KP Point Welder

This a story is about how I made a point welding transformer.

I found an old E-form, 220/24V transformer which had as secondary winding a wire 3mm in diameter. The idea was to replace that with a different winding.

Disassembling was easy. First, I tried to get a smaller number of windings on secondary and test if I can get greater amp output and lower voltage. I read somewhere that voltage in this form of welding should be from 1.5V up to 6V and amps as much as possible.

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Step 1: Rewiring the Old Transformer

I made a wooden mold to help hold the old primary winding. Using the old secondary copper wire (3mm dia) I wound a double parallel winding over existing primary coil. This gave me about 6V on secondary side and test showed that it was not enough power to melt 1 mm steel wire. Additionally the transformer became very hot very fast.

Step 2: Triple Parallel Winding

I tried the same trick but used 3 wires parallel which is equivalent to 21 square mm. Voltage measured 3,84 V on secondary side. I started to assemble all the parts on a board and I started to make end copper connections. I was sure that I got what I wanted from the beginning - Dear Murphy!

Still It was not enough amps.

Step 3: Test Board Misc.

I made a simple construction (test board) to find out how this work. Micro switch was inserted on the top level in a small walnut wooden box (lacquer comes later). Lower electrode was mounted on a aluminium square rod.

Step 4: A/mm2

Finally, I bought a length of 60 mm2 copper wire, removed the insulation and got a smaller diameter. I wrapped this wire with common thin scotch tape. The goal was to put 4 windings in transformer. Insulation was ok because between each loop is max 0.7V. In the end I got 2,54 V at the terminals and hopefully plenty of amps. I finished the new copper terminals for the secondary winding as well.

All this was finished after I went back to read, study and learn from the beginning (see step 7).

Repetitio mater studiorum est!

Step 5: Succes!

I got enough power. About 2.5 V and almost 70 Ampers! 1mm steel wire burned out in seconds. But switching it on and of manually is not accurate. I constantly burn out thinner material.

Sometimes the weld was perfect, but Murphy does not sleep!

Step 6: Intermezzo

In the meantime I needed to weld a small chimney for my tent stove.

I used my welder as is and successfully, hand pressing switch, I made from tin cans 12 cm diameter a smaller 6 cm diameter tubes.

I used an aluminium tube 3 cm in diameter as lower electrode, wrapped this with a tape for isolation, and on one end mounted a screw head as a welding point. The other electrode was freely pressed by hand from above.

Step 7: Triacs an So On

I found an article about how to regulate transformer action on primary side. This was using a triacs similarly as is in a light bulb dimmer or in vacuum cleaners. Lowering power splitting sinusoidal wave. According Fig 11. I collected the necessary components, assembled the circuit, put it in a aluminium box and connected it to the transformer.

Perhaps because of insufficient knowledge, it didn't work as I wanted it to. I could not regulate amps and time duration. I burned out constantly my thin sheets.

Murphy still does not sleep.

Step 8: Conclusion

Let's get serious.

First, learn what the world already tested and wrote. I started with math from the books/web and in a spreadsheet form made a calculator with all the formulas (Proračun transformatora -Transformer calculation) and calculated the dimensions of transformer based on my core. This gave me a figure what my core is capable of. Rewinding the primary and secondary was then necessary.

My frend Mirko Jukl, drew me a simple 555 circuit schematic (Odgoda vremena i trajanje impulsa - Time delay and impulse time) to control time-duration of welding impulse on secondary side. I can choose delay from a few seconds and duration in miliseconds. Small movies show tests with 220V bulb, transformer an when everything was assembled and finished.

And it works!

Because this project was a back and forth process that lasted almost half a year this instructable is a bit inconsistent. Here is the final data of transformer:

Transormer core (E shape) - total square 28 cm2 ... cca 784W.
Primary windings 370 ... for 3,41A ... 1,5 mm dia
Secundar windings 5 ... for 261 A ... 11,54 mm dia

I hope that this DIY project helps somebody to get better results and extend knowledge.


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    7 Discussions


    3 years ago

    Very nice. Thanks.


    5 years ago on Step 8

    tks for the technical data, and the circuits...


    5 years ago on Introduction

    Good work. I guess a larger core transformer, say from a discarded Microwave oven (used backwards, feeding 120 VAC on the secondary to get a much larger current on the thicker wire winding) could be better... but Murphy is always lurking!

    By the way, I have a multimeter very similar to yours, I bought it from Radio-shack in 1996, but I guess it was made for them by METEX. Very good inexpensive, powerful and versatile one. Has served me very well for its price (US$130 in 1996). Nowadays, Radio-Shack is selling only chinese, terribly cheap multimeters, but the METEX is much better. Amclaussen, Mexico City.

    1 reply

    Thanks. Transferred power incl. losses, is determined almost only by the cross surface of inner core.. More cm2 - more power. Microwave transformer is also determined by the above rules. Density A/mm2 is also determined in optimum of 2-4A/mm2 ,respectively, because of overheating. And so on...METEX have a perfect instruments - agree..