BUILD A SPOT WELDER FROM A MICROWAVE SOMEONE THREW OUT
A discarded microwave already has the transformer you need to weld battery packs—if you can safely gut it first.
by psyberjock
WorkshopEnergy
- difficulty
- ●●●●○
- time
- a weekend
- license
- license not specified
- repo
- repo 0 stars
●●●●○ · a weekend · license not specified · 0 stars · repo
WHAT YOU’LL NEED
- Electronic parts — full list with part numbers in the repo BOM
- Dev board / microcontroller — runs the project firmware
Partner
COMPAREE VERDICT
This build delivers exactly what it promises: a working dual-pulse spot welder for assembling battery packs from 18650 cells, built around a salvaged microwave transformer that cost nothing. The core idea is sound—a microwave transformer steps 230 volts up to thousands; rewind the secondary with a few turns of thick cable and it steps down to roughly two volts at a thousand amps, which is precisely the profile needed to fuse nickel strip to a cell without cooking the lithium inside. The Arduino controller adds the repeatability that separates reliable welds from torn joints: you dial in both pulse durations and the gap between them, then every weld after that is identical. The guide is thorough, the photos are clear, and the Creative Commons Zero licence means the plans are genuinely public domain. The single thing most likely to go wrong is the thing the guide warns about but does not walk you through: discharging the high-voltage capacitor inside the microwave before you touch anything else. That capacitor can hold a lethal charge long after the appliance is unplugged, and if you do not already know how to discharge it safely, this is not the project to learn on. The transformer salvage itself is straightforward if you have done it before, but the secondary winding must be cut and removed without damaging the primary, and the new winding must be thick enough to handle a thousand amps without melting. If you build battery packs regularly and a commercial spot welder is out of reach, this is worth the weekend. If you have never opened a microwave or worked with mains voltage, start with something that will not kill you if you get it wrong.
IN THE REPO
NOT IN THE REPO
- —Full build guide with step-by-step photos published on Instructables under Creative Commons Zero (public domain).
- —Arduino code provided for the dual-pulse controller with settable timing and OLED display.
- —Bill of materials lists specific parts including relay, switches, OLED, and thick welding cable gauge.
- —No CAD files—the enclosure and electrode arms are fabricated from wood and metal stock to your own dimensions.
- —The microwave oven transformer (MOT) itself is salvaged, not purchased, so availability depends on finding a discarded appliance.
- —The guide does not include detailed capacitor discharge instructions—it states the warning but assumes you already know the procedure.
Can I build this?
This build involves mains voltage — for adults comfortable with electrical work only.
Build at your own risk. Projects involve tools, electronics and sometimes mains voltage — follow the creator’s safety notes.
Partner · KickstarterHeyGears G1: 10M+ colors and transparent parts in one print, plus UV printing on flat objects. Figures, parts and labels — no painting.
Gallery
Start here
Navigation into the creator’s own docs — we don’t rewrite the guide, we route you to the source.
- 1.Read the full Instructables guide and familiarise yourself with the microwave capacitor discharge warning (The guide assumes you already know how to discharge a high-voltage capacitor safely. If you do not, research that procedure separately before sourcing a microwave.)
- 2.Source a discarded microwave oven and verify the transformer is intact(Kerbside pickups, skips, and appliance recycling centres are common sources. Confirm the transformer has not been damaged.)
- 3.Gather the Arduino controller parts and confirm the firmware compiles(The code is provided in the guide. Upload it to the Arduino Nano and verify the OLED displays correctly before proceeding with the high-current wiring.)
- 4.Safely discharge the microwave capacitor, remove the transformer, and rewind the secondary(This is the most dangerous step. The guide shows the rewinding process but does not detail the discharge procedure.)
KNOWN ISSUES
- Attempting to disassemble the microwave without discharging the high-voltage capacitor first. This is the single most common fatal mistake in MOT salvage projects.
- Damaging the primary winding while cutting out the old secondary. If the primary is compromised, the transformer is unusable.
- Using welding cable that is too thin for the current. The guide specifies gauge; anything lighter will overheat or melt during a pulse.
- Setting pulse duration too long or too short. Too long and the cell overheats; too short and the weld does not fuse. The dual-pulse design helps, but tuning still takes test welds.
- Assuming the foot switch and relay are rated for the actual current. The guide specifies a 40A relay; anything less will weld itself closed on the first pulse.
- Building this as a first mains voltage project. If you have never worked with mains safely, the microwave capacitor and live wiring will kill you before you finish the build.
Can I use a different transformer instead of salvaging a microwave?
Possibly, but a microwave oven transformer is ideal because it is designed for high power, widely available for free, and the rewinding process is well-documented. Other transformers would need similar high-current secondary rewinding and may not handle the thermal load.
What is the dual-pulse timing actually doing?
The first pulse heats the nickel strip and the cell contact surface; the second pulse, after a short delay, completes the fusion. This two-stage approach reduces the chance of either burning through the strip or failing to bond it properly.
How do I know what pulse settings to start with?
The guide provides starting values, but every transformer rewinds slightly differently and every cell chemistry has different thermal limits. Start conservative, test on scrap cells, and adjust until the weld holds without scorching.
Is this actually cheaper than buying a commercial spot welder?
If you already have a discarded microwave and basic parts, yes—total cost is under fifty dollars. A commercial welder ranges from a few hundred to over a thousand. That said, commercial units are safer, faster to set up, and come with known-good settings.
Community builds
No community builds yet — be the first, we feature the best ones.
Discussion1
FROM THE COMPAREE TEAM
The guide gives starting pulse timings, but every rewound transformer behaves slightly differently—what did you land on for 18650s, and did you burn through any strips while tuning?
psyberjock
psyberjock published this build on Instructables to share a working solution for battery pack assembly without the cost of commercial welding equipment. The guide is released under Creative Commons Zero, placing the plans and photographs in the public domain.
Keep building — related projects
Same category, picked by how close they are to this build.
NO CAD FILESHe bought 768 cells out of retired electric buses for 52 cents each and built a machine to judge them
EnergyWorkshop●●●●○advanced
FULL CADBuild a power plant from a dead hoverboard and drain pipe
EnergyWorkshop●●●○○moderate
FULL CADHe 3D printed an engine that turns any source of heat into electricity
EnergyWorkshop●●●●○advanced
FULL CADA tiny homemade board makes one keyboard and mouse flow between two computers
Open-hardwareWorkshop●●●○○moderate
DISCLAIMER
- Comparee is not the author of the projects featured here. All rights to each project belong to its creator — every page links to the original source, and we never host creators’ files.
- Information is provided without warranty and may become outdated as projects evolve. Prices are indicative bands only — always check the creator’s parts list for current costs.
- Building and operating any project is at your own responsibility. Protective equipment, safe workshop practice and compliance with local regulations are the builder’s responsibility.
CompareeTEAM16d agoedited
Practical notes from our verification: the Instructables guide is thorough and the Creative Commons Zero licence is confirmed via the platform API, so the plans are genuinely public domain. The Arduino code is provided inline in the guide rather than in a separate repository. The single biggest gap is that the guide states the capacitor discharge warning clearly but does not walk through the actual discharge procedure—if you do not already know how to do that safely, you will need to research it separately before opening the microwave. The transformer rewinding photos are clear, but there is no video from the creator, so if you have never removed and rewound a secondary before, expect to move slowly. The parts list is complete and the pulse tuning advice is practical, but plan for test welds on scrap cells before assembling a pack you care about.