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
The internal resistance of the caps is small, 0.27mOhm
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.
Yes, stick and MIG and TIG welders usually have lower power consumption than the peak requirement of an industrial type spot welder.
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.
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.
Could be, but I wanted to avoid exceeding the caps' ~1kA rating.
Yes, which is harder than you'd think.
No, that's welding current for common arc welders. Typically ~40V open- circuit.
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.
Cheers, James Arthur
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.
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
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