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  • Sput welder?

    So I came into a pretty large capacitor bank the other day. 9x 300 Farad, 7.5 volt monsters that were gonna be thrown away. I was trying to think of something to do with it, and I think I would like to make a sput welder. Here's the thing, I don't know the first thing about welders. I own one, but that's about it. Has anybody done something like this? I feel like any one of those caps could make me a good one. The voltage might not be quite right (too low) but who knows. I also don't know how to properly charge a cap or even check it's level of charge. I figure all I'd need to do would be to use a transformer to get the AC down to a reasonable level (I just got one of those too, outputs like 16 volts, IIRC, at 1.5 amps AC, I could grab a few high-power diodes to rough it out to get DC to charge the caps). Anyway I figure all I gotta do is short out the cap with a wire to something grounded and boom I have a wire. Any thoughts?

    BTW I have a crapload of tools to derust and really enjoy powdercoating, so that's where this idea came from.

    Any other large coke-can sized capacitor fun projects I would be interested in as well
    You never learn anything by doing it right.

  • #2
    I have a really hard time believing a coke can sized cap is 300 farads, maybe 300 uF... I also have no idea what a sput welder is, so I probably can't help you, but an overcharged capacitor can explode, also hooking them up in reverse polarity, as well as running them at more than their rated voltage.

    Whatever you do, be careful, but I think you may find them to be nearly useless w/ that low of a voltage rating.

    Later,
    Jason

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    • #3
      It says 300,000 mf on the side. It's only 7.5 volts though- that's why I'm thinking it definitely is 300 Farads. I have no idea what the original application was, obviously some sort of huge power supply (could be a filter of some kind, this came from a University so they might have used it to power some benchtop power supplies. Then again it might have just been "sitting around")




      A sput welder temporarily welds a piece of wire to a part that conducts, used for plating, powdercoating, anything requiring a conductive temporary joint that you don't want to clamp. Video:

      http://video.google.com/videoplay?do...16373727477390
      You never learn anything by doing it right.

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      • #4
        Sput is what happens when you try to spit and it dribbles down your chin, instead.

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        • #5
          I thought that was redneck past perfect tense as in:

          ... Aww &%*# I done sput bacca juice on ma dawg!!!
          .
          "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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          • #6
            Not 300 Farads.
            300,000 micro farads. MFD

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            • #7
              Don't know if this is relevant or not.

              Harbor Freight has a close out on "spud welder tips" Looked to be about 1/8" diameter and about 1/2 to 5/8 in length with a domed top on one end and flat cut on the other. They had them for half price. Looked to be something lilke 100 in package for something like $2.50. I don't know if this applies to this thread or not?
              Last edited by Your Old Dog; 09-16-2007, 09:18 PM.
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              It is true, there is nothing free about freedom, don't be so quick to give it away.

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              • #8
                As Mac1 pointed out 300,000 MFD is only 0.3 Farads.
                Now that is still a good size cap. but the low voltage wont do much
                for a welder.
                That must have been some power supply they were used on.
                They are undoubtly electrolytic so be carefull with polarity.
                ...lew...

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                • #9
                  The prefix for micro is the letter mu, it looks like a funny lowercase u.

                  The prefix for milli is m.

                  Millimeters = mm

                  Perhaps on capacitors they do it differently, although I can't see why. I'm not arguing that the value seems large, I could be mistaken, but everywhere else in the world M means milli. Like I said though I could be wrong, but that's where I'm getting my 300F.



                  Anywho, upon more thought, I could just make some kind of what I think is called a "flyback" transformer, I think they can put out a crapload of voltage from a small amount. The transient cap drain should do well in creating a single burst of high voltage through a step-up transformer.
                  You never learn anything by doing it right.

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                  • #10
                    Originally posted by Your Old Dog
                    Don't know if this is relevant or not.

                    Harbor Freight has a close out on "spud welder tips" Looked to be about 1/8" diameter and about 1/2 to 5/8 in length with a domed top on one end and flat cut on the other. They had them for half price. Looked to be something lilke 100 in package for something like $2.50. I don't know if this applies to this thread or not?

                    Thanks for the tip YOD, I think I know what you're talking about, but I don't think thats it. Those are (I think) for body panel work, weld them on and then use a slide hammer to pull out dents. The thing I'm looking for just welds a small wire to something to hang it in an acid bath/powdercoat booth/anything that needs a conductive mount.
                    You never learn anything by doing it right.

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                    • #11
                      .3 farads is pretty impressive but at 7.5 volts don't expect much in watt-seconds charge. Also that impressive bussing is mostly for appearance. The tiny screw and the little bitty post on the cap shows me it's strictly for power supply hold-up while a back-up power supply can be switched in.

                      Work that cap to major energy rates or even major ripple current and you'll most likely blow something. OTH working the time constant and L/C network formulas for one farad caps leads to some interesting numbers.
                      Last edited by Forrest Addy; 09-17-2007, 01:03 AM.

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                      • #12
                        Originally posted by Scatterplot
                        According to http://www.techwr-l.com/archives/970...706-00366.html

                        M means Mega or "10^6: mega M" so apparently,,,, what you've got there is a 300,000 Mega farad or .3 Tera farad capacitor.

                        Hmm, should be able to spot weld billets with that thing.

                        ... but I'm betting it's actually 0.3 farad.

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                        • #13
                          Lol. I figured it being an actual big M was out, so little m seemed closer. Is there any way to measure that easily?
                          You never learn anything by doing it right.

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                          • #14
                            Yes - some multimeters actually have a setting for measuring capacitance, but this is how they do it so you can do it with a normal one and a power source

                            I(t) = Io (e^-t/RC) where Io is the inital current given by IR=E (ohms law)

                            The circuit should be constructed with a resistor of known resistance in series with the capacitor. You need to measure the current flow and the time. Start the timer when the power source is turned on and watch for the current to drop to zero and then solve for C.


                            Basically all you need to do is measure the amount of charge being stored in the capacitor, but since your current changes with time this is hard to do without a computer to log a bunch of data points to simulate the current graph. The definition of capacitance is C=Q/V where Q is the charge stored for some potential. So if you hold the voltage constant and log the current measurements every 1/60 of a second or something, you can then integrate the function of best fit for the curve. Or just use simpson's method to approximate if you dont have a program to find best fit.


                            But it is .3 Farad. A farad is a pretty big unit when it comes to capacitance - most capacitors are nanofarads or microfarads. Very rarely do you see one large enough to be .3 farad so thats still really good for a free capacitor.
                            Last edited by Fasttrack; 09-17-2007, 07:53 PM.

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                            • #15
                              My experience with capacitors is sadly limited. Thanks for the math though, I can definitely do that. Actually there should be a multimeter nearby that will do it easier, lol.

                              Thanks for the info.

                              On one last point, I have seen a few 1 Farad capacitors that were the size of a handful of quarters. Now they were like .06 volts but they were 1 Farad


                              So any more thoughts on the sput welder idea? What kind of voltage do you think I would need to spark a wire onto something like in the video?
                              You never learn anything by doing it right.

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