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spinrow
04-28-2007, 10:09 PM
Looking for information on how to build a capacitive discharge welder. Any plans or references? Thanks Paul

ptjw7uk
04-29-2007, 01:09 PM
Ive been getting the components to build one but the capacitors is the main problem thay need to be the same as flash gun caps able to give all their charge in a very short time. Also unless you use a high vaoltage you need a lot a micro farads capacitance. For small scale components they are the bees knees.

Peter

Brian H.
04-29-2007, 03:41 PM
You might try this site:

http://www.philpem.me.uk/elec/welder/

The welder shown is mainly meant for light-duty jobs such as battery tab welding (or small-gauge sheet), but it is a starting point, at least. I want to build one myself, but I haven't found any capacitors yet.

-Brian

nheng
04-29-2007, 05:51 PM
You can get 100,000uf / 6.3 volt electrolytics from Digikey (probably similar from Farnell in the UK). About $7 each, parallel them with heavy copper bus. Den

darryl
04-29-2007, 07:22 PM
Consider parallelling a number of capacitors to get a better pulse current capability. You should already be considering a capacitor type rated for fast discharge.

J Tiers
04-29-2007, 10:40 PM
Welding goes by ENERGY.... that is what heats the material.

The higher voltages store more energy, but it may not be as easy to get it into the material. The relation for energy storage is 1/2 of (the voltage squared x the capacitance) ..... 1/2(C x V^2).

I would NOT suggest anything like 6.3V.... you probably want something in the 30 to 50V range.

And, yes, many capacitor types are rated for fast energy discharge, but they MAY not be required. The fast discharge ones for flash etc are "sorta" rated differently...... While they are in fact made a little different, they ALSO are rated for "X" number of discharges..... and for flash, that is fewer than you think, possibly in hundreds or a few thousands only. They are NOT necessarily made to withstand semi-industrial use.

It may be as well to get cheaper types and figure on replacing them. The discharge will be a little longer, and not quite as intense, but on the other hand, you won't have as much $$ in them, and can replace sooner and use more.

Also flash caps are rated for 300+ volts, and not a lot of microfarads. If you use lower voltage (good idea from a safety standpoint) you will have far less energy storage. A 300V unit used at 75V instead of 300V will use only 1 /16 of its actual energy storage capability..

A.K. Boomer
04-30-2007, 07:55 AM
Discharge is only half the equation, capacitors are kinda a Battery bank, its not only important to know how fast you can discharge them --- in the case of useing them for welding its important to know how fast you can "charge them" if they take along time to build back up then they are useless for welding...

J Tiers
04-30-2007, 12:53 PM
Discharge is only half the equation, capacitors are kinda a Battery bank, its not only important to know how fast you can discharge them --- in the case of useing them for welding its important to know how fast you can "charge them" if they take along time to build back up then they are useless for welding...

The charge time is definitely NOT a direct function of the capacitors........

It is a matter of the power supply available to charge them. You can charge nearly any capacitor fast enough to work, there are no useful types that inherently take too long to *charge* AND will *discharge* fast enough.

Where the issue that MAY be important will come in is with the "duty cycle". That is how many charge-discharge cycles occur per minute, etc you are going to demand.

If you have one every 10 seconds, that is one thing. If you want many times per second that is likely an entirely different thing.

The primary way that issue shows up is in the "rms ripple current" in the capacitor, and the resulting heating of the capacitor. There is some resistance in the capacitor, (known as "ESR"), and current dissipates power in that resistance. A fast repeated pulse train of charge and discharge may equate out to a current that will overheat the part.

The "rms ripple current" rating will give you a clue to the likely issues there. The only difficulty is that ripple current rating depends to some extent on frequency, for a number of reasons, with higher frequency generally showing a higher current rating. So to "push" the ratings, you need to know the effective frequency content and rms current of your pulse train.

But for intermittent pulsing, that will not be your primary issue.

spinrow
04-30-2007, 10:37 PM
Thanks for your replies and references. What is the material used to join batteries together? Paul

Wirecutter
11-28-2007, 01:05 PM
This is a topic I'm interested in (and currently working on), and I thought I'd give it a "bump".

One common material for joining batteries is nickel or nickel-steel strip. You can buy rolls of the stuff. The idea is to use something slightly less conductive than the (usually) copper top of a battery. That way, when the two electrodes parallel to each other pass the weld current, at least some of the weld current will prefer to go through the strip, into the battery top, and back up to the other electrode. (And of course, you don't want to try to pass the weld current through the battery)

Generally, if you're welding good conductors, you want your electrodes to be not-so-good conductors, and vice versa. Oxygen-free copper is a good choice for electrode material when welding nickel strips to batteries.

The weld head that I'm using has a spring-and-microswitch arrangement inside. The spring tension is adjustable. When enough downforce on the electrodes is achieved, the microswitch closes and is used to trigger the weld. This makes sure the downforce is consistent weld after weld, and allows for predictable results.

What I have right now is a bank of 80 capacitors, each of which being 4500uF at 25v. (about .3 Farads) They're connected together with a bunch of 1/8"x1.5" aluminum strap. The previously mentioned microswitch triggers a couple of big honkin' SCRs connected in parallel, and they serve to transfer the capacitors' charge to the weld head.


It's kind of a Frankenstein arrangement at the moment. I've just run some test welds, and I can get a decent connection with the caps charged up to around 10-12v. I'm considering many improvements, including implementation of a dual-pulse setup. I need to improve the "operator interface" and take some photos.

-Mark

ptjw7uk
11-28-2007, 01:16 PM
I have read (long time ago) that you have to be careful with the SCR's fusing current. In that respect the resistance of the joint being made should be as low as possible and the turn on pulse kept as short possibly using a pulse transformer to get the SCR on as quickly as possible.

Peter

Swarf&Sparks
11-28-2007, 01:24 PM
If you want to dump some serious current (switching arrangements being equal) try a car battery or 2 :D

ptjw7uk
11-28-2007, 01:39 PM
Only problem is welding the leads to the job!! and then being unable to switch it off. Which will be very dangerous as there's a lot of power in car batteries except when trying to start the car in freezing conditions!!
Peter

Wirecutter
11-28-2007, 07:14 PM
Naw, the big batteries are reserved for the gokart.

-Mark

macona
11-28-2007, 07:19 PM
I have worked on the capacitive discharge welder they use to weld threaded studs onto panels. They are a low current variable voltage DC supply from about 50v to 200v. Couple big electrolytics are charged and a big hockey puck SCR is triggered that dumps the output to the gun. Dont know the rating on the caps as they were not marked but they were about 4" in diameter and about 8" tall. Two in parallel.

darryl
11-29-2007, 02:16 AM
I recall a mosfet amplifer that was being demoed to me one day. The guy said that you could actually weld with the output. I offered to bring a couple pieces of metal out so he could give a demonstration, but for some reason he declined. :) At any rate, there was an extremely large current capability there, if you added up the number of mosfet devices and multiplied by their current capability. Which is my point- today there are mosfets that can handle enormous current, and by parallelling several you'd get a switch that could be up to the job. That switch can be turned on AND off, so it could be used with a constant dc source (a car battery). The turn on pulse would have to be a low impedance source for fast turn on and off of the gate drive voltage, and I suppose you'd want to limit the peak current by selecting the appropriate gauge and length of connecting cable.

Of course, if the hockey puck scr has a turn off lead as well as its trigger lead, then it would be capable of working from dc as well.

batt-man
11-29-2007, 02:45 AM
something i know a little about... :)

i do a lot of work with a battery company that has several welders like this (if your interested i can get some pictures over the weekend of the machines)

One of the big things for consistency of weld is the pressure applied by the welding tips. These machines use compressed air + regulators to control the force applied by the welding tips; plus or minus 2psi can mean the difference between a good weld and one that either blows a hole in the top of the cell tab or pretty much falls off (this is when welding cell stacks of 18650 li-ion's)

Another "biggie" is the shape of the electrical pulse - these machines use some big igbt's to enable them to put out a very sharp high-current pulse. With a bit of electronics to make a pulse generator you can very accurately control how much energy is dumped into a joint. It also means your not normally completely discharging your capacitor bank.

Obviously these are expensive (something like 2500 a pop) bits of precision kit but hopefully someone might find it helpful...:D

cheers

Evan
11-29-2007, 07:44 AM
today there are mosfets that can handle enormous current, and by parallelling several you'd get a switch that could be up to the job.

Yep, a trivial design exercise compared to what can be done. We have a LORAN C station about 30 minutes drive from here. The output of the final amps is a 500 kilowatt pulse train at 100,000 hertz, eight or nine cycles of carrier per pulse train switched on and off at ten to twenty pulse groups per second.

They used to use a bank of 4PR1000 vacuum tubes in the final amp which are each the size of a small garbage can. It's all solid state now.

Wirecutter
12-01-2007, 12:26 PM
One of the big things for consistency of weld is the pressure applied by the welding tips. These machines use compressed air + regulators to control the force applied by the welding tips; plus or minus 2psi can mean the difference between a good weld and one that either blows a hole in the top of the cell tab or pretty much falls off (this is when welding cell stacks of 18650 li-ion's)

Another "biggie" is the shape of the electrical pulse - these machines use some big igbt's to enable them to put out a very sharp high-current pulse. With a bit of electronics to make a pulse generator you can very accurately control how much energy is dumped into a joint. It also means your not normally completely discharging your capacitor bank.

Obviously these are expensive (something like 2500 a pop) bits of precision kit but hopefully someone might find it helpful...:D

cheers
Well, I didn't pay too close attention to the pulse shape. Perhaps as a future improvement.

As I mentioned, I'm working with about 1/3 of a Farad. I got some electrode material, and spent time yesterday machining a couple of electrodes. The electrode contact areas are a couple of squares .050" on a side. That's a larger area than earlier test welds, so I had to turn up the power. Now I charge the bank up to about 18.5v. (For the math nerds, that's 1/2CV^2, or about 60 watt-seconds.) I did a number of test welds putting a nickel strip on the top of an old D cell, and they looked good. When pulled loose, the weld nugget stayed on the battery and ripped holes in the nickel strip. That seems fine to me.

The weld head I'm using (Ebay, $50 +shipping) is an old Unitek unit. It's pneumatically actuated through a solenoid valve that runs on 110v AC. The air has adjustments for the up- and downforce, as well as flow restriction for speed. Then there's the trigger adjustment, which is the important one. It's a thumbwheel on the front, and simply adjusts how hard the head needs to press down before the trigger switch trips.

I find the pneumatics a hassle, frankly. Previous welders like this that I've used were cable operated via a foot pedal, and I found them to be much easier to control and keep the hands free. So I'll be working on replacing the pneumatics today.

Oh, and I got the upgraded caps today, but I haven't changed them out yet. The new units are 20,000uF, and 80 of those suckers would give me 1.6 farads, which is about 5x more than I've got. I may not need it yet - I'll see. I think I'd need to upgrade the SCRs for those puppies.

Pictures? Ok. This shows the weld head, caps, and homemade stand. You can also see the gauges and regs for the pneumatics.

http://i70.photobucket.com/albums/i106/wires99/CD%20welder/welder1a.jpg


Here is the power supply and meter, and a side view of stand:

http://i70.photobucket.com/albums/i106/wires99/CD%20welder/welder2a.jpg


Like I said, it's a little "Frankenstein", but it's doing the job. I have much larger pictures, but I sized them down so they won't bloat page width or hog bandwidth. Now I've got to get back to work.


-Mark

Wirecutter
12-01-2007, 12:30 PM
something i know a little about... :)

Obviously these are expensive (something like 2500 a pop) bits of precision kit but hopefully someone might find it helpful...:D

cheers

Oh yeah. I priced out the current model of the welder I'd used in days gone by. The Unitek 250DP, which has recently been discontinued, priced out at US$8500 for the power supply alone, and can go for over $1000 on Fleabay. That's why I decided to build one.

-Mark