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Spot Weld Anything (Even Copper)!

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There's hundreds of welding Instructables around, many of them quite good. I'm writing this one here to share a technique I've developed that works quite well, even on copper tabs (for which it was designed).

The theory is simple- I've got a handful of ultra-capacitors (2.5 Volts, 2600 Farads) wired in series, then discharged through the material. The devil is, as always, in the details.

Anyone who's tried will tell you copper is a pain to weld- for many of the same reasons you probably *want* to be using it in your project. It's got about five times the thermal conductivity of iron, which basically means you need to pump in a lot more heat to overcome heatsinking. When it comes to resistance welding (which is how spot welding works, unlike other electric welders, which use arcs), you've got even more pain coming- the conductivity is another 5 or 6 times higher. So your welder needs to output 5 times more current just to create the same amount of heat.

Put all this together, and it means you probably need a welder that's over 20 times more powerful than a typical steel spot welder.
 
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Step 1: Bill of Materials

I'll hardly claim that I've got the best setup here- but it does work, so this is probably a good place to start. To replicate my setup, you'll need:

* 4 ultra capacitors.
-I used Maxwell "Boostcap"'s, rated for 2.5V at 2600F. I believe mine are surplus from electric buses regenerative braking systems. I got mine for about $10 each from the Electronics Goldmine (http://www.goldmine-elec.com/), but it seems they're sold out at the moment. You can find them on eBay, too. Make sure they're rated to have low series resistance (in other words, they should be able to provide a lot of current- mine should do 600 amps).

* 3 lengths of heavy gauge wire, with suitable ring terminals on each end. I used scrap I had around- I think it's about 6 gauge, roughly 2 feet long. Each is a different color in my setup.

* thick copper tubing, about 3/8" ID, 1/2" OD. You need two pieces, each about 3 inches long.

* a graphite block(s), enough to yield two pieces about 0.5"x0.5"x3.0"

* two big hunks of copper. I used a 1/2" plate roughly 5"x5", and a cube about 3" to a side. It's not critical.

* Some sort of DC power supply- I used a standard benchtop variable DC supply. It's limited to 3 amps, so charging takes a long time. You could do much better.

* Some means of clamping your electrodes: I used some G-10 fiberglass stock (not the best choice, but fairly temperature resistant and non-conductive), with holes drilled for the electrodes, and a rudimentary hinge. Again, you could do better than my quick-and-dirty solution.
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wuj124 months ago

Kinda like using a carbon arc welder

dsandds20032 years ago
Most of thes welders i have seen use a 12 volt power supply. They also a kick box capacator 1,2 or 3f @ 12 volts. They also use a triac to charge the capaciator for a slpit second.
Just some added thoughts for this idea.
I am planning on building one myself soon. Beem trying to fing a resonable priced capaciator for the project as the 1f caps are NOT cheep. Had thought of trying some older computer caps, but rather use 1 cap.
This is a great idea as i could use one for some of the repair work i get.
The tips are another problem finding, Have seen some use thick copper ground to a point. Still working on some other items to use instead.
Keep up the great work and keep us posted on your progress!!
static2 years ago
Personally I would have start using only one capacitor,adding additional capacitors in parallel until the desired results where achieved. Reiterating a comment by another by wiring the capacitors in series you have reduced the total amount of capacitance available, I agree with bobrigewitch's math. Capacitors block DC, so there is no current flowing through your series connected capacitors. While you should see the supply voltage across the capacitor bank, you would see a voltage drop across the individual capacitors. With those capacitors in parallel they wail contain  10400 joules of energy when charged. When connected in series they will contain 2031.25 Joules, both figure calculated ar 2.5 volts. The parallel bank will deliver 2031wats over 1 second, the series configuration 30 watts. Placing trust in available on line calculators.
HarnessedDevilry (author)  static2 years ago
I'd recheck your calculations. The rules for adding identical caps in parallel is that the capacitances add. the rules for adding them in series is that the capacitance decreases to 1/x (where x is the # of caps). The upside is that when you series them, the voltage limit goes up by a factor of x. The energy stored is 0.5C*V2, so you better have conservation of energy if you take 4 charged caps and put them in series- and that's exactly true. In parallel, you're multiplying by 4; in series, you're multiplying by 42, then dividing by 4. So in both cases you've got 0.5*2600*4*2.52= 32500 Joules.

I'd suggest rereading the hyperphysics and wikipedia articles on capacitors:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capac.html http://en.wikipedia.org/wiki/Capacitors

"Capacitors block DC" is a general rule of thumb that fails in this instance. When your physics book says this, it means that they'll block DC over a suitably long timescale- enough for the cap to charge or discharge (i.e. timescales greater than R*C). see, for instance, http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html#c1
klixtopher2 years ago
Nice 'Ible. Really like the use of graphite. One note of correction though. You mention that there not enough voltage to injure. That statement is simply not correct. It's not voltage that kills but amperage. There's enough power in a AA battery to kill you, but due to factors of resistance etc. you can handle them safely. Stun guns and Tazers operate off of simple household batteries and deliver extremely high voltages without (necessarily) being lethal - due mostly to the amperage being so low. While increasing voltage usually means more dangerous, it's really the amperage that'll get you in the end. Thanks again for the 'Ible.
HarnessedDevilry (author)  klixtopher2 years ago
You're right that small batteries have enough power to harm; however, there is absolutely no way to accidentally release this energy in a form that can be harmful. The devices you mention actively "step-up" the voltage from a harmless 1.5 V (in the case of AA/AAA batteries) to many hundreds of volts.

However, saying that this makes a AA potentially lethal is akin to claiming that a hunk of steel is a deadly weapon because you might be able to turn it into a gun. This is strictly true, yet absurd. In both cases you must actively take several complicated steps, impossible to perform ignorantly or by accident, in order to create this hazard. My system physically cannot pose more of an electrocution threat than a standard 9V battery.

Now, there's a large risk of accidental heating (e.g. you could badly burn yourself by touching a metal watch band or ring across two terminals), but that's not what you're talking about.
I wasn't trying to say that your project necessarily posed a threat, just that your rationale for why it didn't was flawed. It's clear you have a good grasp of what's going on. I just didn't want others to think that low voltage = no danger. Cheers.
In context of this project, "the voltages you're dealing with here are not enough to hurt you via electrocution" is correct, and personally I wouldn't have brought it up. Respectfully I'm finding variations of "it's the current that kills, not voltage remarks to instructables rather tedious by now. Generally the statements appear to be mindless parroting of something another that "sounds official", however you seem to have a clue as to what will lead to a hazardous condition.
Do you think such an apparatus could spot weld wires/contacts to a NiMH battery? How would you set it up?
I was simply going to solder wires to replacement cells for a cordless drill but the battery vendor scolded me, saying the heat would hurt the cell chemistry. He said they'd rebuild my pack using spot welds. I learned later that this repair would set me back twice what the whole drill kit was worth.
I think battery tab welding is probably the most useful potential application with this sort of welder, although I haven't tried it yet- it requires a rework of my electrode holder I haven't been able to do yet.
Do you think set-up would be one electrode clamped to the battery on the pole of interest and the other arranged to squeeze the tab to the battery during the actual weld?
If so, the negative end shouldn't be too tough but the positive will take a bit of thinking.
Just took a look...we're not the first to do this stuff. The following link is an interesting practical explanation from a company that makes welders:
http://www.youtube.com/watch?v=GGTGIlT6JvM&feature=related
Based on this and the other links I surveyed, your machine adaptation is less hard than I thought it might be.
NiCAD, NiMH, and many other battery variants all use multiple cells, almost always welded in commercial products. This sure as hell ain't the first battery tab welder, but I believe one of few that can handle copper, and that doesn't cost >$10k. You'll note they use nickel in that video (much easier to handle, but not as good)

I'd do it the same way they do- my same two electrodes, mounted so they are nearly parallel, i.e. with the tips very close to each-other. . Mount the whole thing to a spring-loaded arm that swings down, pushes the tab against the battery, and switch 'er on. Ideally with some precisely timed pulse triggered via foot pedal.
Have you tried the core of batteries or electric motor brushes for your contact ends?
Certain types of batteries have graphite cores, which would probably be acceptable alternative sources of graphite. I'm not quite sure about electric motor brushes. Either way, you probably want the heavy copper running as much of the circuit as possible. The whole system needs to have total resistance in the milli-ohms, so every last bit of resistance matters.
thewmas2 years ago
I have some gouging rods that I got from a friend, I would think that woud work as well. Give me feed back if you think so
copper clad 002.jpg
HarnessedDevilry (author)  thewmas2 years ago
Other people have also suggested using these; I'm not quite sure. I'd suggest giving them a try, and if they don't work, just use my system. It's not too bad to make them yourself. Let me know if they work!
You're going to have to forgive my ignorance. Just what, exactly, is a "gouging rod", and how is it used?
thewmas kmpres2 years ago
http://www.engineersedge.com/materials/graphite_gouging_rods.htm

you might have to copy/paste, or just google gouging rod uses
skrubol2 years ago
You could run Zeiner's in parallel with each cap to limit the voltage across them. Picking the right value to successfully protect the cap without limiting your voltage too much would take some math though.
evangill2 years ago
Why did you choose to wire the caps in series?
I had the same question. I don't know much about welding, but wouldn't having the caps in parallel allow for better release of current? Also, wouldn't this solve the issue of potentially overcharging the caps you talk about later?

My only thought is that you need a higher voltage than 2.5 V for some reason...
You need somewhere around 1-3 volts in the workpiece, but due to resistance of the leads and the ESR of the caps, you need a lot more voltage coming from the power source.
lperkins2 years ago
The best thing to do would probably be to set up a switch system to let you charge them in parallel and discharge them in series. A spark-gap setup might work for that, but you'd have to try it.
HarnessedDevilry (author)  lperkins2 years ago
The "charge them in parallel, discharge in series" approach had occurred to me, and it does a great job at killing the differential charging problem. However, I think it introduces two problems:

First, power supplies are generally more capable of handling volts than amps- it's easier and cheaper to source 20V @ 2A than 2V @ 20A.

Second, the switching mechanism is ugly due to all the amps you've got. No normal knife switch would handle this sort of thing- you'd weld the contacts. My impression of spark-gaps is that they work for high voltages, not high currents, so I'm not sure how much use they'd be here.
Spark gaps would definitely work better for high voltages than high currents since you need enough volts to bridge the gaps. If you could find a dielectric to go between them that lost most of its resistance once current started flowing and regained it once it stopped that might work though.

To make a switched version you'll probably need to make your own switches. You want the contacts to have a large surface area, and grease them with a good, conductive grease to help prevent them from trying to weld themselves together.

You could, of course, simply modify your current connection bars to be easily removable for the discharge half of the switch. The charge half could be standard 20A switches since I'm sure you can keep your charging current under that.

There is another project running around building a spot welder out of a microwave transformer. If you have one of those lying around you might be able to tune it to output the voltage you want, and I'm sure it could probably handle the charging current.

Cool project. Could you please translate "mils" into metric please :-)
As far as I'm aware, 'mils' is metric - just short for millimeters (or 'a lazy way of saying' that, as Waste Of Space posted 17 minutes after you).

However, that would suggest that HarnessedDevilry is claiming to be able to spot-weld 2 *1-inch-thick pieces of a metal that "has about five times the thermal conductivity of iron" AND "5 or 6 times" the electrical conductivity!

My limited (ie. practically non-existent) knowledge suggests this is not likely, so I plugged '25 {mils}' into Waste Of Space's first formula, above, and got a thickness of 0.635 millimeter(s)...
HarnessedDevilry (author)  karlpinturr2 years ago
Sorry for the confusion. A "mil" is one thousandth of an inch (0.001").
russ_hensel2 years ago
Any hints on where to get *graphite blocks.
HarnessedDevilry (author)  russ_hensel2 years ago
I got mine on eBay, as scrap blocks. just do a search for "graphite stock". I'm sure there are other, more reliable sources too.
Electric starter motor (DC) brushes are carbon blocks and I have seen carbon brushes in some electric hand drills too.....
You might want to look into "carbon arc electrodes". You can get 50 in a box and they're already cylinders.
"Heavy Duty" batteries (not alkaline) use carbon rods as their centre electrode. Wear gloves for disassembly and clean them thoroughly.
Graphite rubbing blocks are used a blade guides on bandsaws - you can usually fing them near the replacement bandsaw blades
Very cool concept. I'm glad to hear it works good. Although I am also curious as to why the caps are in series. Capacitors in series effectively increases the plate distance, therefore decreasing capacitance. (same rules as resistors in parallel) so your actually only getting 650F. And paralleling them would give you your expected 10,400F, capable of handling more amperage (the heat generating portion) AND make charging easier. If their 2.5V, supply 2.5V to them and you can never overcharge them (a cheap analog voltmeter might be a good idea to monitor voltage). Other than that great idea and proof of concept :).
kmpres2 years ago
Needs refinement, but nice proof of concept. I like the ultra cap idea, thanks for the source information. They're safer and less toxic than using lead-acid car batteries and cannot fail catastrophically (explode) if accidentally overcharged. They will, however, degrade to an open circuit depending on the extent and duration of the overcharge. The zener diode idea of wa7jos is good as well, as is his suggestion to use it to stop the charge current entirely when the max voltage is reached. That will require a cut-off circuit of some kind, like a relay on the power supply's mains line, to work effectively. If done correctly that could work as a safety fuse allowing you to adjust your regulated power supply to any lower voltage you desire without risking an accidental overcharge. And your MOSFET / 555 variable one-shot switch idea is very good. Far better than the hot-rock-copper-brick approach (got a good laugh out of that one!). Now that we know the concept works, can we see a final product soon?
lperkins2 years ago
It's not volts that cause electrocution. It's amps. Unless you have a pacemaker or a weak heart, of course. It only takes .1 amps across the chest to stop your heart if you're unlucky.
leven lperkins2 years ago
But as ohms law clearly states you will need volts to get amps. and as a typical human body has many thousands of ohms of resistance voltages in this range is unable to drive any current trough you, unless maybe if you come directly from a bath of salt water..
rkrishnan72 years ago
For those who may not have access to a power resistor to safe-discharge the caps slowly, I'd suggest using a soldering iron as a bleeder load. May take a little time, but will do the job gently. Higher the wattge of the iron, quicker the discharge. (You could easily calculate the discharge time knowing the RC time constant, the 'R' of the iron being dependent on it's wattage) I recall working on a old IBM 3203 line printer back in the 80's which housed a huge bank of caps delivering a lot of amps, and at 60V, and the power on sequence failed because of the residual voltage not being bled off - got it working by dropping the voltage by discharging thru a soldering iron!
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