Reply by September 23, 20162016-09-23
On Sunday, September 18, 2016 at 4:19:19 PM UTC-4, Cursitor Doom wrote:
> On Sun, 18 Sep 2016 07:40:17 -0700, dagmargoodboat wrote: > > > No, that's welding current for common arc welders. Typically ~40V open- > > circuit. > > More like 50V - 80V. I have an oil-cooled Oxford welder (a big old > industrial job I converted from 3ph). Current range output is from 40A - > 320A. The OCV is switchable between 50 and 80V; you use the higher > voltage for thinner gauge plate. At 50V and 300A it'll comfortably cope > with 1/2" thick steel plate.
You're keeping nicer company than I've been. Here the hobby / home repair-level welders run on 20A x 110VAC, which likely explains the difference--the transformer ratio that produces 40V from 110VAC would not allow much over 50A welding current. They must use some sort of tricks to get the OCV up... The next level up need 220VAC, which is a special circuit for most of USians. Cheers, James Arthur
Reply by Cursitor Doom September 18, 20162016-09-18
On Sun, 18 Sep 2016 07:40:17 -0700, dagmargoodboat wrote:

> No, that's welding current for common arc welders. Typically ~40V open- > circuit.
More like 50V - 80V. I have an oil-cooled Oxford welder (a big old industrial job I converted from 3ph). Current range output is from 40A - 320A. The OCV is switchable between 50 and 80V; you use the higher voltage for thinner gauge plate. At 50V and 300A it'll comfortably cope with 1/2" thick steel plate.
Reply by mike September 18, 20162016-09-18
On 9/15/2016 6:03 PM, Chris Jones wrote:
> On 16/09/2016 02:14, John Larkin wrote: >> On Fri, 16 Sep 2016 01:38:10 +1000, Chris Jones >> <lugnut808@spam.yahoo.com> wrote: >> >>> It is tempting to make a spot welder from the Maxwell ultracapacitors >>> e.g. the 3000F ones rated for 1900 Amps each. There is something nice >>> about the idea of a spot welder that does not draw large pulses of >>> current from the mains, and a cordless spot welder would be a novelty. >>> >>> I figure that something in the region of 10kA to 30kA output current >>> would make it useful enough to justify the effort. I understand that the >>> welding voltage is normally a couple of volts or so, but if the weld is >>> in an awkward location then cables or tongs might drop more voltage than >>> the weld itself. >>> >>> If you work out the energy required for a spot weld, it is far less than >>> the energy contained in one of those capacitors, let alone several in >>> parallel. Therefore it is going to be necessary to switch off the >>> current somehow. >> >> Why not adjust the voltage on the caps, and just dump it into the >> weld? > > It would force a certain ratio of peak current vs. total energy, which > is not nice, as they have different effects and should ideally be > independently adjustable. Also it would be nice to be able to achieve > arbitrary current vs. time waveforms, as good commercial spot welders can. >
Would be interesting to see your resistance budget calculations, including resistances for: Weld contact area for pristine and not so pristine surfaces. Weld contact area resistance as a function of pressure over the range of CONTROLLED/REPEATABLE pressures to be used. Welded material resistance Electrode to weld material resistance X2 for pristine material and electrode. Electrode to weld material resistance X2 when you don't restore the electrode surface between each weld. Switch resistance Storage capacitor resistance Cable/connection resistance in all the current paths in the loop. Add it all up and, given the cap voltage, you should be able to calculate the min/max current vs time distribution you'll get under OPTIMUM conditions. It goes down hill from there...fast... Making one good weld is not nearly as difficult as making a series of good welds on slightly different setups with different material properties/thickness. I suggest that welding by dumping a low voltage cap is the wrong place to start. Executing the control required to make it repeatable won't be cost effective. Watch out for those little things you forgot to consider. When I was trying to weld with a microwave transformer, the most significant repeatability improvement was to switch the primary current at zero crossings on BOTH ends of the pulse. Inductors are a bitch.
Reply by September 18, 20162016-09-18
On Thursday, September 15, 2016 at 8:47:28 PM UTC-4, Chris Jones wrote:
> On 16/09/2016 05:07, dagmargoodboat@yahoo.com wrote: > > On Thursday, September 15, 2016 at 11:38:13 AM UTC-4, Chris Jones wrote: > >> It is tempting to make a spot welder from the Maxwell ultracapacitors > .... > > > I tried welding with two of those caps in series, just for fun. > > > > Connected with a short length of #12(?) wire for ballast, no joy. > I think you need much thicker wire
Could be, but I wanted to avoid exceeding the caps' ~1kA rating.
> and a lot of contact pressure on the > weld.
Yes, which is harder than you'd think.
> > Regular welders do fine with 80-120A, > That might be the current drawn by the mains input, for a small one.
No, that's welding current for common arc welders. Typically ~40V open- circuit.
> AIUI, for automotive steel spot welding it is nice to have 10kA, and to > spot-weld thick aluminium up to 40kA can be required. For example see: > https://books.google.com/books?id=i0zSBQAAQBAJ&pg=PA96
Nice reference. IM limited E, the difficulty looked to be the resistance of the workpiece itself. I'd hoped to just clamp some less-than- pristine sheet metal and bang it with two short copper electrodes--or maybe do some battery tabbing, in a pinch--but whatever I did wasn't nearly good enough. Too much resistance.
> > ISTM the limitation with these caps > > is getting sufficient amperage through the resistance of the workpiece. > Agreed.
Cheers, James Arthur
Reply by Chris Jones September 18, 20162016-09-18
On 17/09/2016 02:11, Robert Roland wrote:
> On Fri, 16 Sep 2016 10:47:27 +1000, Chris Jones > <lugnut808@spam.yahoo.com> wrote: > >> a lot of contact pressure on the >> weld. > > You want enough force to get good electrical contact, but you don't > want too much force, as you will then squish out the molten metal, > leaving the weld thin and weak. >
Yes, but some homemade spot-welders (like the one I made as a kid) rely on weak contact pressure or dirty workpiece surfaces, to produce a high enough resistance to allow the workpiece to get hot with the weak current available. I found that not to be very repeatable, so somewhat more pressure (at least at the start of the weld) would be desirable. I appreciate your point that more force is not always better.
Reply by Chris Jones September 18, 20162016-09-18
On 16/09/2016 10:35, Winfield Hill wrote:
> Chris Jones wrote... >> What is the cheapest FET per current? > > For cheap, HY1707.
Hrm, yes. Sometimes having people answer my question makes me realise it is not quite the question I was really interested in. I guess I am probably a bit more interested in a small package with a very high current rating, and a manufacturer that I have heard of, and perhaps a little less interested in cheap. Thanks.
Reply by Chris Jones September 18, 20162016-09-18
On 16/09/2016 12:57, Bob Engelhardt wrote:
> On 9/15/2016 8:47 PM, Chris Jones wrote: >> On 16/09/2016 05:07, dagmargoodboat@yahoo.com wrote: > ... >>> Regular welders do fine with 80-120A, > >> That might be the current drawn by the mains input, for a small one. >> AIUI, for automotive steel spot welding it is nice to have 10kA, and to >> spot-weld thick aluminium up to 40kA can be required. For example see: >> https://books.google.com/books?id=i0zSBQAAQBAJ&pg=PA96 > ... > > I have a small MIG (90A max) that runs on a 120v 20A ckt. My 225A stick > welder runs on a 50A 240v ckt.
Yes, stick and MIG and TIG welders usually have lower power consumption than the peak requirement of an industrial type spot welder.
> > My homemade spot welder produces about 2kA (calculated), which is enough > to weld 2 pieces of 1/8" mild steel together. I don't see why 10-20kA > would be needed. Except to keep the time very short & then I don't see > the need to do that. Except for battery tabs.
When I was a kid I build a spot welder by rewinding an old transformer (that had a core about the size of the largest microwave oven transformers), rewound with about 5 turns on the secondary. I could sort of weld things sometimes, if I used only light pressure so that the pieces of steel only made poor contact, so that they had enough resistance to get really hot. When I put a strong toggle clamp on it to hold the electrodes together firmly like on a real spot welder, it wouldn't work at all - the workpiece never got hot enough as the resistance of the transformer windings meant that the current was too small to work with the lower workpiece resistance that results from high contact pressure and clean surfaces. Later on I changed the secondary to one turn of much thicker "wire", but with only about 0.55V per turn, that wasn't enough voltage to weld. My conclusion is that much more power is needed than I had available from that size of transformer. Maybe 2kA would be enough for some things but I don't think I even reached that much.
Reply by Chris Jones September 18, 20162016-09-18
On 16/09/2016 13:17, rickman wrote:
> On 9/15/2016 8:47 PM, Chris Jones wrote: >> On 16/09/2016 05:07, dagmargoodboat@yahoo.com wrote: >>> On Thursday, September 15, 2016 at 11:38:13 AM UTC-4, Chris Jones wrote: >>>> It is tempting to make a spot welder from the Maxwell ultracapacitors >> ..... >> >>> I tried welding with two of those caps in series, just for fun. >>> >>> Connected with a short length of #12(?) wire for ballast, no joy. >> I think you need much thicker wire and a lot of contact pressure on the >> weld. > > Yes, the spot welder I've seen clamped the work like a vise grip. I > think the issue is to get a good contact between the welder and the > metal so most of the resistance is between the two materials so that is > the part that gets hottest. > > >>> Regular welders do fine with 80-120A, >> That might be the current drawn by the mains input, for a small one. >> AIUI, for automotive steel spot welding it is nice to have 10kA, and to >> spot-weld thick aluminium up to 40kA can be required. For example see: >> https://books.google.com/books?id=i0zSBQAAQBAJ&pg=PA96 >> >>> ISTM the limitation with these caps >>> is getting sufficient amperage through the resistance of the workpiece. >> Agreed. > > Isn't it an issue of work piece resistance vs tool resistance? What is > the internal resistance of the caps. Is that the limitation? >
The internal resistance of the caps is small, 0.27mOhm http://www.maxwell.com/images/documents/K2_3V_DS_3001423_EN_1.pdf
Reply by Robert Roland September 17, 20162016-09-17
On Fri, 16 Sep 2016 17:08:10 +0100, Mike Perkins <spam@spam.com>
wrote:

>Isn't that what you require, constant energy?
It sounds plausible, but it does not work as well as constant current or constant power. The problem with a constant energy weld is that the energy will be delivered over a variable time, depending on workpiece resistance. A fast, high current weld will not be equivalent to a slower, lower current weld, even though the energy is the same in both cases. -- RoRo
Reply by Robert Roland September 16, 20162016-09-16
On Fri, 16 Sep 2016 10:47:27 +1000, Chris Jones
<lugnut808@spam.yahoo.com> wrote:

>a lot of contact pressure on the >weld.
You want enough force to get good electrical contact, but you don't want too much force, as you will then squish out the molten metal, leaving the weld thin and weak. -- RoRo