A Battery Spot Welder Made from an Old Microwave and Excessive Mahogany [video] | Hacker News Reader

A Battery Spot Welder Made from an Old Microwave and Excessive Mahogany [video]

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These MOT based spot welders transfer far more heat to cells than capacitive discharge welders. This can result in the damage to heat sensitive components inside the cells which is the same reason that soldering isn't recommended for lithium cells.

To get around those issues, I built a capacitive discharge spot welder for tabbing lithium ion cells a couple years ago: http://robruark.com/projects/welder/welder.html

What is the cause of the excess heat transfer? Is it mainly an excess pulse duration? I notice that the project in the OP was being pulsed on the order of 100s of ms, long enough to get a visible glow. Or is the excess heat caused by the fact that the OP runs at constant current for the entire duration of the pulse, where a capacitive discharge design is going to have the current falling off during the pulse? Or is it a blend of both?

It's excess pulse duration coupled with insufficient current. Capacitors can dump all their energy almost instantly (on the order of 1000s of amperes), but transformers are much more current-limited due to their design. It's analogous to using a press vs a hammer.

I used to get a kick in middle school disassembling disposable cameras (unlimited supply since dad worked in a grocery store with a photo lab), charging the circuit, and discharging on coins.

At my middle school we discharged them on our friends…

I remember very painfully learning that the flash trigger on a disposable camera is not low voltage.

I would try to do that but you could rarely get it to work reliably. I did get it myself a few times and that was always shocking

I quite fancy getting a microwave oven transformer to make a pretty powerful electromagnet.

Another source for a big stepdown transformer : an old arc welder

LTT has a video on a very simple battery powered battery spot welder https://www.youtube.com/watch?v=aHNmVV7THBs

Timing is inconsistent and the contacts will stick and weld together after a bit of regular use. Solid state is better if its designed well. Checkout the kWeld. https://www.keenlab.de/index.php/product/kweld-complete-kit/

I looked at the kweld, came very close to buying one, but ended up getting the Sequre version instead. About the same price, well reviewed, and looks to ship ready to go rather than you having to source pieces. It arrived earlier in the week, I haven't had time to open the box yet. kweld looks solid though. I thought about trying the DIY car battery and starter solenoid hacks, but wanted something a little more accurate.I have around $700 worth of battery packs to repair/rebuild (lawn mower and discontinued electric bike), so I hope to get my money's worth out of it.

In the end of the video he lists off the number of things he did that were fancier than necessary. One of them was the use of an Arduino for the pulse duration and switching. I found myself reflexively commenting, "It's called a 555 timer." I'm confident that the same function could be more minimally implemented with a 555 timer and maybe a couple other supporting chips. But he's probably getting more consistent control of pulse duration and it was probably easier for him to implement as well.

Having digital timer settings would be tricky without a processor, I imagine.

It’s also likely faster to implement as a one-off for a hobbyist and, using a low-cost MCU, cheaper than building it around a $0.10 555 if you want to make a product.

https://cpldcpu.wordpress.com/2019/08/12/the-terrible-3-cent...

He's probably getting a lot more error from using an AC solid-state relay (which probably uses a triac and so probably stays on until the next zero-crossing, thus a 10-millisecond error) than he'd get from using some metal-film resistors, a film capacitor, and a 555. But the Arduino has a much easier time displaying the pulse duration on the LEDs.

nitpicking but as someone with a scientific mind, i found the whole 'i dont know how a transformer works but lets screw with it anyway' mentality to be infuriating.

Electroboom would too.

Check this too , Water Welding Machine

https://www.youtube.com/watch?v=800BnMG8prA

Not pictured: everything is connected to mains voltage. The salt water bucket is just a huge current-limiting resistor to prevent tripping breakers or starting a house fire.

https://en.wikipedia.org/wiki/Liquid_rheostat

Isn't it incredibly dangerous to disassemble microwaves?

Absolutely, for people who do not know what they are doing. Anything with high voltage capacitors is dangerous unless you know how to safely discharge them, and the magnetron in microwaves specifically.

Yeah, you're absolutely right. A bug in kernel programming might erase your filesystem and require you to restore from backups; a bug in handling high-voltage capacitors erases you.

The magnetron can have insulators made of a toxic substance that can be dangerous as a dust. This guy is just using the transformer for it's convenient laminates and primary winding - probably could have used a similar transformer with suitable current handling capacity but dead microwave ovens are fairly common. I have 3

Some beryllium ceramics I heard...

Only dangerous if you grind/crush the very obvious pink ceramic and inhale the dust. https://en.wikipedia.org/wiki/Berylliosis

Modern magnetrons tend to use alumina-based ceramics instead.

As far as I can tell, that's a myth --- beryllia was only used in military radar magnetrons, not consumer microwave oven ones. It is extremely expensive, its other characteristics are not required, and its toxicity requires special handling measures during manufacturing, so alumina is used instead.

Nah. You'll live.

It's less dangerous than servicing CRT televisions/monitors. If you know how to safely discharge the big capacitor and never try to energize (or dismantle) the magnetron, it's pretty benign.

What are the dangers here? What happens if that thin insulation between the copper bars breaks? Can the current jump to you if you use this bare handed?

Well it's only 2 volts. So nothing will happen. Unless your hand is all sweaty maybe. Then maybe you'll feel it. I wouldn't worry.

Agree. You can feel a 9V battery on your tongue, but not on the sweatiest of hands IME. (I still quick-check 9V batteries on my tongue.)

uh... wut

I believe it's the amps you need to worry about; "amps kill."

I believe it takes about 1 amp to kill a person.

This thing is outputting somewhere around 55 amps (if I remember right).

But for a given resistance, you don’t get the amps if you don’t have enough voltage. V=I*R. Don’t they teach that equation any more?

Current availability is not a guarantee of current delivery.

I (current) = V (voltage)/R (resistance)

Typical skin resistance can be as high as 100MΩ (mega ohms) but is often cited as 10MΩ. Even if we assume just 1MΩ (1000000 ohms) skin resistance, then at 2.3V as shown in the video the current would be 0.0000023 A or 2.3 μA (micro amperes).

TIL. Thanks for taking the time to explain!

If the the insulation breaks the tool short circuits and the welding tips don't work. Electricity follows the path of least resistance, so given how close the welding tips are together I doubt it would ever choose to go through you; especially any of the sensitive parts of you. You'd have to be bizarrely well grounded for it to discharge current through your body.

I vaguely remember standing on a rubber mat being standard practice for this sort of tool.

The most dangerous thing here is energizing a MOT with its stock deadly high-voltage secondary coil. If you re-wind the secondary to be low voltage as described in the video, it's no more dangerous than any other project requiring a mains power supply.

It's two volts. You can totally use this bare-handed, no problem. Just make sure you aren't wearing jewelry on your hands.

I think I’d have just hit aliexpress and bought a battery welder for $30 shipped.

This homemade one bodged out of a broken microwave is probably safer.

What's the fun in that?

I ordered one of these and applied all of the recommended "fixes. The unit failed after two spot welds. I would not recommend and my experience seems to be mirrored across the internet.

The Malectrics kit is drasticly superior with reasonable price and resale value tends to be high if you're worried about limited use against cost to aquire.

Fun fact: the cavity magnetron found in micraowaves was one of the most secret inventions of WW2, and guaranteed victory for the Allies.

It was invented to generate 10 cm waves in a compact package, which are ideal for radar. Although Germans and Japanese did early research on the magnetron, the Allies leap-frogged them.

The Americans had a university campus (around 4,000 staff) design production units for literally everything larger than a jeep that moved .

A Brit hand-delivered their prototype to the US, and the US solved some issues and productionized it.

Because the magnetron is made out of a block of metal, there was no way to destroy it after an airplane accident. But the Germans weren't able to adapt it, and admitted it was beyond their understanding at the time.

Ironically one of Germany's most advanced signals airplanes was captured just off a beach in England, but politicking between the navy and army meant it was left in seawater until it completely corroded. (You can identify these special signals planes by numerous antennae mounted on the nose.)

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