How to make a spot welder.

[capwizard]

Re: How to make a spot welder.

yes, AC capacitor was worked good but not enough energy. i need to add more capacity or raise up the voltage.

One thing very strange is i measured charged capacitor which has no AC output only has DC around 80v then slowly drop down the voltage? why?

Does the capacitor has the same rectification function as rectifier diode?

PS: I haven't changed battery for this meter about two years. picture 4.

[Curious.George]

Re: How to make a spot welder.

The original "idea" is sort of flawed, you can't really "charge" a capacitor with AC. The charge that happened to be on the capacitor is pretty much luck of the draw depending on when in the cycle it got disconnected. It doesn't make sense to "store" AC on the capacitor.

But, most of the time, the AC waveform is >> 0V, so the odds are that you will have substantial potential across the cap, regardless of where (randomly) you break the cycle.

On the other hand, I was wondering is whether just using a old solder gun to weld with... Get rid of the resistive tip and use two probes there instead, and depend on that low voltage high current to weld with. It's on the order of a hundred amperes, which is a bit low, but might be enough...

I had recently asked a colleague if I could use a Mig welder "shorted" to the surface to be welded to act as a spot-welder (I was actually looking to weld something substantial -- not flimsy battery tabs). He seemed to think I "needed" to draw an arc for it to work.

(I doubt that but am reluctant to try without some better information)

E.g., in "shop", the spot welder was just a pair of semi-stationary electrodes (brought together by a foot pedal), a giant transformer and an electro-mechanical timer. Turn sweep arm of timer to desired pulse duration, engage power, watch timer arm spin to zero, hear laminations in transformer groan as sparks flew, done!

(i.e., could I do the same with the Mig's transformer?)

[eccerr0r]

Re: How to make a spot welder.

The original "idea" is sort of flawed, you can't really "charge" a capacitor with AC. The charge that happened to be on the capacitor is pretty much luck of the draw depending on when in the cycle it got disconnected. It doesn't make sense to "store" AC on the capacitor.

You have to charge the capacitor with DC no matter what.

On the other hand, I was wondering is whether just using a old solder gun to weld with... Get rid of the resistive tip and use two probes there instead, and depend on that low voltage high current to weld with. It's on the order of a hundred amperes, which is a bit low, but might be enough...

[Curious.George]

Re: How to make a spot welder.

Note that you can just rectify the line voltage and not deal with bipolar capacitors. And yes the capacitor needs to be low ESR...

To be clear, you're proposing rectifying the mains to CHARGE the cap; not to use in place of the cap.

(the point of the capacitor is that it represents a fixed energy reserve; once spent it automatically "shuts off" without any "timing controls")

I think a smarter approach might be to dump the charge from the cap (or even a voltage source) into the load through a step-down transformer (trade I for V). Sort of like a CDI -- but in reverse!

[capwizard]

Re: How to make a spot welder.

you should wire capacitors in series so you end up with 2 negatives or 2 positives .

But can DC capacitor use in AC? From the experience I just got they became really hot. what are those difference?

[eccerr0r]

Re: How to make a spot welder.

I've seen 1F capacitor discharge at ~4V, so to not need a 1F capacitor,
1F * 4^2 = 440uF * x^2
x=191V
So yeah looks about right, need those high voltage in order to not need those low ESR 1F caps...

Note that you can just rectify the line voltage and not deal with bipolar capacitors. And yes the capacitor needs to be low ESR...

[R_J]

Re: How to make a spot welder.

Petehall347, I just looked at the first picture that is NOT the way to make a bi-polar cap. they must be connected like you said, in series (+2+ or-2-) Those caps also likely won't handle the current.

[petehall347]

Re: How to make a spot welder.

you should wire capacitors in series so you end up with 2 negatives or 2 positives .

[capwizard]

Re: How to make a spot welder.

I am so blessed by the God. unlike the video he used 240v and we're in the US only 120v.

I connected two capacitors polarized reverse together I thought I've made an AC capacitor but the fact was I only make a nonpolar DC capacitor. So it blow out with white smoke.

Does anybody know who make the decision USA uses 120v I need to say thank you to that person.

I going to use real AC capacitor from HVAC unit.....to be continued...

[Curious.George]

Re: How to make a spot welder.

The easies video I ever found. I will try it with 120vAC. the capacitor in the video is a 440 volt 20uf AC capacitor. we can make a one with two DC capacitors.

How to make a spot welder.
https://youtu.be/sYR4RuqEIjs

You DO understand that you've now got ~150V sitting on those test leads (lying on the bench)?

[capwizard]

The easies video I ever found. I will try it with 120vAC. the capacitor in the video is a 440 volt 20uf AC capacitor. we can make a one with two DC capacitors.

How to make a spot welder.
https://youtu.be/sYR4RuqEIjs