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
Built withArduino
- difficulty
- ●●●●○
- time
- a weekend
- license
- license not specified
- repo
- repo 0 stars
●●●●○ · a weekend · license not specified · 0 stars · repo
WHAT YOU’LL NEED
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COMPAREE VERDICT
This build delivers what it promises: a dual-pulse spot welder built around a salvaged microwave oven transformer, with an Arduino, a 16x2 LCD and a single-button menu for presets and manual mode, triggered by a hand button or foot pedal. The secondary is rewound with a few turns of thick cable (the author used 1200 amp jumper cable), a 40 amp solid-state relay switches the primary, and a thermistor locks out welding if the cable insulation reaches 80 °C. The author says it can be built for under 50 dollars if you play your cards right, and the guide is released under CC0, though the 3D-printed LCD case he used is based on a Thingiverse design licensed non-commercial. The descriptions are detailed but the author apologises for the lack of step-by-step photos. The single most dangerous step is getting the transformer out: the microwave's high-voltage capacitor can kill you, and the guide warns about it and links another guide rather than walking you through it. If you build battery packs and already know how to work safely inside a microwave and with mains wiring, this is worth the weekend. If not, start with something that will not kill you if you get it wrong.
IN THE REPO
GOOD TO KNOW
- —Full build guide with detailed descriptions (the author apologises for missing step-by-step photos), published on Instructables under CC0; the optional printed LCD case is based on a Thingiverse design licensed non-commercial.
- —Arduino sketch (spot_welder.ino) provided for the dual-pulse controller with a single-button menu, presets and a 16x2 I2C LCD.
- —The materials list names the key parts (40 A SSR, 16x2 LCD, thermistor, buttons, 1200 A jumper cable) but deliberately gives no exact sizes — the author used what he had on hand.
- —No CAD files — the base and electrode arms are cut from scrap wood to your own dimensions, and the LCD case can be printed or built from anything.
- —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.
Parts to buy
11 itemsFrom our check of the build. Exact quantities and part numbers are in the creator’s BOM.
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.
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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 sketch (spot_welder.ino) is attached to the guide. Upload it to the Arduino and check the 16x2 LCD menu works before connecting anything to mains.)
- 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 cable that is too thin for the secondary. The guide gives no exact gauge — the author used 1200 A jumper cable — and anything lighter will overheat 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.
- Undersizing the solid state relay. The guide uses a 40 A SSR on the transformer primary, matched to your mains voltage; a smaller or wrong-voltage SSR can fail on the first pulses.
- 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 controller fires two timed pulses with a gap between them, and you set both widths and the gap from the menu as presets. The guide does not explain the theory, so treat the timings as something to tune on scrap.
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?
The author says it can be built for under 50 dollars if you play your cards right, with most parts recycled. A commercial welder costs more but comes ready to use, with known settings and without the danger of salvaging a microwave.
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Discussion1
FROM THE COMPAREE TEAM
Every rewound transformer behaves slightly differently. What pulse widths did you land on for your battery tabs, and did you burn through any strips while tuning?
psyberjock
psyberjock built this after finding that most spot-welder guides needed hard-to-find parts or custom PCBs, aiming for a simple, robust design from stock Arduino parts and recycled components. The guide is released under CC0.
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- 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.
CompareeTEAM2mo agoedited
Practical notes from our verification: the Instructables guide is thorough and its licence is Public Domain (CC0), confirmed via the platform API. The Arduino sketch is attached to the guide as a file rather than kept in a separate repository. The 3D-printed LCD case is based on NilsRoe's Thingiverse design, which is licensed non-commercial, so if you want to sell welders, design your own case as the author suggests. The single biggest gap is that the guide warns clearly that the microwave's high-voltage capacitor can kill you, but does not walk through the discharge procedure itself; it links another guide on pulling the transformer. The author also recommends connecting the transformer core to the ground prong of the power cord. The parts list covers a 16x2 LCD, a 40 A solid-state relay, 1200 amp jumper cable for the secondary and a thermistor that locks out welding at 80 °C. Plan for test welds on scrap before assembling a battery pack you care about. Correction (4 October 2026): we re-checked this page line by line against the project's own repository, documentation and videos, and fixed errors in earlier versions.