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Tab Welder II Higher V Weld Electrodes DUAL PULSE!!!

Important update: 2-7-08:

Now that the cheapest Capacitive Discharge welder on the market is priced at about $2,500, I figured it is time to build another home-brewed CD welder. 

 I built one about 15 years ago, using about 3 Farads worth of high grade 220,000mfd / 40 Volt (or maybe 63V ?  I don't remember for certain) surplus mainframe computer supply capacitors wired in parallel.  I got them at a great scrap yard in Phoenix ; Davis Salvage for about two dollars each.  

Try THAT today. 
The cheapest I could find  on eBay recently were like 25 dollars each.. Plus $6.00 shipping each.  If you can find some at a reasonable price, by all means get them, rather than the capacitor(s) discussed in the following paragraphs..

The one I built 'way back then' weighed about 85 pounds and didn't work as well as I wanted it to; mostly due to poor electrical engineering and materials choices.

Well, I'm not much better at engineering, and far away from any good salvage and surplus shops, but I HAVE learned a few things, and managed to find all the parts I need at the local Wal-Mart, Radio Shack, and Ace Hardware stores in my little 'Podunk' town, and from eBay.


This first page will show you how to build your own very 'bare bones', but effective, Capacitive Discharge welder for about $100.00.  Much less if you are a good scrounger or packrat and maybe a little more if you don't shop around for the best deals.  In later installments, I'll show some improvements on components, engineering and maybe even aesthetics. 


The key ingredient is a powerful capacitor.  The minimum I'd recommend is 1(real*) Farad.  3F or more would be much better, I think.
Now that good surplus electronics power supply grade capacitors are rare and expensive,  we are fortunate that 'stiffening' capacitors used in car stereo systems are getting cheaper and widely available. 
I got a 1.5 Farad cap off eBay for about 40 dollars including shipping. 

(Most photos can be clicked on for a larger view)

*You have to be kind of careful about buying these capacitors;  There is a LOT of exaggeration of the specifications by the sellers and manufacturers.  I don't have a way to measure it, but I think this one is closer to 1.0 F than 1.5F.  And the supposed '3 Farad' one I got a couple of days ago wasn't even half the power of the 1.5 F above. (HINT: stay away from the Pyramid brand of capacitors.)  The 1.5F in this first installment is a Volfenhag brand. (I've RMAed the 3F and am going to try a 5F POWER ACOUSTIK  brand , {but that is for a future installment}). 

Another thing to be aware of is that most of these car audio Caps are really only safe for a maximum of 16 volts continuous, and maybe a short 20V surge.  That is another thing that the suppliers and manufacturers 'exaggerate'  all too often.  If you can find one that will REALLY run on 20V continuous and 24V surge that is within your budget, get it. (more on that later)

If you are a good 'scrounger', and have a friendly local car stereo shop, you might be able to get a decent 1 to 2 Farad cap, (or a few 1/2F ones to wire in parallel), either free or really cheap if they have any junkers laying around from someone's upgrade or that have 'fried' electronics, but the capacitor itself is still good.

Speaking of electronics:  Most of the audio capacitors these days have a circuit board with various components and a LED voltage readout on them.  While it would be nice to have the voltage readout on your welder, they will at best rob a lot of your welding power, and at worst make your welder not work at all.   So take it off.    The ones I've seen so far, come off easily, just require an allen (hex) wrench and/or a pair of pliers.  No cutting or de-soldering necessary.


The next key component is a DC source to charge up the Capacitor. If you already have a car battery charger, you don't have to go shopping for this part.  For most of this installment, I'm using a Schumacher brand that I have had for several years (still under $20.00 new at Wal-Mart).

It is only rated at 1.5 Amps,  but it works better than I'd expect, recharging the 1.5F cap in about 4 to 6 seconds per cycle.
(the more Amps the power supply puts out, the faster your capacitor will charge and the faster you can do repeat welds.)
And it is kind of neat; an indicator light on this particular charger glows yellow while the cap is charging, and then a green one lights up when it is charged.  That way, you won't try to do another weld with a partially charged cap. 

I have also experimented with using a 16 VDC laptop power supply to charge the cap;  and I like the welds done at 16V better, but even though the laptop power supply is rated at 4.5 Amps, it takes it 2-3 times longer to charge the cap than the lower power car battery charger.

A lab/bench power supply would be great if you have one already or have the budget to get one.

I just got a much higher amperage 12 V (13.8V actually, @ 14 A) power supply that charges the cap really quickly, and have also experimented with a re-wound Microwave oven transformer and rectifier that puts out 18V at about 60 Amps...  (more on those in a later installment.  It should also be possible to slightly modify a desktop, tower, or 'rack' type computer power supply to charge the cap;  I tried it with an old 145 watt one I had laying around, but it just didn't have the 'oomph' necessary; the internal circuit breaker would trip after about 2 seconds.  A 350W or better would probably do the job though.

The next component is a little more esoteric:  a high amperage SCR ( Silicon Controlled Rectifier) AKA thyristor.  While you can get a low power one at Radio Shack, you will have to search further for one that will handle the power you need for this project.  A new one will cost a LOT at a 'brick and mortar' professional electronics supply shop, they can be found on eBay pretty reasonably, although you gotta shop around and maybe wait for a good deal;  some of the sellers try (and sometimes get) near retail for their power SCRs. 

While Philip Pemberton seems to be happy with using a 50 Amp SCR in his ~1/2 farad homebuilt tab welder,, and while it is less bulky than what I'm using,  I think that 100 Amps is the minimum that we want for this project.  I'm currently using a 130A  one I got on eBay for about $25.00 including shipping.  

There are some variations in the specifications of different SCRs (that I don't know enough about, yet), that might give improved performance in this application, but that is for later;  at this point, just grab anything that fits your budget.

It is actually possible to do CD welds without the SCR, but it is tricky to do, and even i you get the technique down, it will be inconsistent and require the dexterity and timing of a professional juggler who is also a concert pianist;  While the SCR does 'waste' some of the power from your capacitor, it makes control so much better.

Those are the three key components: 

Here are the minor parts:

You will also need at least four feet of some insulated, stranded, heavy copper wire.  The current setup is using 10 GA wire I got at my local Checker Auto Parts.  Try your local car parts place or hardware store; should cost 50-75 cents a foot.

However heavier gauge wire is better:  My next upgrade is going to be 8ga super-flex wire.  I got some reasonably priced off eBay along with a bunch of other stuff;  And I also noticed just yesterday at Wal-Mart that they have a 15? foot roll of it in the car stereo department for $30.  And checking the Wal-mart website, they have a very good deal on closeout .  I'm tempted to order some, even though I have more than enough for my current needs. (Current needs...  Ha... No pun intended)

If you have or can get it cheaply, 6 or 4ga welding cable would be great too.  You just need enough to reach from your Capacitor to your handpiece / electrodes.

For your electrodes, you will also need about 1 foot of 4ga solid, bare copper wire. (it is a little less than 1/4 in. or 5mm in diameter. You should be able to get it at your local hardware store, electrical supply store etc.  Ask for 4 gauge copper ground wire.  Should only cost about a dollar a foot, although with the price of copper these days, who knows??.  I already had a bunch I bought at Home Depot a couple of years ago for about 60cents a foot, for another project that I never got around to.  Just straighten it out and grind, file or sand it to a conical point then polish it up a bit with some steel wool or a buffing wheel.  Make two electrodes, about 6 inches long each.  (I'll cover different electrodes and ideas in the future, but for now, this will do.)

I used tungsten TIG welding electrodes in my original CD welder;  While they do have their place in CD spot welding, I find the heavy copper electrodes much more efficient and effective for general work.

You will also need some heavy duty connectors to connect your cables to your electrodes etc. For now, get at least three 1/8 inch galvanized cable clamps;  I paid 40 cents each for them at my local ACE Hardware.  They are neither elegant nor very efficient electrically;  there are much better ways to do it but they are cheap, they DO work, and are easy to find and to use.  I'll cover some of the better solutions in a later installment.

Then, something to insulate the electrodes and provide a better grip.  While the voltages we are dealing with aren't enough to cause much more than a tingle even if you grip the bare electrodes with your bare hand, better safe than sorry.  I used some Delrin (acetal) rod that I had laying around, drilled out and sanded down a bit.  Just because I had both colors in my 'raw materials rack' I used white on one and black on the other so I could have a nice reminder of which electrode went to which pole of the capacitor.  You could use a drilled out wood dowel, plastic, rubber, several wraps of electrical tape, heatshrink, whatever is handy and non conductive.

You will also need a momentary electrical switch of some kind.  Low volts, low amperage is fine.  Even though I have a good 'professional' grade foot switch that I will be using eventually, I wanted to put this first setup together with cheap and easy to find parts. so I used a $2.00 1.5A pushbutton switch from the local Radio Shack.

And some way of mounting the switch so that you can set it on the floor and press it with your foot.  I used a plastic 'project box' that I had laying around.... you could use a short piece of 1X3 or 2X4 wood with a couple of holes drilled in it or whatever you can find around the house.

You will need a battery and a battery holder (or maybe a low voltage 'wall wart' AC to DC transformer?)  to activate the gate of the SCR.  the gate on MOST SCR's operate on 1.5 to 2 volts DC.  Some on 3V, some on 5V.  Check the specifications on yours to be sure.  The one I'm using now works just fine on a AA battery.  I put the battery and holder in the project box next to the foot switch, since it fit there easily, but you can mount it anywhere as long as it is in the right place in the electrical circuit. 

And a 4-5 foot piece of two conductor hookup wire; even if you aren't a packrat like me you can probably find a piece laying around.  Anything in the 22ga. to 16ga. range will do. Anything thicker would be overkill, and anything thinner would break too easily.


A few terminal connectors ($1 worth)  will make your wiring neater, bur aren't absolutely necessary.

And that is about all the parts you need for the crudest, most basic setup.  There are dozens---heck,  hundreds of tiny to major improvements that could be made, from little or no cost, on up to tens and fifties of dollars,  and I'll cover many of them in future installments.

One more thing you will need:  Battery tabbing material.  The best thing is nickel strip from .003 to .007 inch thick.  There may be better, cheaper sources, but so far, the best one I have found is:

If you have the money to spend they will sell you a complete CD welding setup from that page, but if you are going to build your own, they will still sell you the nickel strip.  It comes in coils and precut strips. they have 4 sizes listed.  

For welding Sub C, C or D cell NiMH or NiCd  batteries, or Lithium-Ions of 18 mm diameter or larger I recommend the 0.25"w x 0.005"t x 25'L spooled nickel strip.  

For smaller stuff, the 125"w x 0.005" or 125"w x 0.003" will do.  They also have some 0.5"w x 0.005 for even heavier duty applications.  My first order got here in about three days, so pretty good service.  Although I've been waiting for a week for a return email from their tech department as to what the recommended maximum current / wattage capacities of the various strip sizes are.  I did purposely short out one piece of the .25X.005 about 3 inches long  for a moment with about 6v & 100 amps, and got a nice bright red heat and no meltdown.

Another handy thing to make is to drill out a scrap piece of 2X4 or 2X6 or 2X8  wood with holes about the same diameter as the batteries you are welding.  It will hold the batteries steady while you weld, and if you space the holes right, you can quickly weld just about any configuration of battery packs.

Here is the whole thing assembled and ready to weld.  (I'll draw up a circuit diagram soon, and post it, but I don't have time right now.)

Here is the detail on hooking up the  SCR and the negative electrode:


The way it is set up now, the SCR will keep draining power after the pulse, even if you release the foot switch. So you need to break contact of one of the electrodes manually, very quickly after the welding pulse.

Not a major problem, just something to be aware of. Might be kind of hard on your power supply if you maintain full contact for more than a few seconds.

Wouldn't be a problem if we were dealing with AC, but under DC, the SCR works like a latching relay.


Here is my childish drawing of the circuit schematic for the crudest version:

Sorry I couldn't make it look more professional, Drawing is not one of my competences.


You will, of course, insulate all of the exposed metal, (except for the very tips of the electrodes),  with electrical tape, heat shrink, self vulcanizing rubber tape or something safe, and submit it to Underwriters Laboratories for safety approval before you plug it in and try to use it, won't you?

If you short it out, anywhere other than the electrode tips on semi-resistant material (steel, nickel etc.) you will get a big spark, and possibly a major meltdown, fire, death and destruction.

Just because I don't show something or write about it or am completely ignorant about it myself, doesn't mean it should be ignored.

Lithium batteries especially are inherently hazardous.  Understand the chemical and electrical characteristics of them before you even touch one, much less try to weld one, charge one or use one in any device not specifically designed for the specific battery.  They can blow up, burn, stink, release poisonous gasses, fluids,  particulates and solids.  If you swallow one, it will probably kill you in several unpleasant ways.  Do not stick them in any other bodily orifice.

Nickel Metal Hydride, Nickel Cadmium,  Alkaline, Silver Oxide, Carbon Zinc, Zinc Air, and Lead Acid batteries are only slightly less dangerous in general.  Don't abuse them either.

It is very likely that I forgot something important;  So unless you absolutely know what you are doing around electricity, electrical components, hot and sharp tools, ask an expert licensed electrician and/or electrical engineer.  Make sure that all your wiring is up to code, and get it inspected by your local code and zoning authorities.  If disaster befalls you from following the previous and following instructions, don't call a lawyer and try to sue me.  I spent the last of my money on parts for this thing and on batteries.  I'm broke, and there will be nothing for you or your lawyer.

Here are a few photos of sample welds with this setup:  Hopefully they are all self explanatory, because I'm tired of typing and just plain tired; 

I highly recommend you practice on a bunch of old, dead, alkaline batteries before you try welding a good NiMH or NiCd battery.  And even then, practice on a couple of junk NiXx  before you try the good ones.

Then very carefully try a few old LiIons if you want. the smaller LI Ions seem to have very delicate positive terminals, and are easy to bur through if you use too much power. 

NEVER try to weld 'through' the battery with say, one electrode on the + terminal and one on the - terminal.  The closer your electrode tips are to each other, on the same terminal, the better as long as they don't contact each other.

Some of the photos and demo videos of CD welding on the web (like on the Sunstone engineering website, YouTube etc.) show both electrodes on the nickel tab material when welding.  So far, I have found that, in general,  this setup gives better welds if one electrode is on the terminal and the other on the nickel.  Experiment with which polarity of electrode is in which position.  It seems to make a difference, although I haven't been able to define it precisely yet.

So far, I haven't been able to get ANY decent welds of any material on aluminum with this setup.  One of the newer high-performance Lithium batteries  (A123? Emoli?)  is supposed to have an aluminum 'can', so this setup probably won't work on them.  But we can keep experimenting and may figure out a way to do it.






That's all for now.  I'll try to get the circuit schematic drawn up soon, and then start describing some of the upgrades, done and potential.  and any other related ideas that come to me.

But this should get you started on the right track.

Update #1: 11-11-07

A few new things, not unexpected, but confirmed after more experimenting;

The new power supply / charger makes recharging the cap go nearly as quickly as you can place the electrodes for the next weld.  I opened up the case and found a trimmer potentiometer that allows some adjustment to the output voltage.  Adjusted it to the maximum, so I can quickly charge the capacitor at 14.2 V now. 

The 8ga wire makes a noticeable difference over 10ga. And the super flexible stuff makes manipulation of the electrodes so much easier and more pleasant.

The more amps your SCR can handle, the better. The lot of new SCRs I bought on eBay came in. There were four in there that the seller thought were probably 225A.... I'm guessing those huge things are at least 300-400A, but I can't find any info on the internet using the parts number to confirm. But they make a noticeable improvement in the welds over the original 130A SCR.

Also tried paralleling a couple of 110A SCRs. That works very well too; So, if you have a few lower amperage ones laying around, rather than searching for a reasonably priced big one, you can probably get by by paralleling them. (Although I think ...Guess...that it is probably important that they be all the same part number, or at least the same specifications in all respects.

I changed the position of the SCR in the setup by mounting / connecting it right on the negative terminal of the capacitor rather than on the electrode end of the wire.  That makes the welder work a lot better and makes the  electrode easier to manipulate too.

Drilling and tapping a 1/4in bolt hole in the end of the stud, if you are using a large stud mount SCR, makes it much easier, neater (and probably more electrically efficient) to make connections.

With all of the above improvements I was able to tack weld two pieces of 1/4 inch thick titanium together, with just the 1.5F cap with 14.2V charge. It wasn't a 'structural' weld by any means, but it was strong enough to survive modest handling and a slight amount of torque. Two 1/8 " thick steel plates tack welded together much better than I would have imagined.

Battery tab welds were just about perfect, (at least by MY criteria).

I tried to quickly draw up a circuit diagram, but as exceedingly simple as the circuit is, it looked  really amateurish. So, it is either wait until I have the leisure do try to do something better, or settle for something that looks it was scribbled by a two year old with crayons.

I found a formula for converting farads and volts into watt-seconds (or Joules;  or newton-metres  they all are the same thing) ws=(F/2) X  VČ
Which means that the setup I have now should be (if the farad specs are not vastly over rated) putting out about 140- 150ws.
1.5F/2 X 14.2VČ = 151.23ws  
That is of course the theoretical output;  there are probably losses due to resistance and inductance.


Someone over at one of the EV forums  ( ) got me thinking about using higher voltage and smaller capacitors....  While the basic principles and formula remain the same, there must be an optimum voltage point, although the materials you are trying to weld would be a significant factor.

Years ago when I worked in an aerospace repair facility for a while, the industrial Capacitive Discharge welder there was (if my memory is anywhere near accurate) adjustable from 30V to 40V, with output between 100ws and 225ws.  The one I built long ago ran at 25 volts or so, (again, if memory serves).

Looking closer at the specs of the ones that Sunstone Engineering sells, it looks like their maximum voltage is 16.5, so I don't know what really is the optimum balance between volts and farads and materials

Anyway, Since I haven't been able to spot weld to aluminum with my CD experiments yet, I'm wondering if it might be possible to make a cheap CD welder for doing the aluminum cans of A123 cells by using just a couple of 220,000 mfd higher voltage capacitors charged at around 40 volts. Will have to try that if I can find the caps without spending a fortune, and rewire the secondary of a microwave transformer for the power supply.

More photos coming soon and hopefully the wiring diagram.

Update #2  11-14-07

Well, at least I got the circuit schematic drawn, (such as it is).  Posted above in the earlier description, or click the link.

Update #3  12-3-07

I've improved the system a bit;  A new power supply, a new capacitor, a few design refinements and significantly better welding results. 
You should be able to build a much better unit and still keep the cost under $200.00.  Click the button below to tale a look:

UPDATE#4  2-7-08:Just a heads up:

Had a 36V LiIon battery pack fail 'on the bench' last night.

Took it apart;  (Tubular battery column of ten 32650 5AH cells, inside a tube) to find that one battery tab melted through, (at a sharp 180 degree fold, if that is significant) and two other spots on different batteries were intact but singed the plastic insert around the + nipple of two of the  cells.

Was using the 1/4" X .005 Sunstone Engineering nickel tabbing strip. I don't think I pumped more than about 700 watts through it, and then, only for a moment.

So, for those of us building Lithium or NiMH battery packs for e-Bikes, scooters or anything that uses a lot of watts, it would probably be wise to either use double thickness of the 1/4 X .005 strip, move up to the 1/2 inch wide .005 strip, or try to find a source for .007 or thicker tabbing material.

Fortunately the batteries all seem fine, and all the welds held perfectly.


Tab Welder II Higher V Weld Electrodes DUAL PULSE!!!

Please don't try to contact me for more information.  Everything I know about this is covered on these pages.  For more ideas and information go to:  The guys over there have expanded on my design and tried different approaches.  Maybe you can find out what you want to know over there.


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