The Un-Economy Of Building Your Own Spot Welder

If there’s one thing that brings hackers together, it’s the ability to build something for less money than it takes to buy it. It’s an exercise [Great Scott Gadgets] put to the test because he was playing around with some 18650 lithium cells, and had a huge need to put some tabs on batteries. This can be done by soldering, but to do it right you should really use a spot welder. Here’s the rub: you can buy a spot welder for about $250, and you can build one for a little less. So, the question: should [Great Scott] build or buy a spot welder? This wouldn’t be worth reading if he started off with an eBay order.

[Great Scott] designed this spot welder around a half-dozen supercaps, all securely held together with Kapton tape. This goes through a set of MOSFETs, and everything is controlled through an Arduino, a rotary encoder, and a dirt-cheap OLED display. It’s a simple enough circuit but a bit too much for perfboard, so [Great Scott] laid out a PCB and got a few boards for under $40. A bit of solder and some debugging later, and theoretically a spot welder was created.

After all that work, how did the spot welder work? Well, it didn’t. A slight misstep in the schematic meant this board didn’t have reference ground on the MOSFETs, so all this work was for naught. Of course, the only thing required to fix this board was a second board spin, as [Great Scott] probably bought more parts than necessary because that’s what smart people do. Still, he decided to cut his losses and shelve the project.

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Beautiful DIY Spot Welder Reminds Us We Love 3D Printing

[Jim Conner]’s DIY tab spot welder is the sweetest spot welder we’ve ever seen. And we’re not ashamed to admit that we’ve said that before.

The essence of a spot welder is nothing more than a microwave oven transformer rewound to produce low voltage and high current instead of vice-versa. Some people control the pulse-length during the weld with nothing more than their bare hands, while others feel that it’s better implemented with a 555 timer circuit. [Jim]’s version uses a NodeMCU board, which is desperately overkill, but it was on his desk at the time. His comments in GitHub about coding in Lua are all too familiar — how do arrays work again?

Using the fancier microcontroller means that he can do fancy things, like double-pulse welding and so on. He’s not even touching the WiFi features, but whatever. The OLED and rotary encoder system are sweet, but the star of the show here is the 3D printed case, complete with soft parts where [Jim]’s hand rests when he’s using the welder. It looks like he could have bought this thing.
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Dual-Purpose DIY Spot Welder Built With Safety In Mind

Ho-hum, another microwave oven transformer spot welder, right? Nope, not this one — [Kerry Wong]’s entry in the MOT spot welder arms race was built with safety in mind and has value-added features.

As [Kerry] points out, most MOT spot welder builds use a momentary switch of some sort to power the primary side of the transformer. Given that this means putting mains voltage dangerously close to your finger, [Kerry] chose to distance himself from the angry pixies and switch the primary with a triac. Not only that, he optically coupled the triac’s trigger to a small one-shot timer built around the venerable 555 chip. Pulse duration control results in the ability to weld different materials of varied thickness rather than burning out thin stock and getting weak welds on the thicker stuff. And a nice addition is a separate probe designed specifically for battery tab welding — bring on the 18650s.

Kudos to [Kerry] for building in some safety, but he may want to think about taking off or covering up that ring when working around high current sources. If you’re not quite so safety minded, this spot welder may or may not kill you.

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