 2021, 06:28:15 amCertainly the output impedance of the INA138 is way too high for the ADC and should be buffered, per the datasheet.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 03, 2021, 07:12:54 am
content: Quote from: t0m on March 02, 2021, 06:41:40 pmI don't know what exactly that output impedance number is supposed to mean. I assume it means when the output transistor of the INA138 isnotturned on, otherwise the typical application wouldn't work...Edit: Oh silly me: the INA138 has a high output impedance because it's a current output device.The only requirement here is that the shunt resistor to ground be much lower impedance than the ADC input, which I think we're doing just fine. The effective ADC input impedance is a function of the sampling time, and according to the STM's datasheet, the 71.5 cycle sampling time that sergey's code is using results in an effective impedance that's basically really high.But that's not what I was talking about: I meant the signal that comes off of C75/R40 and connects to the feedback node of the DCDC via a 2.2k resistor.The feedback network of the buck converter is kind of headscratchy to figure out in Sergey's design, since VR1 has an effect that isn't quite obvious--you'd think it changes the nominal voltage of the DC-DC, but it also changes how much that voltage is influenced by the "outer" feedback loop.I've attached the RF board schematic for mamalala's design to this post. You can see it achieves roughly the same thing through an arguably more complex but (imo) easier to understand circuit:The RF feedback signal comes in as RF_DET from the peak detector. The source of this signal is very different but you can think of it as equivalent to the node between R40 and C75 in Sergey's design in that when it increases in voltage, the buck converter decreases its output voltage in responseRF_DET is then buffered and amplified by an op-amp which adds an offset, producing VFBVFB feeds into the DC-DC converter's feedback node via a variable resistance.Now that I type this out and look at the circuit, I think it's not actually quite as simple either way... but the advantage of mamalala's design, I think is that it's much easier to know what voltage range the DC-DC will operate over: Since VFB is produced by an opamp running off of +5 and 0V, its voltage can never be outside the range of 0-5V.I think in both cases you have some feedback signal Vf (RF_DET in the mamalala case, the voltage between R40-C75 in Sergey's case) and your control is something like:Vsupply = Vs0 - k(Vf-Vf0)Ideally if I were designing it I'd want both Vs0 and k to be intuitively adjustable.

author: t0m
author_rank: Regular Contributor
timestamp: March 04, 2021, 01:27:08 am
content: I found a useful post about load voltage and current in inductive heating circuits -->http://www.richieburnett.co.uk/indheat.htmlIn the plots below, the X axis shows the frequency of the tank circuit. The white vertical line represents the resonant frequency of the heating coil. The upper plot shows the amplitudes of the voltage across the load (red lines) and the current through the load (green lines) for a range of values of the Q factor. The sharply peaked lines represent high Q, the smoother lines low Q. The lower plot shows the phase relationships relative to the inverter voltage for the same values of Q.Quoting from the site: "With no workpiece installed, losses are low and Q factor is high. This gives rise to the sharply peaking currents and voltages in the top graph, and the abruptly changing phase shifts in the bottom graph. As a lossy workpiece is introduced the overall Q factor of the LCLR network falls. This causes less resonant rise in the inverter load current and the voltage across the work coil. The resonant peaks become less tall, and broader as the Q factor falls. Likewise the phase of the inverter current waveform and the work coil voltage slew less rapidly for lower Q factors."If I understand correctly, supplying a frequency slightly above the resonant frequency of the heating coil (shaded blue) ensures that it presents as an inductive and not a capacitative load. Under these conditions the load current will lag the inverter voltage by up to 90 degrees, and the load voltage will lag the inverter voltage by between 90 and 180 degrees. Keeping the phase relationships in this stable region avoids stress on the semiconductors in the inverter. Tuning the circuit for maximum power can be achieved by gradually reducing the frequency of the tank circuit towards that of the heating coil.When the iron is heating up, the heating losses to present as a heavy load that damps the resonance in the network. When it hits its Curie temperature, the load will fall and the network Q will rise. The amplitude of the voltage across the load and the current through it will both increase. Also, the phase of the voltage across the load will lag further and approach antiphase with the inverter voltage as resonance increases. The phase of the load current may change in either direction, and slews in a smaller range.Now, if you rectify the voltage across the load and put it through a low pass filter, you get a measurement of amplitude that can be used as negative feedback to regulate the power to the heating coil. As I understand it, this is the basis of the Metcal patent and what is implemented in both the MX500 and mamalala's circuit (in a fancier way). The SP200 uses a measure of load current, suitably buffered, in a similar manner.According to your simulation, the SergeyMax sensor is a negative function of power (I*V) which should provide a more sharply peaked (and presumably more sensitive) feedback function than I or V alone.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 04, 2021, 09:36:44 pm
content: Quote from: t0m on March 04, 2021, 01:27:08 amApologies I could only skim this right now, at work currently but can read it more in depth at a later time.One thing that's kind of buried here is that the Metcal handpiece includes a series L and parallel C hidden near the plug. I was really confused at what I was measuring before I learned that.richard.cs 's pagecontains a table of impedance measurements of the iron tip + handpiece that gibes with what I've seen:RXEquiv XSWRS11Cold:42.3+13j153 nH1.4-16dBWarm (but below Curie temp):55-16j730pF1.1-23 dBHot (above Curie temp):12+24j280 nH5.1-3.4 dBSo when I read this, it seems like if you are using constant current control, then if you are putting 40W into the handpiece in row 2, then you are putting (12/55)*40W = 8.72W into the handpiece when it is above curie temp. This kind of illustrates why constant current is not feasible for more power output, because if you doubled the 40W, you'd also be doubling the 8.72W. My first sergeymax unit drops down to 9W on the display (which is probably more like 5-7W real output) if I hold the tip still and cup my hand around it.The feedback circuit in the SergeyMax design very roughly outputs a voltage proportional to the phase angle between voltage and current: approximately 0 if they're in-phase, negative if current is leading the voltage, positive if current is lagging the voltage. Messing with the LTSpice model, that voltage pegs at its maximum of 1.2V when the load is 12 ohm + 280nH (the "hot" value), and is slightly below 0 at the "warm".A nice advantage of this feedback mechanism is that with the iron in the workstand (which contains a magnet that saturates the heating element) the power drops immediately to sub-5W, which I think is a lot better than what it drops to with the older design (and probably why the older design needs that timeout circuit).

author: t0m
author_rank: Regular Contributor
timestamp: March 04, 2021, 11:47:30 pm
content: It would be interesting to have equivalent measurements for the SergeyMax circuit.The graphs I posted assume an LCLR system, with the heater having a capacitor in parallel with the induction coil. AFAIK that's consistent with what we know about the handpiece.I wonder if Richard's circuit was optimally tuned, because if I understand the blog post, it is saying that the capacitative reactance observed in row 2 would have been putting thermal stress on the power transistor.In the inductive region that Richie recommends, current always lags voltage and the lag tends higher with Q. So if the feedback voltage is proportional to phase it should act to cut power when the iron hits its Curie temperature and the Q factor jumps higher. That makes sense.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 05, 2021, 01:28:19 am
content: Quote from: t0m on March 04, 2021, 11:47:30 pmThese are measurements of the handpiece + tip cartridge, I assume he heated it through external means and measured it with a VNA or similar, but maybe not? In any case if they were correctly taken it is independent of the power supply circuit.Also I built my test load circuit with a capacitance of ~730p in series with a 50 ohm dummy load and it does seem to mimic the iron tip okay.

author: t0m
author_rank: Regular Contributor
timestamp: March 06, 2021, 03:18:22 pm
content: Thanks for setting me straight thereOcela at VRTP.ru recommends putting a noise suppression cap across the DC inputs:https://vrtp.ru/index.php?showtopic=30618&view=findpost&p=837982This one looks like 220 nF / 320V.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 06, 2021, 05:50:39 pm
content: Quote from: t0m on March 06, 2021, 03:18:22 pmHaha wow either someone followed my idea or just had the same idea as me XD same converter module.It looks like that person has tied their dc negative to the housing at the input, which I did not do. I have it separated from the housing via a rubber grommet and plastic washers, and a separate mains earth wire connected to one of the transistor screws. DC negative is tied to he housing closer to the RF side -- via the RF output jack and also via the screw on Q2 for shunting the coupled noise from the tab

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 06, 2021, 05:57:28 pm
content: Quote from: rfmerrill on March 04, 2021, 09:36:44 pmI was trying to work out how to simulate the "hot" impedance since the obvious series solution would require a non-inductive 12 ohm power resistor: I did some calculations and you can get close by putting 330 nanohenry in parallel with 50 ohm. The impedance at 13.56MHz ends up being (12 + 21.36j) ohmEdit: adding another 33 nH in series with that brings it even closer, 12.01 + 24.17j

author: t0m
author_rank: Regular Contributor
timestamp: March 06, 2021, 09:36:30 pm
content: Quote from: t0m on March 06, 2021, 03:18:22 pmHe agrees with you about that... according to google translate:Quote

author: t0m
author_rank: Regular Contributor
timestamp: March 07, 2021, 07:33:50 am
content: Here are the Diptrace files I am using at the moment. I haven't done much except change the format to 100x62 mm and add 100 or so vias. A terminal block on the left hand side accepts (up to) +40V/0V/+10V. The schematic has less than 300 pins so it can be edited with the freeware version. I haven't had much joy exporting Eagle files from Diptrace but it is possible in theory.

author: quadtech
author_rank: Regular Contributor
timestamp: March 07, 2021, 05:27:51 pm
content: I have the L2 routed a bit differently in mine.I was also thinking to move the crystal and the 22pf caps to the top ofthe STM (parallel to P1), to make the routingsimilar to the workaround Sergey had in the blog, but didn't get round to it.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 07, 2021, 05:28:48 pm
content: I've come up on a bit of an annoyance: My load simulation circuits needs to be either directly connected or calibrated for even a short coax lead, because even the 6-inch (15 cm) coax I was using causes significantly different readings on my nanoVNA :/I highly suspect that the whole system is intended for 50 ohm transmission lines despite using what is traditionally a 75 ohm connector. I can't say that for sure though. Using a 50 ohm coax gives me different readings too however.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 07, 2021, 05:30:56 pm
content: In my second unit rather than populating R42 I put a 2512 SMD resistor on the R45 footprint. I am not sure why sergey went with populating the further-away resistor instead of the closer one. I think putting the resistor on the top layer with shorter routing to gate and source might help something, but I don't know.

author: t0m
author_rank: Regular Contributor
timestamp: March 07, 2021, 06:46:18 pm
content: Some comments on my layout: SergeyMax designed his board to only have SMD parts on one side. I loosened up that restriction for a couple of parts like C4 (to simplify the BOM) and L2 (which is very tall for a surface mount part). I'm surprised he didn't use any vias at all, but I went overboard on them and I'm sure I could have used way fewer.

author: t0m
author_rank: Regular Contributor
timestamp: March 07, 2021, 10:34:44 pm
content: I'm thinking of getting rid of the LED and using the spare pin on the microcontroller to measure the feedback voltage. The idea would be: U2 pin 5 -> inverting op amp as voltage attenuator -> ADC. While I'm at it I could put op amp buffers on the inputs to pins PA0 and PA1. Mamalala used a TLV2374 and AFAICT that should be a reasonable choice here too.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 08, 2021, 07:43:37 am
content: Ugh, I really wish I could find a way to limit inrush current that doesn't cause issues. I've tried three different spec NTCs and they all cause the display to glitch out under load.The power supply I'm using doesn't care, I just hate the rather prominent arc when making the power connection on the bench.

author: t0m
author_rank: Regular Contributor
timestamp: March 08, 2021, 05:40:45 pm
content: Could you simply put a fixed resistance in series with C6? Or try a smaller cap?

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 08, 2021, 05:56:02 pm
content: Quote from: t0m on March 08, 2021, 05:40:45 pmI think a fixed resistance would have the same problem as the NTC if not worse. I think the issue is that the NTC isn't staying hot enough to keep its low resistance. Maybe if I moved R17 and thermally bonded it to the NTC, but that seems like an ugly hack.I think a better solution would be an instantaneous current limiter, or a resistance that is switched out once the cap is partially charged. If I could perfectly limit the current to 4A the cap would charge in a few milliseconds.Coming from the complex digital world I always find myself wanting for robust power management where I could do things like switch out the inrush limiting at a certain point in the sequence.

author: t0m
author_rank: Regular Contributor
timestamp: March 08, 2021, 08:10:37 pm
content: Interesting. I would have thought that a bigger problem was that the NTC stayed hot during momentary power glitches and failed to limit the subsequent rush of current as the cap recharged.8 ohms sounds about right to limit the current to 4A on start up. Perhaps put the resistance in parallel with a relay that opens after a delay of a couple of milliseconds? Or switch the shunt path with a low RDS MOSFET?

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 08, 2021, 08:38:16 pm
content: Quote from: t0m on March 08, 2021, 08:10:37 pmThat would mean I'd see worse behavior with the NTC bypassed. The problem is that I picked an NTC rated for the maximum draw of the soldering station (~2-3A) but the draw with the iron in the stand is significantly less, so the NTC will cool down and increase in resistance. Then when the current increases sharply when suddenly applying a heavy load, the NTC does not heat up quickly enough for its resistance to drop, and the capacitor is not big enough to maintain the minimum input voltage which could drop down into the 20s.

author: t0m
author_rank: Regular Contributor
timestamp: March 09, 2021, 03:29:32 am
content: I don't understand. What you're describing sounds to me as if the NTC is limiting the current draw. Isn't that what it is supposed to do?If the problem is a glitchy display, could it be that the metal bezel of the LCD is grounded to the case?

author: Dave8266
author_rank: Contributor
timestamp: March 09, 2021, 04:02:56 am
content: Quote from: rfmerrill on October 10, 2020, 01:16:31 amPart of it may be because graphic OLED displays (and the library software to drive them) are so widely available now.Even if you never draw shapes or graphics, the graphic display is still a superior text output device, as it gives you so much flexibility for font styles and sizes, single-pixel positioning, variable character widths (non-monospace fonts), etc. And they are basically at parity for cost at consumer retailers.

author: quadtech
author_rank: Regular Contributor
timestamp: March 09, 2021, 05:08:44 am
content: @rfmerril - I am thinking if it helps to have smoothen the output voltage change curve of theTPS54560 - maybe increase C24?@t0m - in your PCB, you moved the MOSFET driver to the middle of the board - isn't it preferable tohave it closer to the mosfet as in Sergey's design?

author: t0m
author_rank: Regular Contributor
timestamp: March 09, 2021, 05:58:00 am
content: Quote from: quadtech on March 09, 2021, 05:08:44 amYes -- good catch!

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 09, 2021, 06:12:02 pm
content: Quote from: t0m on March 09, 2021, 03:29:32 amYes, the NTC is behaving as designed. I am saying it doesn't achieve what I want.With the NTC I most recently tried, I measure a resistance of ~6-6.5 ohm when it's cold.The station draws very roughly 200mA at idle. If I connect the NTC across the output of my bench power supply and set it to 200 mA, after a few moments the voltage drop is 0.74V, so it has dropped to 3.7 ohms. It took about 30 seconds to reach that resistance.If I then suddenly increase the supply current to 2.2A, the voltage drop across the NTC spikes to >3V, drops to 1V within 1 second, and after a few seconds it reaches 0.74V and seems mostly settled there, about 330 mohm.What I'm demonstrating here is that the NTC is not an instantaneous current limiter--its resistance reacts very slowly to the current averaged over a fairly long period of time (seconds). When you pick the iron up out of the stand and the system's draw jumps from 200mA to 2.0A, the input voltage presented to all of the regulators on the board will drop suddenly by ~6-7V. The regulators then have to draw *even more* current to drive the same output power, and there will be some momentary instability over the fairly long timescale while the NTC warms up and its resistance decreases.I think NTCs are just ill-suited for what I'm trying.QuoteI was seeing display freezes and blanks very frequently until I bypassed the NTC. Now I am only seeing uC resets much less frequently, so I think I removed at least one component of the problem.Another possible issue is that the cheap DC-DC converter module might just not have enough input capacitance. I wonder if placing the NTC only in series with the RF supply and connecting the 10V converter's main input directly to power might help? Again though, not a big deal as the inrush limiting seems largely unnecessary.Quote from: t0m on March 08, 2021, 08:10:37 pmThis isn't actually the goal of the inrush limiting. I only want to limit the initial large current spike when the cap charges up from zero. The current waveform after that should be much smoother due to the capacitor. If the supply voltage suddenly jumps that's a problem with the power supply and I don't intend to protect it from itself.

author: t0m
author_rank: Regular Contributor
timestamp: March 09, 2021, 09:02:53 pm
content: Quote from: rfmerrill on March 09, 2021, 06:12:02 pmThat makes sense, and is why I initially suggested a resistor in series with the cap.I found your experiments with the NTC quite informative. It seems their use case is really fairly specific.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 09, 2021, 10:17:17 pm
content: Quote from: t0m on March 09, 2021, 09:02:53 pmI usually see them in use on mains AC inputs, where the voltage is much higher and current much lower, so they can be rated for a much lower max current and the resistance isn't as big of a deal. The voltage drop produced by the NTC will be on the order of normal variation in mains supply that you'd expect anyway, and at 120VAC the potential inrush current is really high: an NTC with a cold resistance of 8 ohms could cut the immediate current spike from 50-200A to <15A, which may be way more than what the system will draw normally, but brings it below the 15A rating of household wiring.NTCs' main advantage is simplicity and cost--it's a single component and they're relatively cheap. It has other downsides like not handling quick power cycles.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 10, 2021, 10:08:46 pm
content: Thanks t0m for the inspiration: I discovered that the EastRising/BuyDisplay OLED ties its screw holes to its internal ground. Neither the winstar OLED nor the adafruit LCD do this. Bleh!Edit: it's easy to fix, the screw holes are only attached to the ground plane on the back layer and the thermal reliefs are really easy to cut

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 11, 2021, 12:01:51 am
content: I got rid of most of the glitchiness on unit #2. Things I did:Ditch the NTCuse a shorter and thicker gauge wire for the ground from the output jack (I'm just using two separate wires because I mounted the jack right by the footprint on the boardCut the thermal reliefs on the display module, confirm that ground is not shorted to housing when it's installedMod in the connection between tip detect and the enable on the main supplySwitch the main power transistor back to the "good" silpad from the cheap amazon one (which I used briefly because I couldn't find the good ones)Change the screw grounding at Q2: For this unit I initially tried a fairly thin wire directly soldered to the source pin of the transistor. I switched back to how I did it on the first unit which is to run a thicker wire from the ground leg of D19, but I kept the small wire and just connected it to the thick wire across the shortest distance I couldI still had significant glitchiness after step 4, but some of the steps 1-4 didchangethe behavior.After step 6 I now have no display garbling, no freezes or blanks, and only a rare clean reset when I pull the iron tip out under load.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 11, 2021, 12:35:24 am
content: Since I'm looking at it now, I thought I'd give this info:2x of the BuyDisplay displays (PN ER-OLEDM1602-4Y-4BIT6800) cost US $31.02 plus $25.97 shipping to California and they arrived 7 days after I placed the order3x of the Winstar displays (PN WEH001602AGPP5N00001) cost US $9.79 each but with$59shipping to California (and the shipping is the same for 1 unit) and arrived 11 days after I placed the order.The Winstar displays seem clearly superior, but depending on your situation the BuyDisplay/EastRising one may be easier/cheaper. I bought those directly from buydisplay.com and the Winstar ones from Unisystem

author: t0m
author_rank: Regular Contributor
timestamp: March 11, 2021, 09:40:58 am
content: That is one hell of a shipping charge. Holy crap.Mind you, I'm a total cheapskate. My build is going inside a box that I got for $1 in thelocal junk shop.

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 11, 2021, 06:34:55 pm
content: Quote from: t0m on March 11, 2021, 09:40:58 amYeah as far as I can tell they're the only English-language website where you can order them shipped to a US address, and the only shipping option is FedEx. I'm sure if you're ordering dozens it ends up being totally reasonable...

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 12, 2021, 05:39:54 am
content: Confirming if anyone is curious: the station absolutely freaks out if you don't have the ground wire connected. I wonder if the issues I'm seeing might just be too high impedance to mains earth?

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 25, 2021, 06:24:39 pm
content: Unit 2 is struggling with microcontroller resets and false tip detects now. I wonder if it's "broken in" or if the colder weather is affecting it (like seemed to be the case with the first unit). This is actually somewhat good news as it means that these problems were not the result of the ferrite cores/inductors I was using.I kind of want to try tracing back whatever is causing the microcontroller resets, but there are a few obstacles:The bad behavior tends to happen much less if the housing is open. I don't understand RF enough to explain why, but I can only assume the cause is either due to signal taking a different return path (through the lid of the housing) or the radiated noise bouncing around the housing instead of escaping.The pins I would want to probe, near the microcontroller, don't have a close enough ground to avoid just picking up the radiated noise on the scope probe.That being said, I do have a few hypotheses:The low-pass filter in the tip detect circuit is located really close to the programming header. About 10-50 mA AC current flows through C76 and that might be enough to make the ground plane nearby on the bottom quite noisy. The ground return path for that current passes very close to the reset pin on the programming headerThe 13.56MHz reference signal coming from the microcontroller passes right past the reset pin as well as directly under Q3

author: quadtech
author_rank: Regular Contributor
timestamp: March 26, 2021, 03:22:43 am
content: Perhaps you could try an external 13.56MHz oscillator mounted close to the MOSFET driver IC,and change the firmware to generate an Output Enable signal to turn on the oscillator -https://www.digikey.com/en/products/detail/sitime/SIT8008BC-13-33S-13-560000G/11473743

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 26, 2021, 06:04:19 am
content: It was the goddamn display again.Oh my god I spent hours trying *everything*. Moving grounds around, even added a second DC-DC module to power 5V directly off the inputThen I disconnect the display ground and it works.I connect the display ground to one of its screws and it works even betterwhat the fuck

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 26, 2021, 06:11:15 am
content: So it turns out sometimes you get the best result if the display is grounded *only* to the housing and *not* to the mainboard?!?!

author: rfmerrill
author_rank: Regular Contributor
timestamp: March 26, 2021, 07:05:12 am
content: OK, Unit #2 is now working perfectly after making the following changes from its previous fairly usable state:F connector connected to the board with a short length of RG174F connector wrapped in copper tape on the inside, soldered to coax shield5V linear regulator replaced with second buck converter module, connected as close to power input as possible47nF C0G cap from microcontroller reset pin to ground (same ground as the crystal load capacitorsBoth buck converter modules grounded only on the output side--5V grounded near D13 footprint, 10V grounded near C36C76 relocated to top side of board--connected between L12 lead and top ground planeDisplay grounded by one of its screws on the side closer to the RF sectionResets and freezes on tip removal or heavy load appear to be totally gone... fingers crossed.

author: rfmerrill
author_rank: Regular Contributor
timestamp: September 22, 2021, 01:01:06 pm
content: In case anyone is following this thread and is curious--I've been using one of the units basically exclusively since March, and it hasn't failed me yet. The occasional resets and display blanking when you pull out a tip has not been solved, but it also doesn't really bother me.I have had a few tip cartridges fail (lose their inductance/base station won't drive them) but I can't tell if it's a higher than normal failure rate. I have been using it pretty heavily.

author: WaveletSea
author_rank: Newbie
timestamp: September 25, 2021, 03:06:07 am
content: From minute15:08to min16:45the host of the video shows you the spots where Sergey has done it's mistakes & also how to fix them all.Good luck.

author: rfmerrill
author_rank: Regular Contributor
timestamp: November 02, 2021, 08:32:45 pm
content: Oh... it would be very nice if there were a translation. I don't speak russian xDIt might be simplest to just make a daughterboard with the microcontroller circuit on it with better layout for the microcontroller problems.Also his heatsink is sideways... might not matter, but it's always best to have fins oriented vertically.