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.
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.
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.
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.
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.
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.
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.
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
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.
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?
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).
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.
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.
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.)
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