Build thread: nickel strip spot welder from microwave oven transformer

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5chn3ll
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Build thread: nickel strip spot welder from microwave oven transformer

Post by 5chn3ll » Wed Apr 10, 2019 2:04 pm

There is a surprising amount of information about using microwave oven transformers to build spot welders appropriate for assembling battery modules, but generally they are crappy YouTube videos that don't include actual schematics or even decent images of the electrical connections. There are also a lot of terrifying lash-ups out there just waiting to start a fire; I'm hoping to share something somewhat less fire-prone.

Lithium cells do not like high temperatures. There are plenty of folks who solder their cells - including me, until now - but I want to start building some "serious" packs (a solar power wall and some additional e-bike batteries), so I need to treat the cells properly.

The "correct" way to connect 18650 cells is using nickel strips and a spot welder. Spot welders are not cheap, but you don't need an automotive-grade welder. A misused microwave oven transformer and a few odds and ends will produce a workable welder; adding some relays and a Raspberry Pi or an Arduino can yield the same results you get from retail welders selling for several hundred bucks.

In this thread, I'll document both builds. First, I'll build the minimal solid-state version of the welder - which will work, but it doesn't work amazingly well. Once I have v1 working, I'll start working on the control circuit for the "intelligent version," and then I'll replace the solid-state parts with the upgraded gizmology.

Understeer: You will hit the wall with the front end.
Oversteer: You will hit the wall with the rear end.
Horsepower: How hard you will hit the wall.
Torque: How far you will move the wall.

"Reading tire date codes is for p*****s."
-P. Walker


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5chn3ll
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1.0: The solid-state version...

Post by 5chn3ll » Wed Apr 10, 2019 2:05 pm

This is the star of the show, a microwave oven transformer (geeks on the Internet just say "mot"). Power from your outlet goes in, and as Jeremy Clarkson would say, witchcraft happens and something different comes out.
1.png
1.png (362.69 KiB) Viewed 305 times
Normally, there are two windings: a heavier primary winding and a much thinner secondary winding. The purpose of the secondary winding is to decrease amperage and increase voltage. To use the transformer as the guts of a spot welder, you need to ignore the guy who designed it and replace the super-fine, really long secondary winding with a REALLY THICK, really short cable. Doing so will have the opposite electrical effect of the original secondary: lots of current, very low voltage - something like 1 to 1.5 volts, and current above 1000 amps.

My wife was not thrilled about this part of the project: removing the secondary winding. I used a chisel to cut through the loop on each side of the transformer. Once the wires were cut, I drilled out a small cross-section of the secondary wiring (it's really packed in there). Once you have managed to create some wiggle room, use pliers to pull out the remains of the secondary winding.
2.png
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Be VERY cautious to avoid damaging the coating on the primary winding. That's the part that plugs directly into your wall socket, so it's best to avoid shorting it out. When in doubt, test it out - verify the coil's resistance before and after removing the secondary.

In the top photo, you can see some red wire - your MOT may include a third coil between the two main windings. Remove this winding as well.

When you're done, you will (hopefully) have something that looks like this:
3.png
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I picked up some stumpy cable from Home Depot - someone else had it cut and then decided not to buy it, so I got about 30 bucks worth of cable for $2.

The new secondary wiring is 1.5 loops - as soon as it's safe to turn on, I'll fire up the transformer and test the output voltage.
4.png
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I spent a little while figuring out the best way to get the electrodes attached to the secondary. After staring at it for a while, I decided to use a stainless hex nut on each end of the winding. The nut is threaded onto the very end of the copper conductors. I soldered around the union between the nut and the conductor, and then I wrapped the entire junction in copper tape.

That copper tape is pretty slick, by the way - both the tape and the adhesive are conductive, so you can pretty much tape circuits wherever you need them. In this case it's just cosmetic - getting the solder hot enough to properly flow without it filling up the hex nut resulted in not-so-pretty work, but it should do the trick.
5.png
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Understeer: You will hit the wall with the front end.
Oversteer: You will hit the wall with the rear end.
Horsepower: How hard you will hit the wall.
Torque: How far you will move the wall.

"Reading tire date codes is for p*****s."
-P. Walker


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5chn3ll
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2.0: Digitally-controlled pulse length and safety (Arduino/RPI)

Post by 5chn3ll » Wed Apr 10, 2019 2:05 pm

More stuff.

Understeer: You will hit the wall with the front end.
Oversteer: You will hit the wall with the rear end.
Horsepower: How hard you will hit the wall.
Torque: How far you will move the wall.

"Reading tire date codes is for p*****s."
-P. Walker


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32wildbilly
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Re: 2.0: Digitally-controlled pulse length and safety (Arduino/RPI)

Post by 32wildbilly » Wed Apr 10, 2019 2:22 pm

5chn3ll wrote:
Wed Apr 10, 2019 2:05 pm
More stuff.
You really are the person that Back to the Future writers based Dr. Emmett Brown on aren't you!!??
MV5BMGU1NDkyMzUtMjE1OC00NDM3LWI1ZDYtM2ZiYzYwZDM5ODM3XkEyXkFqcGdeQXVyNjUxMjc1OTM@._V1_SX1777_CR0,0,1777,967_AL_.jpg
MV5BMGU1NDkyMzUtMjE1OC00NDM3LWI1ZDYtM2ZiYzYwZDM5ODM3XkEyXkFqcGdeQXVyNjUxMjc1OTM@._V1_SX1777_CR0,0,1777,967_AL_.jpg (166.24 KiB) Viewed 307 times

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5chn3ll
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Re: Build thread: nickel strip spot welder from microwave oven transformer

Post by 5chn3ll » Wed Apr 10, 2019 2:37 pm

No, man, I am quite seriously just a hack with Internet access.

Understeer: You will hit the wall with the front end.
Oversteer: You will hit the wall with the rear end.
Horsepower: How hard you will hit the wall.
Torque: How far you will move the wall.

"Reading tire date codes is for p*****s."
-P. Walker


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32wildbilly
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Re: Build thread: nickel strip spot welder from microwave oven transformer

Post by 32wildbilly » Wed Apr 10, 2019 2:56 pm

5chn3ll wrote:
Wed Apr 10, 2019 2:37 pm
No, man, I am quite seriously just a hack with Internet access.
Well, Dr. Brown is kind of a Mad Scientist, which is what I am referring to... :?

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gnat
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Re: Build thread: nickel strip spot welder from microwave oven transformer

Post by gnat » Wed Apr 10, 2019 3:02 pm

#whatcouldpossiblygowrong

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32wildbilly
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Re: Build thread: nickel strip spot welder from microwave oven transformer

Post by 32wildbilly » Wed Apr 10, 2019 3:27 pm

gnat wrote:
Wed Apr 10, 2019 3:02 pm
#whatcouldpossiblygowrong
This...?
58c9b5f16ff80159008b5360-750-563.jpg
58c9b5f16ff80159008b5360-750-563.jpg (82.9 KiB) Viewed 296 times

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5chn3ll
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Re: Build thread: nickel strip spot welder from microwave oven transformer

Post by 5chn3ll » Wed Apr 10, 2019 4:01 pm

There are plenty of people who use a similar rig for fractal wood burning - which involves both 1000 amps AND salt water.

To that I say, Good Day!
gnat wrote:
Wed Apr 10, 2019 3:02 pm
#whatcouldpossiblygowrong

Understeer: You will hit the wall with the front end.
Oversteer: You will hit the wall with the rear end.
Horsepower: How hard you will hit the wall.
Torque: How far you will move the wall.

"Reading tire date codes is for p*****s."
-P. Walker


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gnat
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Location: If you knew, you'd move

Re: Build thread: nickel strip spot welder from microwave oven transformer

Post by gnat » Wed Apr 10, 2019 4:33 pm

Heh. This weekend I found build instructions for a plasma cutter. I want a plasma cutter. I'll spend the money for a real one though...

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