Homemade IGBT MIG welder with ATX PSU parts. [blown yet again][blown count: 5] - Page 1
Forum section: electronics / projects

author: Refrigerator
author_rank: Super Contributor
timestamp: June 21, 2019, 09:04:11 pm
content: For a while now i've been thinking of building a welder. I've recently bought a rusty old car to restore, which makes it a perfect opportunity to actually make the welder.That and the fact that i spent all of my spare money on the car and i couldn't afford a decent welder even if i wanted.MIG welders are most well suited for car body repairs so i'll be making that.For the brains i'll be using an ATX power supply with the primary BJT push-pull circuit replaced with an IGBT push-pull circuit, including a new transformer. MIG welders work at constant voltage so the ATX PSU controller will need minimal modifications for my purpose.Basically what i'm making is an ATX PSU on steroids.An ATX PC case will be used as the case for the welder, to which a secondary ATX PSU will be added, this PSU will provide power to the CO2 solenoid and wire feed motor.With the original transformer no longer in place the welder controller will no longer have voltage rails such as -12 and -5, without which it will shut down because undervoltage, this will also be taken care of by the secondary ATX PSU.The green enable wire will be connected to the trigger, this way there won't be any voltage unless it's time to weld.Just so happened that i recently replaced the old LPG injectors in my car so i will use the old ones to control the CO2, there being 4 solenoids in total means that i can have controlled gas for stuff like purge or whatever else.Biggest problem was sourcing the diodes, i wanted something that could take the current and was fast (because SMPS) and i just so happened to spot these 150EBU04 which should work just fine.Since the cheap ones on ebay are probably chinese knock offs i'll be doubling them up for redundancy (same with IGBT's), so i went to order 4 of them but i forgot to type 4 in the quantity so only one was ordered, no problem i'll order the other 3 from a different seller with a similar listing (same pictures, same diode) to see which one ships quicker. ( i do that often )Big mistake, the one i ordered was the right part but the other three came in this dumb package (included pics) that i'll have to make work somehow. I scraped one with a knife to see if it's actually copper (it was) and did a quick forward voltage test, which was about 0,55V at 6A if i remember right, so not terrible.If they blow so be it i won't know until i try.Any thoughts on this? Some advice perhaps?

author: strawberry
author_rank: Super Contributor
timestamp: June 21, 2019, 09:52:46 pm
content: overcurrent protention circuit, current sense transformer, prefered current mode controller, large enough heatsink...BJT ATX current sense circuitry relay on base current variation and IGBT wont work in such circuitDiodes in MIG welder have to hold high peak currents.I used 150EBU04 for transformer MIG welder repair and one out of four blewup. I think these were fake??your diodes could be fake as package dimentions does not match IR datasheet spec.Mount it on heatsink and monitor junction temperature rise .

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 21, 2019, 10:08:08 pm
content: Whats your transformer design look like so far? Any idea of operating frequency?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 21, 2019, 10:18:46 pm
content: FYI... you can't just slap things together and expect them to work.The ATX supply controller isn't a terrible starting point, but you need to provideallthe interfaces required:1. Auxiliary supply2. Voltage sense/feedback3. Current fault (or sense)4. Gate drive1. Supply is usually from the motherboard 5V standby supply, alongside the main converter. Usually a smaller transformer and a single-transistor driver (either an old fashioned 1T blocking oscillator circuit, or the more modern equivalent with an offline regulator IC which may be transistor-sized, or DIP or SOIC).To power the gate drive, you will need a bigger supply. Standby is typically 5V 1A or something like that; expect to need 12V or more at a few 100mA.2. This should be okay, wired in the same way (a resistor from the output(s) to the feedback node, with common ground tied to the output). But beware of voltage and current surges even over short lengths of that ground -- use star-grounding and lots of filtering, TVS protection, and shielding preferably, to keep the controller comfortable.3. This is your first complication. If the original circuit has a current fault (the usual case), it won't be very useful for you as such. It's usually a latching function, so as soon as you spark up (or maybe during one of the myriad sparks that makes up a running bead -- GMAW draws intermittent load current), it just stops.This should be changed to a current limiting function, so that if current rises over the limit, PWM throttles back, maintaining the arc without destroying the power supply.Bypassing or omitting this function will lead to destruction. You may still be able to run some beads, but the bucketful of burnt transistors and other components won't be very economical.4. This is your second complication. You can't simply replace BJTs with IGBTs and expect anything to happen at all, let alone survive more than one switching cycle without exploding.So the original driver circuit and transformer is useless to you. Well, you could perhaps rewind the transformer as a GDT (gate drive transformer), but you still need something to drive it in the way IGBTs expect to be driven.You will also be using quite large IGBTs (or MOSFETs), to deliver the required kilowatts. These have continually improved over the years, but you're still looking at 100-200nC of gate charge required.What's a nC? A nanocoulomb is a unit of charge. Charge is current * time. It takes current to change the gate voltage -- the gate has a capacitive characteristic, so gate voltage and charge are proportional -- and to move the gate in some amount of time, requires current. For a switching frequency of 100kHz or so, you need switching under 200ns or so.So we calculate the required peak gate current as:Qg (gate charge) / t_r (rise time) = IpkActually peak current may be a bit higher than that, because waveforms, but it's in the right ballpark. An ampere peak, in fractional microseconds, also requires low inductance.What is inductance? Inductance is the voltage drop associated with a change in current:V = L * dI/dtSo if we're driving an amp of current into the gate, in much less than 200ns (because that current needs to rise, and then fall), and we can only afford to drop say 5V in the process, we need much less than L = (5V) * (200ns) / (1A) = 1uH.This sets the maximum leakage inductance for the GDT.So we already know we need a GDT of some level of performance.5. The last, and major, concern you will have, is constructing the new inverter and rectifier entirely from scratch. There isn't anything you can reuse from the original circuit -- the BJTs are too small, the rectifier and transformer are too small, the capacitors... You will be using larger components, so even if you copy the layout and scale it for the new components, the layout will be larger.Stray inductance limits us again here. When one transistor turns off, all the current it was carrying, has to drop to zero, and the load current has to find another route (usually a clamp diode). This change in current, must not produce an excessive voltage drop -- else transistosplosions -- and so we again can find a maximum inductance.Say the inverter is supplied by 360VDC and delivers 20A peak, and turns off in 200ns; to use 600V transistors, we only have 240V of headroom, and so L < (240V) * (200ns) / (20A) = 2.4uH. (Probably, a time of ~50ns would be more representative -- transistors sharpen risetimes, so if you're getting 200ns at the gate, the collector will be less. We also don't want to run with zero headroom, so a voltage excess of 50 or 100V would be much more comfortable. Which gives a much safer limit of L < 0.3uH.)What gives inductance? Length. Simple as that. The inductance of free space is 1.257 uH/m.Because of geometry effects, the inductance/length of a typical wire is lower, maybe 0.3 to 0.6 uH/m. Circuits also contain loops -- there's no meaningful notion of the inductance of a free wire -- so we need to count the full loop path length.For an inverter, the loop length starts at one transistor, through the catch diode (again, usually the opposing inverter transistor -- note you'll need co-pack IGBTs here), through the supply bypass/filter cap, back to the one transistor.So this is why even merely scaling up the existing layout, isn't a sure thing. You're scaling up all the lengths, so Lstray goes up proportionally. (Incidentally, a proportional scaling keeps the geometric factors constant, so this is actually a very accurate representation. Whereas using different sized traces or spacings would change the geometric factors some.)You likely won't succeed by bolting transistors to a heatsink and wiring them with hookup wire -- even with short leads, you'll incur loop lengths of fractional meters, and 0.1's of uH.Example from an early project I did:Two pairs of IGBTs, half bridge configuration (two in parallel, each side). The two copper plates are +/- DC bus (the three red caps, and two blue ones, bypass them together). The little plate in the middle is the output node (leading to the bank of many red caps, and the load, out of frame). The stray inductance of this configuration was around 20 or 30nH (0.02-0.03uH), pretty good.Another good method is to use a 4-layer board, with solid pours: DC+ on top, DC- on bottom, and OUT in the middle. This can get Lstray in the single digits nH range. Not an extreme you need to go to, but it's pretty cheap (< $200) considering how effective it is.Then there's the transformer. You can't use the ATX transformer, or core. You might be able to use a dozen of them wired together, if you happen to have identical supplies to gut. More likely you will be winding one new? Big cores like ETD59 and 87+ mm toroids are easily purchased, so that's fine. The wire needs some attention. You can't use solid wire at this frequency -- it'll get way too hot, skin effect. Preferably you'd use Litz cable, assembled from a gazillion strands of 30 AWG or finer. You can also use foil/tape/strip/sheet, but how well depends on the ratio (if you have lots of turns stacked up, foil will be worse than round wire; it has to be interleaved, always primary facing secondary, never primary facing primary, to work out).Here again, we have length -- winding wire length -- and so, stray inductance, also called leakage inductance. This matters to the rectifier, depending on how you're arranging it.If you opt for a FWCT configuration, then the leakage between ends of the secondary impacts the peak voltage the diodes will see. That is, when one diode turns off, its current flow has to divert to the other diode, and this can only be done through the length of the winding between them.You can easily exceed the 400V limit of those diodes, even for a mere 50V nominal output, with a careless transformer winding. So it's very important.You have some savings with a FWB configuration: there's always a pair of diodes to grab either polarity of flyback voltage. Downsides: a. you're burning twice the voltage drop in the diodes -- a big deal at 100A+; b. the diodes are clamping into a filter choke, which doesn't really clamp at all, it's just another inductance, right?... So for this reason, you may need a large TVS (or some means of hard voltage clamping) at the rectifier output, or across the diodes.Other clamping means would include an RCD clamp snubber -- a catch diode dumps the flyback into a beefy capacitor, and a resistor bleeds away the accumulated charge. Note that diode-capacitor loop needs to be very short, otherwise the series inductance will defeat its purpose.If you have enough voltage overhead, an MOV can be used. For a 50V output and FWCT configuration, the nominal reverse voltage per diode is somewhat over 100V. A 100VDC-nominal MOV could be used, which will have a peak breakdown voltage around 300V (MOVs are not actually very good at clamping, they're just economical), and so 400V+ diodes could be used safely.And obviously, all the output connections need to be beefy -- you'd probably need a custom fab PCB to do this (heavy copper). A regular proto fab PCB could be used with a lot of soldered-on strap to increase ampacity. Takes a bit of faffing to do. Or it can be constructed from cut and formed sheet, insulated with plastic sheet and clamped with plastic hardware.6. Other things.If you're confident in modifying a circuit so extensively, why settle for the -12 and -5V undervoltage circuits? Why not bypass or remove them? Or reuse the comparator for something else, perhaps? (I...dunno what; some dumb timers or something?..)What filtering network will you choose? A traditional choke-input, then capacitor filter is what the controller is expecting. The capacitor may work against your load, however (the arc has a negative resistance and tends to oscillate with capacitors and cable inductance). You could use just a choke, but the controller will have to be adjusted for that; it also won't be able to regulate the voltage very well, because that's what a choke does, it allows voltage to vary while resisting changes in current. In particular, the flyback voltage when the arc breaks, will destroy pretty much anything directly connected to the output node.You could put some capacitance on there, or some R+C damping, to provide some filtering and dampening of ripple and flyback, but this will dissipate quite a lot of heat (even if it's handling 10% of full output power, that's hundreds of watts continuous!).Probably a modest amount of electrolytic capacitance will behave anyway, but be careful here too, because those caps will heat up quickly if you are expecting a lot of ripple current from them. Again, if you're running a ~10% ripple fraction, that's still hundreds of VA, and dozens of watts the capacitors will dissipate -- you'll need many more capacitors than are found in an ATX supply to handle that.So, all in all, this is an ambitious project -- it is not an impossible one, but it will lead to the discovery of much new arcane knowledge, and it will take a long time to complete (years).If you're really looking for a learning experience with a shorter payoff, consider something smaller in scale. Or, if you can't push yourself to concentrate on a single project foryears-- I certainly can't, I still have some of that above-referenced project floating around, over a decade later...Or consider something closer to completion already -- pick up a shitbox inverter welder, and see how it's made. Tweak it for better performance (water cooling for high duty cycle?), or add control features. Once you have more experience, you'll be able to take on a project like this in much less time (maybe months?). Maybe not a welder from scratch, since you'll already have one then, but maybe something related like an induction heater, or play with pulsed high voltage (Tesla coils) for not much practical purpose but much amusement instead.Good luck!Tim

author: Refrigerator
author_rank: Super Contributor
timestamp: June 22, 2019, 10:12:54 am
content: Quote from: T3sl4co1l on June 21, 2019, 10:18:46 pmI plan to have this one finished in a month.The IGBT's i'll be using are H20R1203, 20A, 1200V, 211nC gate charge.Peak gate current turns out just over 1A, which means the wimpy TO-92 drive transistors (basically in all cheapo ATX PSU's) will be replaced for something better.Why do i plan on leaving -12 and -5 rails in? Well not anymore i don't, reason being the previous donor PSU had a fancy controller with internal voltage sense comparators.I've decided to use the good old TL494, 200mA on output means i can use some fat transistors to drive the GDT, and the external comparator means much less hassle.A snubber on the secondary of my transformer should keep the diodes from breaking down.Diodes and IGBT's will be connected point-to-point with thick wire, no need for a PCB this way, the only PCB i'll be making will be for the gate drive circuit for each IGBT.For my transformer core i'll be using the core from the back of a big old CRT, the core is pretty big and i've seen someone make a working welder with one.CRT TV's also happen so have a big coil of wire around the CRT (the demagnetization coil or whatever it's called), i'll use it as litz wire on my secondary.Also any advice on removing the enamel off a wire, but without burning because i've tried it and it doesn't work one bit.I think i've seen someone use a wire wheel so i'll give that a try.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 22, 2019, 11:35:24 am
content: Quote from: Refrigerator on June 22, 2019, 10:12:54 amIgnorance is bliss, and all that.Quote from: Refrigerator on June 22, 2019, 10:12:54 amSo... what is the original ATX PSU being used for, then?Quote from: Refrigerator on June 22, 2019, 10:12:54 am"A little dab'll do ya," as the old commercial jingle for [I can't remember what] went.Quote from: Refrigerator on June 22, 2019, 10:12:54 amFinally a question that isn't completely mental... There are basically 3 ways to remove enamel insulation semi-practical for the home-gamer: 1) abrasion, which works for any type of wire; 2) dipping in a pot of alkali metal salts dissolved in molten sodium or potassium hydroxide (don't get any in your eyes!); 3) dipping in a pot of molten solder.***This reply might seem a bit flippant (because it is), but the fact is this is a terrible idea with almost zero economic justification (as Tim already pointed out in a reply he probably regrets spending so much time writing). The two-switch forward is commonly used in ATX PSUs and is a good choice of topology for either a CC (TIG, stick) or CV (MIG) welder, but you will need a core area in the 6+ cm² range, which is at least 3x bigger than any core you'll find in a COTS power supply. Then there is the matter of the output choke, which typically requires a core area of around half that of the transformer. You can make bigger cores out of smaller ones IF they have rectangular cross sections (basically, some U, E and toroidal shapes), but a flyback transformer from an old TV? Fuhgetaboutit.If I were to do this - and believe me, I wouldn't - it would probably take me a couple of months, minimum, and cost at least $500, and that is with the benefit of several square meters worth of shelving containing SMPS parts and a lab decked out specifically for the task.If the goal is to learn something, well, you'll certainly do a lot of that, but it has been my experience - and probably not too dissimilar to all but the most genius among us - that learning proceeds far more efficiently when you take on incrementally more difficult projects, rather than leaping straight into a monumental one.

author: strawberry
author_rank: Super Contributor
timestamp: June 22, 2019, 11:58:19 am
content: for $500 worth of parts you could start to compete with miller

author: Siwastaja
author_rank: Super Contributor
timestamp: June 22, 2019, 12:12:52 pm
content: I remove the enamel by burning the wire end with the open flame of a cigarette lighter for a few seconds, then carefully wipe the burned enamel off with a fine (500-2000 grit) sand paper. The copper does get a little more brittle due to the heat, but this hasn't been a problem for me.I have learned a lot by taking monumentally complex projects, but OTOH I have never talked about them in the forums or asked for help because everybody would have just told me not to do it. If you have to start with a lot of questions, it may not work out that well. An iterative process where you have a good idea where to start and then develop questions "why this happens?" tends to work better.

author: Refrigerator
author_rank: Super Contributor
timestamp: June 22, 2019, 12:17:28 pm
content: Quote from: MagicSmoker on June 22, 2019, 11:35:24 amThe flyback core was my initial idea, i've seen people use four smaller cores to make one larger core, but what i decided to use isn't that.As mentioned it was from the back of a CRT, and as far as i'm aware there are no TV's where the flyback transformer is mounted to the CRT.I'll include a pic below, also a pic of the coil i was talking about (hand for reference), it's about 7.5 mm thick, probably around 7 mm without the tape, which should be plenty.Edit: finally remembered, the coil is called a degaussing coil.

author: strawberry
author_rank: Super Contributor
timestamp: June 22, 2019, 01:53:38 pm
content: complex projects like this ?https://www.eevblog.com/forum/projects/oscilloscope-pong-for-1-or-2-players/500/

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 22, 2019, 01:55:14 pm
content: Quote from: strawberry on June 22, 2019, 11:58:19 amNot when you are buying those parts in quantities of 1-10 from the usual distributors. Also, $500 worth of parts does not get you a Millerdesign... And I know at least anecdotally of what I speak, as I have a cute little Miller inverter TIG/stick welder that is almost 20 years old (!) and still working great. It is a real work of art inside and uses a phase-shifted full bridge topology; something that was still very much considered "experimental" at the time.EDIT - apparently Miller didn't design this model (MaxStar 140 STR), either... so not even buying a Miller will get you a Miller design... Oy vey!Quote from: Refrigerator on June 22, 2019, 12:17:28 pmOh dear...

author: Refrigerator
author_rank: Super Contributor
timestamp: June 22, 2019, 02:27:04 pm
content: Quote from: MagicSmoker on June 22, 2019, 01:55:14 pmIt will work, i promise

author: strawberry
author_rank: Super Contributor
timestamp: June 22, 2019, 02:42:48 pm
content: my asumtion is based on my calculations for 230V 3 phase inverter. 300$ is enough to make decent welding inverterinteresting question is, how are you going to make wire feed mechanism?

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 22, 2019, 03:39:47 pm
content: I think its possible to cobble together a working prototype of this in a month if you approach it seriously, have a basic understanding of the design issues (especially switching transients destroying things), and can get the magnetics close enough for a 30 second weld.Can we organize the design plan a bit?Topology: Is this a 115VAC rectified to DC switched into xfrmr for 24V out or something like that?Magnetics (transformer): What is the proposed design? Core and number of turns of what material? How hot will the core and windings get? What is the operating frequency?Switches: What IGBT? Why IGBT? What temperature will they get to?Heat sinking: How much power needs to be dissipated and how will it be done?Switch control: Can't use the existing IGBT control circuit?Current control: So does this need some kind of current feedback servo loop or not? And how will that be done?Switching transients: How will these be snubbed?Full bridge: What diodes and how hot will they get? What sort of capacitance and inductance will be used to smooth the DC for the torch?Bussing: How will high current be routed as far as bus bars while keeping things very short and balanced?Its possible to use a completely unregulated voltage source like two car batteries to run a mig welder. But that only applies if the voltage source is near the desired welding voltage. Just something to keep in mind.http://readywelder.com

author: Siwastaja
author_rank: Super Contributor
timestamp: June 22, 2019, 04:34:22 pm
content: Quote from: cur8xgo on June 22, 2019, 03:39:47 pmYes I agree, but using a degaussing coil as a core material might not be a good start.Nevertheless, do it! It will be fun.

author: Refrigerator
author_rank: Super Contributor
timestamp: June 22, 2019, 04:49:53 pm
content: Quote from: cur8xgo on June 22, 2019, 03:39:47 pm30 seconds is plenty considering i will be welding car panels and anything more than a couple seconds will warp the panels and ruin my day.Quote from: cur8xgo on June 22, 2019, 03:39:47 pmQuote from: Siwastaja on June 22, 2019, 04:34:22 pmDo you mean the XY deflection core? The degaussing coil does not have a core it's just a very long coil with lots of thin wire, basically litz wire if you cut through it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 22, 2019, 04:58:59 pm
content: The deflection yoke core BTW is some horrible kind of material, not sure what. Seems to have lower permeability, Bsat and higher losses than the usual stuff. Maybe stability of permeability is the tradeoff? Dunno.A stack of four FBT cores isn't bad. There's plenty of room to get all the wire you need in there, and while the core area could be higher, you aren't using that much more wire in exchange.You will want to pay careful attention to how much interleaving is done on the transformer windings. Primary and secondary need to be as close as possible*. Probably you'll need many sections of primary and secondary in parallel, to get the total current rating up, while using fine-ish stranded wire (degaussing coils are probably okay, but you'll need a lot of them to finish the transformer).*And I really meanas possiblehere, not just a wave-of-the-hands "probably good enough". You could wind a thousand twisted pairs and connect them in parallel, and get about the right transformer impedance here (<< 1 Ω!). Anything else you do will have a higher impedance, and therefore more leakage inductance. Foil windings, and multifilar (flat cable, lots of strands in parallel, broadside) are your friends here.Tim

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 22, 2019, 05:15:46 pm
content: IGBT because the high voltage right? Otherwise wouldn't MOSFETs be less loss and easier to drive?I'm interested in exactly how current will be limited to allow sheet metal usage. It might actually be easier to make a welder which welds thicker material than thinner material in this respect.Torch + feed drive systems are available from communist china on aliexpress and ebay FYIThat diode is about 0.8V @ 100A. Wouldnt a single secondary with a full bridge and four of these 0.5V @ 80A (parallell) waste less power?https://www.digikey.com/products/en?keywords=DSSK%2080-0045BCan you elaborate on the SMPS transformer reusage plan?Can you also talk about frequency of operation? I am actually really interested in how you are going to get the transformer right.Can you draw up a basic schematic so we can all talk about it? This is a cool project

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 22, 2019, 05:30:09 pm
content: Mig wire feed system $19https://www.ebay.com/itm/Black-24V-DC-10W-Mig-Welder-Wire-Feed-Motor-Mig-Wire-Motor-Mig-Wire-Feeder/153325626788Mig torch + cable/connector $64 (oops I mean mig "touch" apparently)https://www.ebay.com/itm/MIG-Welding-Gun-Torch-Stinger-Replacement-Miller-169598-250-Amp-M-25-15FT/392266137032

author: Refrigerator
author_rank: Super Contributor
timestamp: June 22, 2019, 05:36:05 pm
content: Quote from: T3sl4co1l on June 22, 2019, 04:58:59 pmDamn, i thought i hit jackpot when i found that core, turns out not. I really wanted to use it because it had a lot of space for windings.Also i counted my FBT cores and i have 5x132mm2, 3x143mm2and 2x176.6mm2cores.Meanwhile a typical 350W ATX PSU core has a 95mm2area.I also wanted to find a picture of the one done with a deflection yoke but couldn't find it, i did, however, find one that used two FBT cores for an inverter stick welder/plasma cutter:https://www.edaboard.com/showthread.php?374338-Homemade-Arc-inverter-welder-And-Plasma-cutter-powerThe FBT cores might be a bit of a squeeze form my welder transformer but i'll try.Quote from: cur8xgo on June 22, 2019, 05:30:09 pmI've already found a MIG torch for 30€ locally (by locally i mean in my country) and i'll use a windshield wiper motor for my wire feed for maximum DIY points.The torch i'll be using:http://irankiai-fb.lt/lt/suvirinimo-irangos-dalys/14708-suvirinimo-zarna.htmlThere are other ones for 30€ as well but this particular one should be easier to hook up without having to order a socket.

author: Refrigerator
author_rank: Super Contributor
timestamp: June 22, 2019, 06:23:54 pm
content: Quote from: cur8xgo on June 22, 2019, 05:15:46 pmDriving them would be pretty much the same because they both have a gate, MOSFETs would have lower losses but i have CPU heatsinks to spare so it won't be too big of a deal to get rid of the heat.Quote from: cur8xgo on June 22, 2019, 05:15:46 pmI could add a timer to the welder for short welds, current will be handled through a current transformer.Quote from: cur8xgo on June 22, 2019, 05:15:46 pmThat's assuming 100% duty cycle (in welding terms) on some thick metal, if i remember right MIG welders work in the 60-ish amp range.Quote from: cur8xgo on June 22, 2019, 05:15:46 pmI'll unwind it to see how the boys at chinatown do it and toss it away.Quote from: cur8xgo on June 22, 2019, 05:15:46 pmI looked online and it seems like 22kHz and 33kHz are are the most common switching frequencies, my board being from a 350W supply i expect the latter.Quote from: cur8xgo on June 22, 2019, 05:15:46 pmNot much to draw except the gate drive, for SMPS schematics you can check out this:http://danyk.cz/s_atx_en.html^^^^excellent source for anyone tinkering with ATX PSUs.I'll also include a my gate drive schematic.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 22, 2019, 07:26:36 pm
content: Okay dumb question....why an inverter? Why not just drive a 60hz transformer and full bridge the output?Or go totally bare bones and just have a transformer in a box like harbor freight does with their 125amp 110v mig welderI guess now I am wondering why AC powered inverters exist at all....is it just because switching faster could reduce transformer size vs 60Hz?

author: Refrigerator
author_rank: Super Contributor
timestamp: June 22, 2019, 07:34:17 pm
content: So far i've unwound two ATX transformers and they're pretty much the same, i only counted turns on primaries and the 5V secondaries since those were the main windings so to say.Transformer consists of two 19T primaries, while 5V secondaries are 4T each sandwiched between the two 19T primaries.Let's say i use four of my smaller 132mm2cores for one 528mm2transformer core, that means it's got about 5.3 times more cross sectional area than the ATX PSU core.So basically my primary will now be 19T/5.3 = 3.58 ~ 4T and secondary (because i need 22V, 22/5=4.4) 4T*4.4/5.3=3.32 ~ 3T.I'm pretty sure those numbers i just came up with are a pretty good recipe for some magic smoke, but that winding ratio should mean about 25% duty cycle.Quote from: cur8xgo on June 22, 2019, 07:26:36 pmI have a german made AC buzzbox and it's so heavy that you need two people to carry it, you can still pick it up by yourself but it's dangerously heavy.Meanwhile inverter welders are small, easy to carry and have all kinds of features.Edit: i could add a full bridge with SCRs with some control circuitry to control the output current and voltage but for above mentioned reasons i won't, also with a CO2cylinder it would be even heavier and more awkward to carry around, not to mention the wire feed and spool, i would be well over 100kg.

author: Refrigerator
author_rank: Super Contributor
timestamp: June 22, 2019, 07:55:23 pm
content: Did some calculations with an online calculator and i think i'll settle for a 10T primary and an 8T secondary.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 22, 2019, 08:23:03 pm
content: Quote from: Refrigerator on June 22, 2019, 07:55:23 pmWhat kind of open circuit voltage do you get with a 10:8 ratio on a 240VAC rectified input?

author: Refrigerator
author_rank: Super Contributor
timestamp: June 22, 2019, 08:50:39 pm
content: Found another degaussing coil laying around and this one is huge, only about 5.5mm thick (5-ish mm without the tape) but boy is it long and according to the link below it should be able to handle well over 50A and since i'll be using two secondaries it should be enough.http://www.jst.fr/doc/jst/pdf/current_rating.pdfAlso i think i'll be doing a modified interleave winding with the primary sandwiched between my secondaries.Quote from: cur8xgo on June 22, 2019, 08:23:03 pmOpen circuit voltage doesn't matter because PWM. Also because of the way SMPS PSU works you have to divide your 230VAC peak voltage by 2.A 10:8 ratio would mean about 17% duty cycle at idle to get 22V.230*1.41 = 324V(peak)324*8/10 = 259V259/2 = 130V22/130 = 17% duty cycle.Actually 17% duty cycle seems pretty low, i hope my peak current doesn't get too high.

author: xavier60
author_rank: Super Contributor
timestamp: June 22, 2019, 08:55:43 pm
content: There happens to be a nice design for a 2 switch IGBT welder in this thread.https://www.eevblog.com/forum/beginners/component-role-in-a-circuit/msg2502276/#msg2502276It uses the UC3845 current mode controller IC which is better suited than the TL494.Quote from: MagicSmoker on June 22, 2019, 01:55:14 pmInteresting, I have been keeping alive a few Castolin/Eutectic PowerMax 2 welders that also have the phase shift topology. Maybe the same as the Miller.https://www.elektroda.pl/rtvforum/topic2554501.htmlhttps://www.mig-welding.co.uk/forum/threads/eutectic-powermax-2-faulty.36400/

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 22, 2019, 09:19:40 pm
content: QuoteThere is an output choke right so thats why?

author: Refrigerator
author_rank: Super Contributor
timestamp: June 22, 2019, 09:36:25 pm
content: Quote from: cur8xgo on June 22, 2019, 09:19:40 pmThat and also some capacitors.Quote from: xavier60 on June 22, 2019, 08:55:43 pmYeah i've found quite a few of those:https://www.hlektronika.gr/forum/showthread.php?t=53608https://www.homemade-circuits.com/smps-welding-inverter-circuit/http://danyk.cz/svar_en.htmlBut i want to make mine my way.

author: strawberry
author_rank: Super Contributor
timestamp: June 22, 2019, 10:54:00 pm
content: How about sinhronous rectifier?modern IGBT can run 50..100kHz not just 20..30kHzI think, bipolar gate drive is better than unipolar

author: xavier60
author_rank: Super Contributor
timestamp: June 23, 2019, 01:55:37 am
content: I suspect that higher frequency makes the output choke unnecessary. I can't find the choke in the design linked earlier.And my old welders don't have a choke. The rectifier output pulses are 100KHz, I think it relies on lead inductance.

author: tautech
author_rank: Super Contributor
timestamp: June 23, 2019, 02:09:41 am
content: Quote from: xavier60 on June 23, 2019, 01:55:37 amChokes in MIGs are more common in single phase plain transformer types however an old 3 phase unit I had didn't have a choke at all.Typically the cleaner the DC the better they weld.

author: james_s
author_rank: Super Contributor
timestamp: June 23, 2019, 03:37:16 am
content: I get the distinct feeling that you have no idea just how difficult of a project this is going to be. Inverters require meticulous attention to detail, especially high powered inverters. One thing just slightly out from perfect and the whole thing goes bang in an instant.If you want a welder just buy one, if you are on a budget you can buy used welders, sometimes for not a whole lot of money.If you want an advanced engineering project then go ahead and try to build one, just realize that you will spend more money on parts and almost certainly get inferior results to the cheapest Chinese welders on the market.Also how experienced are you at welding? MIG is one of the easier ones to learn but it still takes a great deal of practice to get good at it.

author: Refrigerator
author_rank: Super Contributor
timestamp: June 23, 2019, 08:11:11 am
content: Quote from: james_s on June 23, 2019, 03:37:16 amI learned to MIG weld when i was 13, it's basically a hot glue gun for metal.Assuming the welder is correctly set up it takes about 30 minutes to learn MIG.

author: Refrigerator
author_rank: Super Contributor
timestamp: June 23, 2019, 10:06:57 am
content: Quote from: strawberry on June 22, 2019, 10:54:00 pmSynchronous rectification would over-complicate things.

author: DannyTheGhost
author_rank: Contributor
timestamp: June 23, 2019, 10:28:43 am
content: Quote from: Refrigerator on June 23, 2019, 10:06:57 amSometimes you need to complicate your design when you don't want to dissipate over 50 watts from rectifier

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 23, 2019, 11:36:04 am
content: Quote from: xavier60 on June 22, 2019, 08:55:43 pmMy Miller was supposedly made by Fronius, but Fronius lost the contract with Miller soon after because it couldn't keep up with parts demand. I opened mine up to poke around inside even before the warranty was up and then again about a year ago to check the ESR of the elkos inside and everything was still within spec. Then again, I probably don't put more than about 10 hours of use on it any given year...Quote from: xavier60 on June 23, 2019, 01:55:37 amThe "choke" might be the leakage inductance of the transformer, usually by specifying an unusually large gap for a core that really shouldn't have one in the first place (e.g. - forward mode transformers). Note that this definitely falls in the "ugly hack" category, and not something I would personally try to replicate just to save on the cost/volume of a proper choke.

author: Refrigerator
author_rank: Super Contributor
timestamp: June 23, 2019, 01:10:11 pm
content: The transformer is wound, i took care to make sure the primary is properly isolated from the secondary.Although as much as i tried i could only fit 7.5T in the secondary, no big deal.I couldn't find appropriate litz wire for my primary so i went outside, screwed to screws into a board spaced about 1.7m apart and made some. In the end it was quite a squeeze but i think it looks pretty ok-ish.Now i can begin building the primary switching circuit.

author: Refrigerator
author_rank: Super Contributor
timestamp: June 23, 2019, 01:15:32 pm
content: Quote from: DannyTheGhost on June 23, 2019, 10:28:43 amA CPU cooler can handle that no problem.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 23, 2019, 02:53:58 pm
content: Quote from: Refrigerator on June 23, 2019, 01:10:11 pmvery interested to see how that performs? so thats just magnet wire and you twisted it to make litz?how did you decide on litz vs foil?

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 23, 2019, 02:54:32 pm
content: Quote from: Refrigerator on June 23, 2019, 01:15:32 pmYa I dont think you will have an issue getting rid of 50W from TO247

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 23, 2019, 02:55:57 pm
content: Quote from: Refrigerator on June 23, 2019, 08:11:11 amYep even a poor mig welder can get the job done for simple projects....I've taught people who had zero mechanical aptitude and never even knew what welding really was how to FCAW in about 30 minutes..then they practiced on their own for about an hour and were able to put tubing together to make things. Its motivation that is really key

author: strawberry
author_rank: Super Contributor
timestamp: June 23, 2019, 03:32:39 pm
content: Synchronous rectification is not complicate. Same gate drive sigal for rectifier mosfet gates

author: Refrigerator
author_rank: Super Contributor
timestamp: June 23, 2019, 04:44:15 pm
content: Quote from: strawberry on June 23, 2019, 03:32:39 pmI'd have to order new parts and wait for them to arrive = nope.

author: Refrigerator
author_rank: Super Contributor
timestamp: June 23, 2019, 04:46:24 pm
content: Quote from: cur8xgo on June 23, 2019, 02:54:32 pmDon't forget i have two diodes in parallel.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 23, 2019, 05:56:55 pm
content: Quote from: Refrigerator on June 23, 2019, 04:46:24 pmYa even easierI've been doing it at 125A to 150A through TO-247 50% duty cycle (full bridge) for 30 to 60 seconds at a time dozens of times. The heat sinks arent even that big maybe 5-10 sq inches? There is forced air but its about a foot away and is cooling the switches first. They heat sinks get warm but not hot. I have not yet measured junction temperature or calculated the heat sink efficiency to see if its reaonable but it definitely works so far. 0.54V @ 155A btw (per leg of FB)

author: Refrigerator
author_rank: Super Contributor
timestamp: June 23, 2019, 07:30:58 pm
content: I thought up a pretty good way to get the most out of my diodes.Basically since the weird TO-247 diodes came in a package that's meant for electrical isolation i decided to solder the diodes straight to a copper plate and then bolt the plate to a heatsink, which i pulled from a Dell Dimension.This way the diodes will not only dissipate the heat better but they will also pass current through the aluminum heatsink.

author: strawberry
author_rank: Super Contributor
timestamp: June 25, 2019, 01:20:37 pm
content: https://www.edaboard.com/showthread.php?57813-SWITCH-MODE-ARC-INVERTER-WELDER-SCHEMATIC/page2&p=862700&viewfull=1#post862700arc welder with deflection ferrite core

author: Refrigerator
author_rank: Super Contributor
timestamp: June 27, 2019, 03:46:35 pm
content: After some fiddling i have the rectifier diodes sorted and IGBT gate drive sorted as well, only a few finishing touches left.Next will be the GDT, but before that i think i'll remake the TL494 board. Keeping the ATX PSU board as a control board seemed like a good idea at first but i might as well go all in and remake it on some proto-board. The TL494 datasheet is a little confusing at first but i think i have it figured out.I also remembered that i have a Delta fan from a Dell dimension, 1.6A at 12V makes 19.2W so it ought to move some air.Quote from: strawberry on June 25, 2019, 01:20:37 pmExactly what i was looking for, but they mention that the core loses inductance after some time, perhaps it's heat related?

author: strawberry
author_rank: Super Contributor
timestamp: June 27, 2019, 07:41:29 pm
content: MIG/HF TIG 200A inverter3phaseUC2834 current mode controller with built-in gate drivershalf bridge 40A IGBT 0.66mJ@150 1.6V heatsink SK442-100E65 CF297 2 sets in paralel ( dont know if they are good cores)

author: Refrigerator
author_rank: Super Contributor
timestamp: June 27, 2019, 09:25:03 pm
content: Quote from: strawberry on June 27, 2019, 07:41:29 pmWell, according to the links below, i think one core should be enough.http://www.cosmoferrites.com/Downloads/Product/CFR%20Cross%20Reference%20Chart%202011.pdfhttps://www.tdk-electronics.tdk.com/inf/80/db/fer/e_65_32_27.pdf

author: strawberry
author_rank: Super Contributor
timestamp: June 29, 2019, 04:27:27 pm
content: fan 120x120mm 24V 11.3W 3.7m^3/minair speed approx 5m/sSK442-100 0.22K/W @5m/s5kW HB inverterigbt 0.6K/WAlO wafer ~0.3K/W(0.6+0.3)/2+0.22=0.67K/WIGBT loss 0.66mJ @400V 20AIGBT 1.6V 32W@20AIGBT 43W@65kHz43*2+32=118Wtemperature of silicon at room temperature118*0.67+25=104CDiode 0.94V@60A trr25nsDiode 0.9K/WRectifier contains 6pcs in totalRectifier 200ARectifier loss 200*0.94=188WSK442-100 0.22K/W @5m/s0.9/6+0.22=0.37K/Wtemperature of silicon at room temperature188*0.37+25=94.5C

author: Refrigerator
author_rank: Super Contributor
timestamp: June 30, 2019, 06:44:01 pm
content: I've got my drive circuitry nearly finished now, it's a mega-bodge on some proto-board but it does work very well.One problem that i've come into is frequency instability, seems that even small changes in RTmake massive difference in operating frequency.Using a 2n2 CTi get 12 kHz with a 5k1 resistor, 100kHz with a 2k2 resistor and about 50kHz with a 4k7 resistor, i might put a 10 turn trimmer in place of RTto set the frequency more accurately.So far the gate drive transformer was the fiddliest part to make, finding the right core took longer than i'm willing to admit.In the end i settled for a core from a common mode choke from the same ATX PSU.I made it a 1:1 transformer with the same construction as in an ATX PSU, bifilar wound secondary sandwiched between two primaries, 10 turns each.Gate drive signals look super nice, i used the signal on one of the GDT drive transistor bases as a reference to check if my gate drives were correct.

author: xavier60
author_rank: Super Contributor
timestamp: June 30, 2019, 10:10:47 pm
content: Does the 5k1 resistor actually measure 5k1?. It seems to be the odd one out.

author: Refrigerator
author_rank: Super Contributor
timestamp: June 30, 2019, 10:42:37 pm
content: Quote from: xavier60 on June 30, 2019, 10:10:47 pmI've already tossed it in the bin so i can't measure it, but i replaced RTwith a 2k 20 turn trimmer and a 3k3 in series and the lowest frequency i can go to is 55kHz, so the 5k1 resistor might not have been 5k1 afterall.Checked all of the gates of my IGBTs to make sure they're all being driven properly and everything seems fine at 55kHz.The inverter part is basically finished now, i'll just have to put it all in a box tomorrow.Next will be the wire feed and speed. I wanted to make a closed loop wire feed mechanism to keep the speed constant but i think i'll do it the easy way with a buck converter.

author: strawberry
author_rank: Super Contributor
timestamp: July 01, 2019, 06:23:55 am
content: you should set dead time for IGBT. TL494/KA7500 is determined by DT pin voltage

author: xavier60
author_rank: Super Contributor
timestamp: July 01, 2019, 10:44:31 am
content: Quote from: strawberry on July 01, 2019, 06:23:55 amThe maximum duty cycle would be something less than 50% with only one output used.What bothers me about the TL494 is not being certain that double pulse suppression has been added.

author: xavier60
author_rank: Super Contributor
timestamp: July 01, 2019, 11:23:47 am
content: Gate drive for a one or two switch forward inverter is complicated by the changing symmetry of the drive waveform with change of duty cycle.I remember with one welder that I repaid, the drive transformer had a large inductance and was driven by a single MOSFET in flyback mode. The flyback voltage was clamped by Zener diode across the primary.I wonder if the same drive that's normally used for half-bridge forward inverters can be used. The negative pulse will just be ignored by the IGBTs.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 01, 2019, 12:40:41 pm
content: Quote from: xavier60 on July 01, 2019, 10:44:31 amOne thing i noticed when testing the circuit out is that the voltage on the open collectors of the TL494 does not go down to 0V but rather to something like 0.7V, which explains why there are almost always one or two diodes between the base drive transistor emitters and ground in the ATX PSU schematics.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 01, 2019, 04:14:50 pm
content: Well i seem to have run out of solder for today but i finished the bridge rectifier and while i was at it i added a simple relay circuit to bypass the PTC resistor after two seconds, this way i won't cook the PTC.Pretty much the same thing done as in big old CRT TVs.

author: strawberry
author_rank: Super Contributor
timestamp: July 01, 2019, 06:14:18 pm
content: thermal compound is not used with silicone alike thermal padsdid you mean NTC ? PTC wont limit inrush current

author: Refrigerator
author_rank: Super Contributor
timestamp: July 01, 2019, 08:23:11 pm
content: Quote from: strawberry on July 01, 2019, 06:14:18 pmYes, i meant NTC.The pads are reused from the ATX PSU and it looked like most of the contact was made right around the screw hole and not lower down on the TO-220 so i added some thermal paste to help the diodes out a little bit.

author: techman-001
author_rank: Frequent Contributor
timestamp: July 01, 2019, 08:40:15 pm
content: Quote from: Refrigerator on June 23, 2019, 08:11:11 amYou have a very interesting blog, exactly the kind of thing I love to read because you engineer your projects using materials that you have, to make what you want, without buying everything of the shelf.Kudos on your moving coil meter project, I was amazed it even worked! I spent 3 years repairing and calibrating moving coil meters in instrumentation and automotive. One rugged automotive method used for a meter bearing is a small tube around the moving coil shaft that has a viscous liquid such glycerin in it. This method adds a lot of damping but seemed to be low friction because the shaft 'floated' in the bearing. It only had one longish bearing of this type with the other end floating free.Keep up the excellent work, you have a long and interesting career in front of you in my opinion.

author: strawberry
author_rank: Super Contributor
timestamp: July 02, 2019, 06:29:53 am
content: For beginners it is nice to have parts you can blow without worries..downside is large space filled with PCB boards and staff

author: Refrigerator
author_rank: Super Contributor
timestamp: July 02, 2019, 03:36:46 pm
content: Quote from: techman-001 on July 01, 2019, 08:40:15 pmI actually started the blog because i couldn't find anyone else doing the kind of stuff i was doing, and i thought surely there are more guys like me out there.So now if anyone's interested in the kind of projects that i'm into they can find my blog.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 02, 2019, 05:35:44 pm
content: Finished the wire speed controller, I had some 555's laying around somewhere but couldn't find them.No big deal because i found a neat little LM393 PWM controller schematic on google and made the speed controller with that.Drew the traces with a permanent marker and etched the board, soldered everything on top because i couldn't be bothered to mirror the layout on the other side and drill the holes.I put dots around the circuit with my marker, those dots are there in case i ever wanted to experiment with feedback so that i'd have pads to solder to.Planned to use a TO-220 MOSFET but the only ones i had were IRF640 and IRF614, which would get way too hot, so i bodged in a NTD70N03R that i pulled from a dell motherboard.My PWM is tuned to 30kHz and the MOSFET stays ice cold.For my wire feed i'm using a VW windshield wiper motor, which seems to have some good torque so the open loop speed control might not be as bad as i had anticipated.

author: strawberry
author_rank: Super Contributor
timestamp: July 02, 2019, 06:07:20 pm
content: is it posible to use TL494 as PWM controllermore detailed info for TL494:http://www.ti.com/lit/an/slva001e/slva001e.pdf

author: mikerj
author_rank: Super Contributor
timestamp: July 02, 2019, 06:59:45 pm
content: Quote from: Refrigerator on July 02, 2019, 05:35:44 pmThat going to be a very heavy duty wire feed, but it's a nice choice for an easily found/cheap part to repurpose. Getting/making suitable rollers and assembling then in an accurate, mechanically rigid assembly is key to a decent wire feed, and typically is where all the cheap MIG welders have problems (flexible plastic rubbish held together with self tapping screws).

author: Refrigerator
author_rank: Super Contributor
timestamp: July 02, 2019, 08:27:55 pm
content: Quote from: strawberry on July 02, 2019, 06:07:20 pmYeah i saw that but the LM393 circuit is much simpler.Quote from: mikerj on July 02, 2019, 06:59:45 pmWhile i was looking up wire feed mechanisms on the internet i found quite a few that use a 24V windshield wiper motors in them.The roller problem was sorted with the help of a cheap skateboard bearing that i welded shut to keep it from spinning, this one will be mounted to the windshield wiper motor and will function as the drive roller.One problem that i came into though is that the bearing is so cheap that the races are not even hard, my dull file bites pretty well into it.I might try quench hardening to see if it helps because as i assume a soft metal drive roller won't last long.

author: tautech
author_rank: Super Contributor
timestamp: July 02, 2019, 08:53:05 pm
content: Quote from: Refrigerator on July 02, 2019, 08:27:55 pmYes because the 24V wiper motor is a good match for MIG welding voltages and simple speed control circuits are needed as the motor is only energised when the handpiece trigger is pulled.Quote from: Refrigerator on July 02, 2019, 08:27:55 pmMore likely you annealed the hardening from it by welding it.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 02, 2019, 09:03:22 pm
content: Quote from: tautech on July 02, 2019, 08:53:05 pmI dumped it into water right after welding but i guess it wasn't hot enough to harden.

author: techman-001
author_rank: Frequent Contributor
timestamp: July 02, 2019, 10:30:41 pm
content: Quote from: mikerj on July 02, 2019, 06:59:45 pmReally? what utter junk. Chinese I assume ?

author: Refrigerator
author_rank: Super Contributor
timestamp: July 03, 2019, 01:21:35 pm
content: Wire feed is done and i'm getting really excited, seems my MIG welder might be finished sooner than expected. (knock on wood)Also the copper pads i left around the speed controller circuitry came in handy as i had to add an enable transistor.For that i changed the inverting and non inverting inputs of the second comparator so that my duty cycle went up as control voltage went down.Next i added an NPN transistor that would pull one of the potentiometer leads to ground this way enabling the speed controller as soon as a high signal is received.Last thing i added was a 47µF cap between the collector of the aforementioned transistor and the control voltage, this way there's a period of high duty cycle on startup, which made a big improvement on the motor speedup.

author: strawberry
author_rank: Super Contributor
timestamp: July 03, 2019, 04:33:22 pm
content: I am not sure about post flow gas for MIG

author: Refrigerator
author_rank: Super Contributor
timestamp: July 03, 2019, 05:34:22 pm
content: Construction begins on none other than good old plywood, since wood can absorb moisture and become slightly conductive i separated any wood from high voltage with a sheet of plastic.Now i'll have to find the PS3 power supply that i had laying around somewhere and bodge*ahem* i mean add a variable DTC feature into my TL494 circuit board.Feeling a bit nervous as i inevitably start thinking about all the things that might go wrong.Quote from: strawberry on July 03, 2019, 04:33:22 pmNot necessary as far as i can tell, many MIGs don't even have it.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 04, 2019, 10:55:26 am
content: Getting a little tight in the ATX case but everything should fit just fine.I might be able to fire this up either today or tomorrow.Ps: the plywood is bowing but that's only because it's just a mock-up to see how everything fits.

author: mikerj
author_rank: Super Contributor
timestamp: July 05, 2019, 10:10:00 am
content: Quote from: techman-001 on July 02, 2019, 10:30:41 pmOf course. They arevery cheapbut you get what you pay for.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 05, 2019, 11:05:28 am
content: Just tested the beast out, but not on 230V yet.I connected the 12V supply to all the control circuitry as well as the bridge rectifier for the inverter and ran a quick test, shorted the output with with my DMM set to amps and got 1.3A.Now i can hook up the scope and set my deadtime, after that everything will be packed into the ATX case and closed shut.Also i'm not sure whether or not to put a choke on the output of the rectifier, i found a nice FBT core that has brass bolts holding it together and i can fit 3 turns of my 35mm2welding wire in it.

author: techman-001
author_rank: Frequent Contributor
timestamp: July 05, 2019, 11:41:37 am
content: Quote from: mikerj on July 05, 2019, 10:10:00 amOf course you do, but if it only lasts a short while because it's junk, is it even worth buying ?My Father always said to me, buy the best you can afford, that way you will only buy it once, and save money over buying junk again and again.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 05, 2019, 12:19:05 pm
content: Quote from: techman-001 on July 05, 2019, 11:41:37 amDoes it even have a spring or does it rely on the flex in the plastic to keep the pressure constant?And that motor looks wimpy as. For a spoolgun this sort of thing would be ok-ish but not for anything else.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 05, 2019, 01:59:08 pm
content: Getting quite messy but if everything goes to plan i'll have it running today.With the DTC cranked all the way up i was able to get 4.4A on the output (that's running on 12V) but i toned it down to limit max duty cycle to about 85% just to be safe.Also came up with a neat way to connect the welding cable to my diode bridge.My old LPG injectors have a coil resistance of 4Ω, which is going to draw 3 amps from my 12V supply. My 12V supply can handle up to 23.5 amps so that's not a problem but i'm worried the solenoids on those injectors won't last long with 36W going into them.Good thing the coils are removable so i can perhaps retrofit a coil from a different solenoid such as a solenoid from a washing machine for example.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 05, 2019, 09:48:48 pm
content: The maiden voyage is going to be tomorrow but all the tests are now complete.The way i connected my voltage pot meant that if i disconnected it the voltage would be regulated to 2.5V (my reference voltage).And that's almost exactly what i got at the output when testing on 12V + 100Ω load (because it would max out at 4V with the pot).This means that my voltage is properly regulated.All that is left is to finish is the mains wiring (circuit breaker, cable, plug), then i also need to add a resistor to load the inverter output a little for stability reasons.After that i'll make a driver thingy for the gas solenoid (probably a small MOSFET on a wire) and the welder will be ready to make some sparks (the good kind, i hope).

author: tautech
author_rank: Super Contributor
timestamp: July 05, 2019, 09:56:08 pm
content: Quote from: Refrigerator on July 05, 2019, 09:48:48 pmMaybe not, this connection needs improvement IMO:Decent lugs are not that expensive and I have on occasion used small diameter copper pipe crimped onto the welder lead with the edge of a nut as the anvil and squashed tight in a vise.Likewise the lug flat is formed in a vise and then drilled to requirements.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 05, 2019, 10:57:22 pm
content: ^ to me it looks fine, all the strands were locked in place, the copper plates were cleaned and the M8 bolt was cranked as far as the aluminum would let me.Perhaps the washer is a little too small but i have mo worries about this connection.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 06, 2019, 11:09:14 am
content: It blew up.Well, if failure is the best way of learning then i'm in for a whole lot of knowledge.Basically what happened is everything was fine until i blipped the trigger and it instantly blew three IGBT's, two low side and one high side.Took the opportunity and cracked one apart to see how big the die was and it came in at about 3.5x3.5mm with two bond wires per leg.Both low side IGBT's completely blew apart, but the one high side IGBT only had a shorted gate.I'm thinking perhaps the high side IGBT failed first and stayed on then while the two low side IGBT's were turning on the amount of power they had to dissipate was astronomical and they blew up. (that's what my inner Sherlock says anyways)Now, why would the high side IGBT fail in the first place? Damn voltage spikes i bet.I'll be remaking the IGBT board to incorporate more protection against voltage spikes and i'll also have to test the remaining IGBT to see if it's still good.If it's good then i'll still have two remaining IGBT's and will have only one per side, but that should still be enough.If not i still have some Toshiba MOSFETs that would get the job done as well.

author: techman-001
author_rank: Frequent Contributor
timestamp: July 06, 2019, 11:14:53 am
content: Quote from: Refrigerator on July 06, 2019, 11:09:14 amI think failure is the ONLY way to learn, at least it always has been for me

author: magic
author_rank: Super Contributor
timestamp: July 06, 2019, 11:42:10 am
content: Is there no way ofgraduallyramping the power up and watching the waveforms on a scope?

author: Refrigerator
author_rank: Super Contributor
timestamp: July 06, 2019, 12:35:15 pm
content: Slight change of plan.I cracked open a failed (but genuine) RJK5010 MOSFET (20A, 500V) and it has a die size of 6x6mm = 36mm2.Assuming that 20A/36mm2= 0.55(5)A/mm2then the IGBTs i had would be 12.25mm2* 0.55(5)A/mm2= 6.8A (yikes).6.8A is a far cry from the advertised 20A so i will not even try to plug these back in but rather i'll go the MOSFET route.BTW i knew the IGBTs were not genuine but i didn't think they were THIS bad (i expected something like 10A).My Toshiba MOSFETS, on the other hand, are genuine, i know because i pulled them from a LG plasma TV.They're 2SK2698 (500V, 15A, 0.35Ω), i had four but used one to replace the blown RJK5010 mentioned above (and fixed a plasma TV).So i'll have only one per side but that's not a problem as long as my gate drive is correct.I also checked the switching transistors from the ATX power supplies i took apart and they're like 10 or 12A rated so in theory using them would have been better than the fake IGBT's!I actually might make another welder just to test these transistors out, mini spot welder for battery tabs perhaps?

author: Refrigerator
author_rank: Super Contributor
timestamp: July 06, 2019, 12:37:45 pm
content: Quote from: magic on July 06, 2019, 11:42:10 amMy scope only goes to 50V max (10x probe) but i do have a small variac that i will use for my second test.

author: strawberry
author_rank: Super Contributor
timestamp: July 06, 2019, 02:22:46 pm
content: RC snubbers for IGBT and rectifier

author: Refrigerator
author_rank: Super Contributor
timestamp: July 06, 2019, 04:44:00 pm
content: Quote from: strawberry on July 06, 2019, 02:22:46 pmI already had those but i assume they were insufficient, now i have a new board with a RC snubber and a 470V MOV right on the MOSFETs.Also i soldered on much bigger screw terminals because the smaller ones had a problem with stripping the threads or being a pain to fit the wire in.

author: profdc9
author_rank: Frequent Contributor
timestamp: July 06, 2019, 06:48:41 pm
content: I have made boards for building half and full bridges. They are intended to be used with dual resonant solid state Tesla coils, but they can work for other IGBT based bridges as well. For example:https://github.com/profdc9/DRSSTC-PCB-Pack/tree/master/full-bridge-transistorThe whole project is athttps://github.com/profdc9/DRSSTC-PCB-Pack/There are spots for the IGBTs, DC link capacitors, snubbers, TVS diodes, gate drive transformer inputs, etc. to help minimize the inductance of the layout.Dan

author: Refrigerator
author_rank: Super Contributor
timestamp: July 06, 2019, 09:56:58 pm
content: I tried running the inverter on 12V with my new MOSFETs and was getting completely nothing.Turns out at least one secondary diode is shorted, could this be why the IGBTs blew?Well as they say when it rains it pours and i happen to be absolutely drowning right now (in crap diodes and IGBTs that is).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 06, 2019, 10:33:57 pm
content: Current mode control won't save you from counterfeit transistors*, but it will save you from failed diodes and shorted loads. I don't recall a control scheme being posted in this thread so I'm reiterating this.*If you have to ask, they're counterfeit. If they're too cheap to believe, they're counterfeit. If they're from eBay or Ali or Amazon, they're probably counterfeit. It's very little labor to grind down the face and laser a new label. Anything from 2N3904s to 2N3055s to power modules and ICs.Tim

author: Refrigerator
author_rank: Super Contributor
timestamp: July 07, 2019, 08:09:52 am
content: Quote from: T3sl4co1l on July 06, 2019, 10:33:57 pmActually the one diode that was the closest looking to the original did give different measurements, but i suspect the TO-247 ones are probably some 40V 20A shottky diodes or something along those lines.I'll pull them apart to see how the die looks.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 07, 2019, 09:05:49 am
content: The plot thickens...Pulled the diodes apart and they're very different inside.The diode closest resembling the original had a die of 5x7mm and was connected by four bond wires.But the TO-247 diode was entirely different, it had a die of 6x6mm and was connected with a big copper tab bonded directly to the die.So in terms of current handling the TO-247 diode should actually be the better choice because it not only has a bigger die (36mm2vs 35mm2) but also the better connection to the die (copper tab vs four bond wires)Also for a quick comparison, assuming he bond wires in the RJK5010 MOSFET and the aforementioned diode are the same then then it having four would mean that it can only handle about 80A.But then again the TO-247 didoes aren't that great if they short out and die before any real current gets to go through them.Meanwhile i had more of those TO-247 diodes arrive so i might test the reverse breakdown voltage on them.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 07, 2019, 10:25:08 am
content: Did a quick reverse breakdown test and the voltage is 35V at 135µA and 65V at 400µA.That makes it a 30V diode, no wonder it blew, in a half bridge configuration this thing would see at least double the output voltage.

author: strawberry
author_rank: Super Contributor
timestamp: July 07, 2019, 02:20:41 pm
content: ESAB Arc251i uses current mode cotroller but they do not use this function by some reason. but rather relay on custom 30A semikron module (blows time to time).sherman and similar chinese use sg3525 or tl494, 2..or more paralel IGBT in bridge configuration. even seen paralel output transformers.without output inductor. if welding wires tangle/twist inductance will drop below minimum and overload IGBT

author: Refrigerator
author_rank: Super Contributor
timestamp: July 07, 2019, 03:56:17 pm
content: Quote from: strawberry on July 07, 2019, 02:20:41 pmGood thing that i put the inductor in then.What about the rectifier diodes?

author: strawberry
author_rank: Super Contributor
timestamp: July 07, 2019, 07:02:21 pm
content: https://elektrotanya.com/esab_caddy_arc-251i_810-xxx.pdf/download.html#dlrectifier 3pcs dual diode module (not popular brand)Esab arc 251i only drawback is wimpy plastic caseKEMPPI mastertig 2200controller protected and filtered I/Odiscrete diodes dual TO-247IGBT moduletransformer ~4x U corebig heatsinksMiller maxstar 200 SDcustom controller (bunch of OP and logic)single custom integrated module semikron SKIIP (IGBT , 3F rectifier, tyristor, PFC)DC link voltage 810VDCMigatronic navigator 3000controller ML4828ISrectifier two pcs DSEI 2x101 06IGBT bridge can be seen in picture under PCB (4pcs)Sherman MIG 200controller sg3525 and lot of LM324 and wiresSherman MIG 261M/4Rno triger circuit protection (control borad VCC overvoltage can happen) , weak funktion switches prone to corosionbunch of discrete IGBT

author: Refrigerator
author_rank: Super Contributor
timestamp: July 07, 2019, 09:05:40 pm
content: Went to Aliexpress and found out what the diode was supposed to be and it's a RURU15060 (or 15080, 15090, 15070 whatever) and this whole diode business seems like a bit of a rabbit hole.The 150EBU04 marked diodes in the TO-218 package are cheaper than the RURU15060 but even the RURU15060 fakes are made in the TO-247AC package.Ps: one good thing about aliexpress is that the reviews do help out quite a bit, especially when buyers include pictures.

author: strawberry
author_rank: Super Contributor
timestamp: July 08, 2019, 04:36:26 pm
content: https://www.aliexpress.com/item/32815709325.htmlfound this wire feed motor, is this any good? are all euro type connectors same size? and what kinda torch to choose?

author: tautech
author_rank: Super Contributor
timestamp: July 08, 2019, 08:15:57 pm
content: Quote from: strawberry on July 08, 2019, 04:36:26 pmYou'd need to check it can reliably feed 0.6mm wire unless you're only gunna use larger gasless wire.Their cheap plastic feed motor does 0.6mm:https://www.aliexpress.com/item/32816596600.html?spm=a2g0o.detail.1000060.2.2cc235328Qu7wf&gps-id=pcDetailBottomMoreThisSeller&scm=1007.13339.128551.0&scm_id=1007.13339.128551.0&scm-url=1007.13339.128551.0&pvid=08073933-1557-49fb-89e5-ec18ecf29bc9QuoteThe one from the same supplier looks fine:https://www.aliexpress.com/item/32823078876.html?spm=a2g0o.detail.1000060.1.2cc235328Qu7wf&gps-id=pcDetailBottomMoreThisSeller&scm=1007.13339.128551.0&scm_id=1007.13339.128551.0&scm-url=1007.13339.128551.0&pvid=08073933-1557-49fb-89e5-ec18ecf29bc9

author: Refrigerator
author_rank: Super Contributor
timestamp: July 08, 2019, 08:21:08 pm
content: QuoteI wonder how many mm2of conductor is in that cable for that kind of a price.

author: strawberry
author_rank: Super Contributor
timestamp: July 09, 2019, 02:51:41 pm
content: or this SSJ-4Chttps://www.aliexpress.com/item/32826045278.htmlI dont know what size fits best for 5/15kg spoolI think 0.6mm roller could do the job.15AK MIG torch seems got thin copper wire for 180Ahttps://www.aliexpress.com/item/32849629353.html

author: mikerj
author_rank: Super Contributor
timestamp: July 09, 2019, 03:19:38 pm
content: Quote from: strawberry on July 08, 2019, 04:36:26 pmIt certainly looks a lot most robust than the all plastic versions, though the lack of 0.6mm wire support is a little annoying for use as an upgrade for cheap MIGs.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 09, 2019, 04:13:10 pm
content: Quote from: strawberry on July 09, 2019, 02:51:41 pmlooks like 20-25mm2wire.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 09, 2019, 04:48:51 pm
content: Got 30% of my money refunded from the previous crap diodes and bought four of these diodes:https://www.aliexpress.com/item/1720411447.html?spm=a2g0s.9042311.0.0.14194c4dvOxU16According to user reviews these should handle 30A per leg and breakdown voltage is around 170V, there are also some people who tested the recovery time and such. Two in parallel on each side of the half bridge should in theory handle 120A.Too bad the shipping from aliexpress is notoriously slow so i'd have to put this welder project on hold for a while, bummer.Anyhow, i developed a LM393 SMPS controller for two switch forward application that i will be using for my mini spot welder project.

author: strawberry
author_rank: Super Contributor
timestamp: July 09, 2019, 07:12:58 pm
content: https://lcsc.com/product-detail/Schottky-Barrier-Diodes-SBD_IR_80CPQ150PBF_80CPQ150PBF_C3044.html

author: Refrigerator
author_rank: Super Contributor
timestamp: July 12, 2019, 08:50:55 pm
content: Quote from: strawberry on July 09, 2019, 07:12:58 pmLooks pretty good at first glance, but the shipping and handling fees make it impractical for small orders.

author: strawberry
author_rank: Super Contributor
timestamp: July 15, 2019, 08:14:05 am
content: Welder 200A rectifier with two RC snubbers 10n(MKS4 2kV) 2.2R(4x 10R wirewound)Y2 10n 300V8x 60A 300V fast diodes TO-247

author: Refrigerator
author_rank: Super Contributor
timestamp: July 23, 2019, 05:38:41 pm
content: The diodes are finally here, took 11 days from being shipped to arriving at my door (the record so far is 9 days from ordering to arriving at my door)That aside i'll go put the diodes in and fire the welder up.Also i noticed that i need to put a freewheeling diode in the welder because i added the inductor.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 23, 2019, 09:22:32 pm
content: Assembled the welder back together and did a quick test on 12V, the output measures 4V and i get 3.8(-ish) amps when i short it out with my DMM.So my welder is back and ready to rock and roll, this time i'll have to whip out the variac to test it out.Before assembling i also measured the diodes by connecting them in a voltage feedback loop in a high voltage boost converter.This way the output of the boost converter would be whatever the breakdown voltage of the diode is.All of the diodes came in at around 130V, considering the current flowing through them is much lower than rated at 150V i think these diodes should be allright.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 24, 2019, 11:47:45 am
content: It blew up again, goddammit.Had my brother hold the trigger pulled while i had my DMM measuring the output along with a current clap on the mains input.As i increased the voltage the output settled at 19.54V but my current clamp was measuring between 1 and 2 amps idle.That's about 300W going into my welder somewhere.My Brother then had the bright idea to release the trigger and pull it again, which instantly blew the welder up.I hope that only one MOSFET blew because i only have one spare.The diodes are shorted out yet again so i guess 150V was not enough.Anyways i'm off to aliexpress to find some more, at least there's some improvement i guess.

author: patrick1
author_rank: Contributor
timestamp: July 24, 2019, 02:40:04 pm
content: the idea might corrupt your plan abit, - but away... how about a RC LIPO or LION Pack in paralell with the output.... super caps might be ok too.....btw i love your ambition ... but its verging on insanity... a home made reliable transformerless? welder.--- i want too say too, the best arc welding experience i have ever had, is with car batteries, - 24V... it so smooth like butter. so nice.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2019, 03:53:08 pm
content: Some voltage and current monitoring sensors, and protection latches say, might be in order, yet?Tim

author: Refrigerator
author_rank: Super Contributor
timestamp: July 24, 2019, 04:18:21 pm
content: Quote from: patrick1 on July 24, 2019, 02:40:04 pmThere's no insanity here just a lot of learning (through even more failing).Also i'd rather not use batteries for welding, good batteries are expensive and bad batteries wouldn't work.Anyways, i took the diode bridge apart only to find out that just one of the diodes had failed.One of the diodes in the double diode shorted out throwing the magnetic field in my transformer out of balance and into saturation and blew the MOSFETs right out.I did some calculations and assuming i get 4V out with 12V in i get a ratio of 1:3, this means that with 340V (rectified 230V) i would see around 113V.Since it's a half bridge the diodes would see at least double making it 226V that's not including any current spikes so i should take 250V as my bare minimum.Also digikey came in handy with their filters, with them i can find actual part numbers rather than write "80A diode switching" in the aliexpress searchPs: cracked the failed diode apart to see what's inside. There are two silicon dies, each 2.5x2.5mm ( 2x6.25mm2in total) both with two bond wires each.Two bond wires should be ok at 40 amps, the silicon die has a current density of 6.4A/mm2, in comparison the TO-247AC (150EBU04) diodes were 4.16A/mm2.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 24, 2019, 04:20:32 pm
content: Quote from: T3sl4co1l on July 24, 2019, 03:53:08 pmI had a current sense transformer with current limiting circuit ready but forgot to put the CST onto the power transformer primary wire.

author: patrick1
author_rank: Contributor
timestamp: July 24, 2019, 06:49:29 pm
content: depending on how many transistors you have too burn ;-D.... i would suggest trying to put a motor start capacitor across the transformer primary ... something between 1-10uF. ...its really hard too protect transistors from this kind of abuse, and im pretty sure a simple diode wont do the job, even a super fast one, - i have never been satisfied with ultra fast dioes for snubbing, - its plain too see on the scope, they only work 80-90% of what you want them too do... especially if you have a fast offtime.and ferrite inductors are your friend. like the little torroids you have before mains SMP circuits... i would put them everywhere, even one or two turns of your power wires, before and after the transformer, and after the ESR caps, before the arc. stick them everywhere ;-D. - it has got many of my ambitious projects back on track several times...

author: xavier60
author_rank: Super Contributor
timestamp: July 24, 2019, 06:53:38 pm
content: A schematic would make it possible to follow what is being done.

author: strawberry
author_rank: Super Contributor
timestamp: July 24, 2019, 07:16:37 pm
content: isnt that your voltage is too high for MIG welder. arc voltage should be 30V max

author: Refrigerator
author_rank: Super Contributor
timestamp: July 24, 2019, 07:45:42 pm
content: Quote from: strawberry on July 24, 2019, 07:16:37 pmIt's an inverter welder, the diodes see high voltage (not mains of course) impulses, which then get smoothed out by the inductor, very similar to a buck converter.The 4V i got were at whatever the max duty cycle (because it was undervoltage) i set the welder to was (85% i think), assuming 1:3 ratio between primary and secondary voltage i would get 113V when running on mains at that max set duty cycle.If 113V = 85% then to get 21V (common MIG voltage) the controller would reduce the duty cycle to roughly 16%.Also since during the test on 12V the welder was basically unloaded (except for the 100Ω idle load resistor) i can assume that 4V was also my peak voltage and therefore i can also assume that 113V would be the peak voltage on mains.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 24, 2019, 07:54:48 pm
content: Quote from: xavier60 on July 24, 2019, 06:53:38 pmI planned to release the schematic in my blog as soon as i finished the welder because i kept changing the design and component values constantly.But i do have a crude one drawn (as a guide when soldering), although i doubt that it would be very useful.Ps: please excuse the crudity of the schematic, last time i used KiCAD was years ago.

author: strawberry
author_rank: Super Contributor
timestamp: July 24, 2019, 09:36:44 pm
content: should be ~45Vfor scratch start stick welder 113V is fine

author: Refrigerator
author_rank: Super Contributor
timestamp: July 24, 2019, 09:52:25 pm
content: Quote from: strawberry on July 24, 2019, 09:36:44 pmGoogle says 12 to 30V for MIG

author: magic
author_rank: Super Contributor
timestamp: July 24, 2019, 10:02:02 pm
content: I think the part around the power transformer, primary switches and secondary rectifiers would be more relevantIn particular, secondary diode voltage is something you should know, not guess. What's the turn ratio? How is it rectified?

author: Refrigerator
author_rank: Super Contributor
timestamp: July 24, 2019, 10:22:42 pm
content: Quote from: magic on July 24, 2019, 10:02:02 pmI don't quite remember the transformer parameters but they're here in this thread somewhere along with all the other details.Secondary is configured in a half bridge (center tap ground), primary in a push pull.Mains is rectified into a capacitor bank with two series capacitors, where the transformer goes in the center, everything is very similar to an ATX PSU.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 24, 2019, 10:49:36 pm
content: Whew, spent some time surfing through aliexpress, reading all the reviews and found a few gems.The image is from the reviews and it's quite hilarious, as there's an SMD device stuck into a TO-247 package.

author: magic
author_rank: Super Contributor
timestamp: July 25, 2019, 07:26:31 am
content: Quote from: Refrigerator on June 22, 2019, 08:50:39 pmQuote from: Refrigerator on June 22, 2019, 08:50:39 pmWait, so is the primary 2×10T and the secondary 8T in total, i.e. 2×4T?Because if not and it actually is 2×10T and 2×8T (or center-tapped 10T and 8T in total) then you got the math wrong. You apply primary voltage to only one 10T winding at a time, but both 8T secondaries react, producing a combined 1.6×Vin across the secondary rectifier.Is this the circuit you use?

author: xavier60
author_rank: Super Contributor
timestamp: July 25, 2019, 07:39:28 am
content: It is the typical half bridge PC PSU topology,The transformer turns ratio is best calculated from the primary turns divided by the turns of one of the secondaries.

author: magic
author_rank: Super Contributor
timestamp: July 25, 2019, 07:58:26 am
content: OK, thanks for the correction. I got to "primary is a push-pull" and stopped reading thereIf the primary is 10T and the secondaries are 8T each than it's 330/2·0.8=130V indeed. So the diodes are killed by transients or their dubious provenance

author: xavier60
author_rank: Super Contributor
timestamp: July 25, 2019, 08:03:10 am
content: Quote from: magic on July 25, 2019, 07:58:26 amAnd undamped ringing would come close to doubling that.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 25, 2019, 08:40:52 am
content: Quote from: xavier60 on July 25, 2019, 08:03:10 amThe snubbers on the diodes are another thing i forgotI had them on the crap 150EBU04 diodes.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 25, 2019, 10:53:15 am
content: Took the MOSFETs out and they're both blown.Cracked one open to check the die size and it's 5.5x5.5 = 30.25mm2, 0.5A/mm2.

author: magic
author_rank: Super Contributor
timestamp: July 25, 2019, 02:56:27 pm
content: So what actually is the point of having ±65V open circuit secondary voltage for 22V output? Do you expect 33% efficiency under actual load?I don't know what can be realistically expected from such converter, but 33% seems rather extreme. SMPS work better if transformer ratio is close to ideal and duty cycle close to 50%. Isn't it be better to cut the secondary in half and parallel the two parts for 32V output at lower impedance, still giving margin for 66% efficiency?PWM surely is magic, but you pay for it right here.

author: Refrigerator
author_rank: Super Contributor
timestamp: July 25, 2019, 04:02:59 pm
content: Quote from: magic on July 25, 2019, 02:56:27 pmI can remove a few turns in the transformer but not now, i still want to test the current setup out.

author: magic
author_rank: Super Contributor
timestamp: July 25, 2019, 05:39:38 pm
content: Not my circus, not my monkeysHigher turn ratio means lower peak secondary voltage, less primary current and even less resistive losses for the same amount of trafo wire.https://www.tme.eu/en/details/idw100e60fksa1/tht-universal-diodes/infineon-technologies/Is this good enough? 600V, 100A and 200W continuous at Tc=100°C. Should be hard to destroy unless you forget the snubber againAnd you are guaranteed it's not a 1N4148 inside and you get a proper datasheet corresponding to the actual silicon in your hands, no guessing, no smoke, just calculate your power lossess and heatskinking and it works.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 14, 2019, 07:03:16 pm
content: A little update on the whole project.I've since blown the welder up two more times.Each time getting closer to proper operation.With the new diodes i have (IDW100E60) and new MOSFETS (IRF460) the welder does power up and survives a trigger pull just fine.No load with the trigger pulled (operational) the inverter draws 50mA from the mains, which i think is not too bad.But the MOSFETs popped as soon as the welding wire touched the metal scrap i tried to weld.I then doubled up on the MOSFETs, previously there were one per side, now there are two per side (a total of four) and also removed two turns off the transformer (10 primary, 5.5 secondary).Did some tests, everything seemed ok-ish, i then separated a few strands of my negative welding wire and shorted them out to the positive to see how well it melts wire.It lasted about 30 seconds before blowing up yet again, although a lot more spectacularly than before.So for once i decided to do some actual research and calculations (groundbreaking stuff, i know).Couldn't find much info on FBT cores, but Bmaxon them appears to be in the range of 2000 to 3400 gauss.I took 1500 as my Bmaxto be extra safe and calcuated a primary of 10.7 turns (rounded off to 10, because generous Bmaxestimate).From that i calculated my secondary, which turned out to be around 3 turns.I'll rewind my transformer in the same way i did before with the primary between two secondaries, but i'll split the secondary wires to make them bifilar.So my transformer will be wound with the primary between two bifilar secondaries.

author: magic
author_rank: Super Contributor
timestamp: August 14, 2019, 09:11:47 pm
content: That's progressWhat's the switching frequency, duty cycle, dead time, average input current and primary inductance? Here some math needs to be done: checking if idle magnetizing current of the primary is sane, calculating FET Rds(on) losses.It seems like your topology shouldn't subject FETs to significant overvoltages because of the clamping action of the opposite FET's body diode, but that's only if everything is laid out tightly and the current loop through the FETs and the capacitor bank is short.BTW, I have no idea what will happen but you could wrap one turn of cable around the transformer, bring the voltage further down with resistor dividers and hook it up to the scope. It might give at least some approximate picture of voltage waveforms at the working windings.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 14, 2019, 10:35:08 pm
content: Did some estimates and decided to go with 10:2.5 setup on the transformer.Also the bifilar thing didn't work out because the wire was too tangled to separate.With only 2.5 turns on the secondaries it was a bit of a challenge to wind, in the end i decided to loosen the twisted strands of my wire to spread them out, this way it would cover most of the surface. Copper sheet would be best for situations like these.Anyways, i'll assemble the welder tomorrow and will see how well it works this time.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 14, 2019, 10:39:57 pm
content: Quote from: Refrigerator on August 14, 2019, 07:03:16 pmCongratulations!

author: strawberry
author_rank: Super Contributor
timestamp: August 14, 2019, 11:19:41 pm
content: there is no 2.5

author: coppercone2
author_rank: Super Contributor
timestamp: August 15, 2019, 05:51:09 am
content: been waiting like 3 years to power up my IGBTs, looks like I made the right call doing nothing

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 15, 2019, 01:56:38 pm
content: Quote from: coppercone2 on August 15, 2019, 05:51:09 amAs an old colleague of mine used to say: "it's urgent not to do anything".

author: Refrigerator
author_rank: Super Contributor
timestamp: August 15, 2019, 03:44:47 pm
content: Tested the welder but not fully.Seems either i made a mistake when rebuilding or the previous failure did more damage than expected, because at a certain input voltage the MOSFET half bridge starts oscillating.Above 130VACif i pull and release the trigger the MOSFET half bridge starts oscillating at around 90kHz, while my switching frequency is set to 50kHz.I think i'll remake the MOSFET board and if that doesn't help i'll remake the TL494 board along with the GDT.Otherwise everything seems to be looking pretty well so far. Under no load output reaches 19V at about 130VACinput and with a 1.3-ish Ω load it maintains 14V, which might be limited by the input voltage.With the 1.3Ω load at 14V the ripple on the output is 1.4VRMSI haven't yet tested the welder on anything above 140VAC because i fear it might blow up.Also my variac is only rated up to around 300W and starts buzzing pretty loud on anything above 1A.After operating with the said load for a few minutes all of the heatsinks are cold.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 16, 2019, 09:06:49 am
content: Hooked up my scope to the gate of one of the low side MOSFETs to capture the waveform during oscillation.I also made a quick and dirty current clamp with an inductor core and a clothespin, with 100 turns and a 100Ω resistor it should give me 1mV/1mA.Yellow - gate voltage, Red - transformer current.Any idea where this could be coming from? Connecting a 23.5Ω load on the output stops the oscillation.

author: magic
author_rank: Super Contributor
timestamp: August 16, 2019, 02:18:32 pm
content: I would guess the PWM controller just drives it that way.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 16, 2019, 02:25:25 pm
content: I decided that the 23Ω load resistor is an ok fix and ran some tests at full voltage.For these tests i made a 2Ω load resistor out of some old welding wire, but 2Ω was a bit much so i stepped it down to 1Ω and later ~0.75Ω for my tests.Checked the voltage range and it goes from 18V to 25V, testing was done at 25V with the aforementioned resistances.I let the welder do its thing for about 15 minutes and checked the temps. Both heatsinks were warm, could say almost hot but not bad, considering there was no fan plugged in, that's with the 1Ω load.Too bad i don't have a temp probe so i can only go by feel. Of course i unplug and discharge everything before i go poking my fingers inside.Before i do any more testing i'll go make a temperature controlled PWM speed controller for the fan i'll be using because it uses a PWM input and is very loud at full power (afc1212de).Edit: had to downsize the image.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 16, 2019, 02:34:34 pm
content: Quote from: magic on August 16, 2019, 02:18:32 pmWhen the trigger is released the GDT drive transistors are turned off along with the controller output (DTC pulled high) so it's not the controller.I guess it was because there was no current going through the GDT that allowed the oscillation, anyways it's fixed now.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 16, 2019, 06:43:56 pm
content: I think it's ready for a true weld test, but i'll do that tomorrow, because it's dark already and all the mosquitoes have come out to bite me in the arse.It's quite amazing what a little airflow can do, with the fan barely idling the MOSFET heatsink is only lukewarm and the diode heatsink is ice cold when running just shy of 700W.I also "accidentally" melted the load resistor i made.When i shorted the melted ends out i got some hissing arcs.One thing i'm not feeling too confident about was the squeal coming from the inverter during moderate loads (above 1kW).Last time right before it blew up it started squealing, which is why it worries me so much.

author: magic
author_rank: Super Contributor
timestamp: August 17, 2019, 06:14:07 am
content: Are both FETs still driven from two opposite windings of the same GDT? This oscillation waveform makes no sense to me

author: Refrigerator
author_rank: Super Contributor
timestamp: August 17, 2019, 06:35:50 am
content: Quote from: magic on August 17, 2019, 06:14:07 amThe GDT has two isolated outputs, both of which are inverted.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 17, 2019, 09:34:31 am
content: I got it to make a few snotty welds before it died (i died a little inside too) but all is not lost.Turns out the low side gate discharge transistor shorted out and prevented the low side MOSFETs from turning on.Since i'm all out of suitable PNP transistors i redesigned the gate discharge to be handled with a NPN transistor.I basically flipped the PNP circuit over and put an NPN transistor instead of the PNP transistor, confirmed that they both work identically with Ltspice.Now all that is left to do is to make a new MOSFET board.So far all the testing was done without gas, because i don't have any at the moment and also my current sensing circuit needs some tweaking.

author: tautech
author_rank: Super Contributor
timestamp: August 17, 2019, 09:38:27 am
content: Get some gasless wire.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 17, 2019, 11:14:05 am
content: Previously i had a pair of SS8550 transistors, one per side.The collector of the SS8550 shorted to the emitter but the base seems fine so i assume the collector current was too high.The MOSFET board was made for one MOSFET per side but i added two more so i guess discharging both gates at once was a little too much to ask, even if the transistor is rated for 1.5A.All MOSFETs had a 10Ω resistor on the gate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 17, 2019, 03:23:36 pm
content: Yeah, when MOSFETs die, they fail three-way shorted. Expect high voltages on all three pins. Standard procedure is to check gate resistors and replace drivers when transistors fail. Even stuff on the other side of the transformer may be affected.Instead of attempting to weld -- which is clearly not working, and only serving to rack up a larger pile of failed components -- why not stage some tests, measuring peak voltages and currents on the transistors under given input and output conditions? Example, low input voltage and full output load current; shorted load; full input voltage and no load or light load; etc.The measurements taken in such tests can be extrapolated to the full operating input voltage and output current, and you can determine whether it is safe to operate at full power, or if the design needs further refinement.Tim

author: Refrigerator
author_rank: Super Contributor
timestamp: August 17, 2019, 05:18:41 pm
content: Quote from: T3sl4co1l on August 17, 2019, 03:23:36 pmThanks for the advice but the pile of blown silicon has grown once more.The MOSFETs were not blown when the gate discharge transistor failed, but i got lazy and inpatient and thought that a bigger transistor would solve the problem, which was not the case.The result: two more blown MOSFETs, both low side MOSFETs in this case (high side MOSFETs are ok). Seems the welder wasn't switching correctly.Also the load resistor wasn't there just to be burned, while the welder was busy making toast i poked around and measured everything i could with the limited amount of equipment i have (one DMM, 1985 USSR clamp meter and a USB scope).I'll put this project on hold because there's other projects i want to try out (2 switch forward circuits, current transformers and some other stuff), also i'm out of parts.I think i might also rebuild the welder with a new controller and a better layout. I don't like the GDT and i think photocouplers would be the better way to go, the GDT is finicky and rewinding it is a pain in the arse. With an external power supply for the gate drive circuits i could adjust many more things.So far i'm about 30€ in and i think this project was worth it even if it doesn't work just yet.