tig microwelder feasible? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: October 17, 2018, 08:31:42 pm
content: So if you are doing tig welding thats under 10 amps, for doing various foil joining jobs, I thought a home made power supply might actually be feasible with out all the difficulties that come with R&D assosiated with high power electrical control systems. It could run off batteries and for very low currents even possibly use a linear regulator thats well heat sinked (i.e. welding aluminum foil thats thin is like <2A).does anyone have any ideas? Maybe a custom built unit could actually work better then a big welder trying to control the bottom end of its current regime.For jobs other then foil you can probobly do a decent job with a oxy/acetylene microtorch that uses one of those jeweled outlets.Say this could be jobs smaller then 5 thousandths, which is something like 4-8A peak depending on the metal I think.Even a highly regulatable 2A peak one would be interesting.Electrical efficency is non issue here, just quality of joints made.

author: PointyOintment
author_rank: Frequent Contributor
timestamp: October 17, 2018, 09:45:39 pm
content: Hack a server or even ATX power supply?

author: coppercone2
author_rank: Super Contributor
timestamp: October 17, 2018, 10:09:29 pm
content: I want it good. I guess you would want a constant voltage power supply followed by some kind of modulatable current regulator. I think that would work best for a precise job.I am not sure where the tricks are though. I never used a tig, I don't know how the arc strike works. Do you have a strike circuit consisting of a inductor and some kind of fast isolator switch to connect the main output when the plasma channel still exists but the potential is low enough to not hurt the electronics? I thought maybe a tube could work there as a pass ?I assume when the gas is ionized you still have some period of time with low potential where there is still a effectively low impedance path of plasma present.. it persists a bit I think? I don't know if there is a useful plasma half life after the HV decay based on a thermal time constant?You might need a stable HV rail with a pulse circuit to act as a striker to create such a plasma channel?What else is there?time to open a arcane blue tome..

author: coppercone2
author_rank: Super Contributor
timestamp: October 17, 2018, 11:38:44 pm
content: the extinguishing of a hot arc seems to be f airly electrically slow for modern partsOK, reading about it some, it seems that it has to do with the arc dimensions and it seems to be related (unknown if dominant factor) to diffusion of hot plasma into the air and vise versa, it kinda makes sense to me that its cooling by diffusion. I don't think the main cooling is from photon emission or canceling collisions within the plasma itself if its a partially ionized gas thread? and possibly based on the free path lenght average how much collides with the unionized gas to lose energy to it as the diffusion envelope area gets larger in proportional to volumeI think if I figure out the density of the plasma maybe you can just estimate the cooling by considering the plasma life time long compared to the rate of gas infusion into the plasma thread so you might be able to do a simple calculation based on minimum allowable impedance.I wonder if you can estimate a theoretical spark volume or something to make starter arc time constant estimates accurate to an order of magnitude so that experiments can be organized.

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2018, 01:17:37 pm
content: So to narrow the problem down, you would need something like a 100V voltage source (bipolar), followed up by a current source with a frequency response of like 400Hz to a square wave, so say 4KHz to get decent edges up to the 10th harmonic.I think like 500 watts would be a reasonable cutoff limit, past 5 amps you can probobly get good performance from a regular mid range TIG machine and the metal would be thick enough to join with other processes like soldering or even welding without a problem (my dhc2000 gas torch can do something like 0.011'' minimum thickness), and i Don't know how fine those jewelers torches can do.I think the arc voltage is like 70V, so a 500W supply should have some head room if your steady with your hand.I am imagining some kind of tricked out water cooled howland current source that has some kind of open circuit protection. The amplifier signal part could be a APEX part thats good to 350V and then some kind of buffer can be used after it.What special provisions are there ?relevant:https://www.eevblog.com/forum/projects/power-amplifier-design-for-100hz-to-150khz/

author: langwadt
author_rank: Super Contributor
timestamp: October 18, 2018, 02:00:06 pm
content: Quote from: coppercone2 on October 18, 2018, 01:17:37 pmtig arc voltage is more like 20V

author: rstofer
author_rank: Super Contributor
timestamp: October 18, 2018, 02:32:06 pm
content: If you want to weld really thin material or any gauge of Aluminum, you probably don't want to use 'scratch' start of 'lift' start. You really want a controllable high frequency start.https://www.everlastgenerators.com/blog/hf-start-vs-dc-lift-start-tig-vs-scratch-startYou also want to be able to modulate the current with a pedal and this typically involves a saturable reactor. Or it did way back when... Today the systems typically use some type of inverter.In any event, you can find a lot of reference material on Google.There's a reason that usable TIG Welders start at several hundred dollars and work up - quickly.

author: taydin
author_rank: Frequent Contributor
timestamp: October 18, 2018, 02:56:56 pm
content: A TIG welder is mostly a constant current source, with a fixed upper voltage limit. Note sure what the voltage upper limit is, but I remember reading it as 80V for some welders.But I think during arc start, the voltage gets much higher. My Miller TIG welder's starting arc easily jumps around an inch or so. I always get reminded about it, because I sometimes accidentally hit the pedal before having the torch positioned, and get tasered by it

author: langwadt
author_rank: Super Contributor
timestamp: October 18, 2018, 03:26:00 pm
content: Quote from: taydin on October 18, 2018, 02:56:56 pmit is basically a high voltage high frequency , transformer coupled in series with the welding power

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2018, 09:01:53 pm
content: Yes in my first two or three posts I discussed the HF start.I am more concerned about the modulatable current source.I am concerned about this scenario:Plasma channel forms, current source is correct, but wind/motion/splatter/etc causes the arc to momentarily break, which causes the voltage to go to its maximum span, with a partially formed plasma arc thats basically there for thermal reasons.If a arc/plasma channel that exists is extended due to potential, does the HF spike form, as it does with a spark being formed in the first place without a initial plasma channel? Or does it electromagnetically ellongate like a lava lamp or something, so there is no HF or less of a HF transient created vs a non-formed spark?I am worried that the feedback will end up getting a bunch of HF garbage on it in the case of this kind of 'intermittent' connection and cause the circuit to malfunction or other wise damage it.I would like a series connection to the load, without galvanic isolation. I don't want to deal with transformers.Should I protect both the output and the feedback path together, or layer in different protection on the output and the feedback path ?The start circuit is basically a gimick that needs to be tested (like a motor run/start) to make sure it does not damage the current source... It does tie into feedback though, perhaps passive protection could also help shield the arc from the starter circuit.My idea for the stater circuit was basically to have a HV rail connected to a pulse circuit and a high power fast transmission gate network, either consisting of fast robust semiconductors or vacuum tubes, which would work like this:1) Current source is isolated by fast switch from the electrode2) HV rail is passed through to the electrode, by DC pulse3) the pulse is configured so that a respectably hot conductive plasma channel can form between the electrode and the work piece. For safteys sake it might be better to use a charge pump to limit energy rather then to just short the rail out (with a controlled impedance, but still)4) once the start arc formed is sufficient, large to have a useful thermal time constant, the high voltage rail is switched off5) the pass gate is opened to connect the current source to the plasma channel/work piece network, at this point its in weld mode6) it stays in weld mode and during weld mode can be adjusted with the foot petal or thumb switch or whatever control you have and whatever settings (polarity, frequency, offset, waveform,etc)But anyway, I would like to keep it linear since I know how to manage the heat and its all familiar to me, its so esoteric that Idon't care to work on a bipolar switching supplyI am working on the following:1) the isolation switches choice (it seems that the big boys use more of a passive network from patents but I only glanced at them). But, consider the fact that this thing is like 5 amps MAX, so you don't really have to use high-power design for protection.. you are given the opportunity to use something like a make before break switch since these currents and voltages are kind of sneezeable with modern semiconductors, if anything you can limit the size of passive protectors and make the circuit more ideal during operation without parasitic protection elements required by something that can put out 200A/90V2) the howland current pump buffer design (transistors type/model, bias levels, etc)3) feedback filtering (i.e. something like a active LPF on the current feedback, time constant that should be held to for the control loop)4) any possible problems created by trying to turn a kind of unique APEX op-amp into a composite current pump, possibly problems with the topology5) additional feedback filtering evaluation (i.e. common mode choke on the current shunt, bulk inductor/ferrite on the output signal, pass capacitor, TVS, maybe HV GDT and related impedance network)The ionization potential of various weld gasses is like <30V. I am not sure why those welders go up to 80V, maybe its just voltage drop across longer arcs? Not sure what to do. I will look in a book about it.

author: jmelson
author_rank: Super Contributor
timestamp: October 18, 2018, 09:09:00 pm
content: Quote from: coppercone2 on October 17, 2018, 10:09:29 pmNo, they have a Tesla coil that delivers HF signal to a series inductor in the main welding circuit. So, the HF is added in series to the welding circuit.The HF is REALLY strong, so strong you can actually weld tiny stuff with it - kinda sorta. I have a huge Lincoln square wave TIG machine, VERY old-school, but it works very well down to a few Amps.Jon

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2018, 09:14:07 pm
content: Are you sure thats a good idea? The HF might cause punch through of the foil and I don't think the metalurgy is well defined for that sort of thing.I thought by using a isolated arc start thats parallel you can figure out how to grow the minimum possible hot arc with the lowest voltages etc to really control the situation.It also sounds like you need custom magentics for that design, I really don't wanna go there unless you can find problems with my idea (other then small price I don't care for a one off, unless its gonna save me like 1000$)On the other hand it might be better if you don't need to swtich because it can be theoretically faster and be able to put less energy into the starting arc.Do you have a circuit of your old welder?

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2018, 09:16:44 pm
content: I assume it can be thought of as a magnetic pulse boot strap through a transformer?

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2018, 09:19:50 pm
content: I don't know how I feel about boot strapping the entire precision circuit and power supply with a RF pulse.... it sounds kind of like a bad idea...

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2018, 09:34:40 pm
content: I even thought to float the arc starter from the circuit entirely some how, with a separate ground..and for the control loop, based on how the arc behaves, do you really want a constant current source? It seems like you would want some kind of PID-ish feedback or heuristics based on how the arc wisks around and shit.. don't know if you can fight the fluid/plasma dynamics that form as the metal is vaporized and oil deposites and burned off etc though. might be just too complicated to try to control.. but the thermal time constants seem much lower then with conventional welding because everything is thin and heat is drastically sucked away more on the XY axis rather then depth.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 18, 2018, 10:43:03 pm
content: The reason the spark start is used is to get a high enough surface temperature on one side of the arc to form a cathode spot and obtain electron emission. A very high Dc start voltage generally does not provide enough Di/Dt to get a cathode spot and ionize enough air to get a self confining arc going.So one of the big expenses and more difficult design problems is to keep the start pulse or start HF out of the current regulating electronics. I spent many an evening designing DC coupled low pass filters and protection circuits for this task.Steve

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2018, 10:49:00 pm
content: how come the HF is better at doing that? Because of the skin effect?I thought it was just to make a conductive path.. from what you are saying its actually to kinda heat some point on the surface to be welded like a induction heater? Not saying you are wrong its just non intuitive for me, I only ever used mig/stick for electric welding.What exactly is happening? The only reason I can think of why HF would work here is because it would not penetrate as much so it would make metal vapor on the surface that would aid in plasma conduction? (since you don't want to vaporize the electrode).In that case would it be good to negative bias the HF so that the ions are drawn towards the electrode too?

author: LaserSteve
author_rank: Super Contributor
timestamp: October 18, 2018, 10:50:22 pm
content: A few systems we have at work use a low current and a retracting sharp pin. A few tens of mA are flowing into the future weld area and the pin is pulled back by a very well controlled linear solenoid.A few microseconds after the sense current starts declining, a IGBT fires and dumps in the ignite spark, the inert gas pulses in, then a second IGBT dumps capacitor one into the weld probe, then the main regulated weld current is gated in. Used mainly in pulsed welders for jewelry and thermocouples and test sample wire bonding. The timing is incredibly tricky.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: October 18, 2018, 10:54:22 pm
content: Yes, you need a glowing orange to white hot emission spot in TIG, ion and electron bombardment do the heating, as well as localized resistance heating.Steve

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2018, 10:55:03 pm
content: Would you say the start HF does a job with more ideality (less heating that negatively effects the bond area)? The way you put it sounds like either way could possibly work if you time it right or figure out a way to protect the current source.Do you have things like filters which get shorted out electronically (slow) compared to the instantaous response of the filter, kind of like shorting out a inrush limiter in a power supply? Or is HF prefered because you can filter it out easily (easy of electrical control implementation), rather then any physical process control reason?

author: langwadt
author_rank: Super Contributor
timestamp: October 18, 2018, 11:03:24 pm
content: Quote from: coppercone2 on October 18, 2018, 10:49:00 pmI'd hazard a guess; to generate the high voltage you need some form of AC, using high frequency means you can use a smaller transformer and keep it out of the main supply with a filter and it is not quite as dangerous if you get shocked because of the skin effect

author: langwadt
author_rank: Super Contributor
timestamp: October 18, 2018, 11:04:33 pm
content: Quote from: LaserSteve on October 18, 2018, 10:50:22 pmis that for a fully automated weld? it sounds a bit like "lift start"

author: coppercone2
author_rank: Super Contributor
timestamp: October 18, 2018, 11:13:15 pm
content: Quote from: langwadt on October 18, 2018, 11:03:24 pmThat would be electrical ease of implementation and safety reasons.But, once the arc reaches the metal, generated by whatever means, does the very surface of the metal get hotter with less penetration with the RF start?, meaning less uncontrolled energy enters the weld area? If it only heats the very surface of the metal, then ion emission could occur and allow it to switch into its intended mode of operation with less energy delivered. But I am not sure if I am getting it backwards, because the energy might spread over a greater area with RF/HF if I did get this idea backwards, meaning you need more energy to spot heat something with HF.What I kind of imagined is if you use negative DC to strike an arc, it would heat the metal, then drag the ions into the plasma stream to make it more conductive because it would become a metal plasma rather then just a ionized gas plasma, which I think is more conductive.If the AC thing is still true, then maybe you can use a -DC biased with HF waveform (for a short time period), so you can heat the surface towards emission/vaporization, then form a metal plasma channel, then turn on the high steady state or low frequency (400Hz or so acording to manufacturers) weld current?the idea sounds a.c.m.e. though from eng prospective..

author: LaserSteve
author_rank: Super Contributor
timestamp: October 18, 2018, 11:20:36 pm
content: Its a very sophisticated lift start with lots of programable pulse parameters. It aborts about 1 in 3 welds on its own without doing visible damage. It uses a very controlled current ramp tranistion and pulses to sustain the start. I need to put a scope on it some day.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: October 18, 2018, 11:27:04 pm
content: Hf start gets used because 32 Kv PIV 2 Kiloamp diodes with no forward drop only exist in Star Trek plasma conduits. ;-)

author: LaserSteve
author_rank: Super Contributor
timestamp: October 19, 2018, 12:03:50 am
content: Hf start gets used because 32 Kv PIV 2 Kiloamp diodes with no forward drop only exist in Star Trek plasma conduits. ;-)Lift start does a great job of ruining your carefully shaped Tungsten TIG tips, and they can get stuck in the weld at start when doing it by hand.I have Xenon Arc lamps, Ion Lasers, Pulsed Flashlamp based lasers , and welders in my life. All but one device uses either HF start or a cap dumped into a coil to make a 100 Khz or greater exponential decaying AC start pulse.Cap dumped systems usually have a serloies injection ignition transformer that carries the whole load current, a series inductor to block the RF and a specially constructed metal canned mica capacitor across the power supply to protect it by providing a return path for the RF. Many of these devices have spark gaps to keep their DC supply out of tbe HF starters. All the cap dumped systems depend on the plasma voltage clamping and adsorbing the ignite spike to protect the PSU. If the plasma doesn't form in many of these devices, the psu and /or any blocking diode is toasted.Most of the non welding devices have DC boost voltages across the load to aid in Cathode Spot Formation.The pulsed flashlamps have series ignitors, 3 KV 100 mA simmer currents and IGBTs, SCRs, or Thyratrons to dump in the capcitor into the ignited load.IF dc start were practical for loads above 100 mA most of these devices would have it.Hf start toplogies work, are proven, act as their own load switch for pulsed loads, do not need exotic diodes and the ringing wave creates very high peak fields. Lc filters do a great job in place of expensive diodes.The one DC started lon Laser I have results in hard starting plasma tubes that get replaced early. The same tube that gets removed lives another few thosand hours if placed in tbe same model laser with HF start installed.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: October 19, 2018, 12:11:14 am
content: It takes immense amounts of heat to obtain a decent vapor pressure in metals. These temperatures are far, far above the melting point in most cases.Hence the helium or argon in TIG carries most of the load

author: coppercone2
author_rank: Super Contributor
timestamp: October 19, 2018, 12:24:43 am
content: Ok, it sounds like injection and shunting is the way to go from what you said.What does this transformer look like? What turns ratio does it have, and what core material is used?What capacitor voltage/capacity is used? Do they use a charge pump type circuit?How is a spark gap used to protect the supply? Does the spark gap circuit just run with a steady arc along a gas discharge tube during the operation of the welder, basically saturating the core transfer flux? I assume its loaded out immediately after. It seems like it would be best to short the transformer out on the weld source side right after ignition.

author: langwadt
author_rank: Super Contributor
timestamp: October 19, 2018, 01:04:15 am
content: Quote from: coppercone2 on October 19, 2018, 12:24:43 amthe capacitor, inductor(transformer) and sparkgap is what makes the HF oscillator

author: coppercone2
author_rank: Super Contributor
timestamp: October 19, 2018, 01:08:52 am
content: yea, but I want to know practical values to start design simulations or experiments around that will be useful for welding..

author: LaserSteve
author_rank: Super Contributor
timestamp: October 19, 2018, 01:09:07 am
content: Welders used to use what amounts to a small 2 KV neon sign transformer designed for overload duty arcing into a spark gap that drives the primary of a tiny ruggedized tesla coil. The secondary of the tesla coil arced into a gap connected to one of the welding leads.An inductor the size of Manhatten and some shunt caps keeps the HF out of the rectifiers.That is the old way and it is still widely used.Steve

author: coppercone2
author_rank: Super Contributor
timestamp: October 19, 2018, 01:24:01 am
content: what is the new highly engineered esoteric expensive way (if there is one suitable for hand use not solenoids etc requiring a custom torch)?

author: coppercone2
author_rank: Super Contributor
timestamp: October 19, 2018, 03:46:05 pm
content: Out of curiosity, do you think its possible to use series semiconductors to act as a high voltage block switch with your knowledge of the time constants ? I am still reading the relevant chapters in 'gaseous conductors' to figure out if its possible *but based on your solenoid description its at best hard*I don't like the idea of a monsterous choke or relying on a capacitor to shunt a power supply for some reason. Even if its not perfect, maybe you can reduce the size of the passives significantly?I think the correct term I am looking for is 'arc deionization'.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 21, 2018, 01:01:02 am
content: Think this is easy? Read their blog!:http://www.arcpig.com/index.htmlHere is a modern controller...https://patentimages.storage.googleapis.com/be/4b/40/b50c91641429b3/US6388232.pdfHere is a conceptional design, its missing a great deal of protection circuitry:http://www.wseas.us/e-library/conferences/2006prague/papers/513-109.pdfAnother concept:https://patentimages.storage.googleapis.com/16/b9/d7/b1e113ce83ff20/US5296665.pdfAdd on starter, assumes the welder ships with a series inductor, protective arc gap, and HF bypass caps in the output.https://www.millerwelds.com/files/owners-manuals/O611L_MIL.pdfSteve