DIY Spot Welder Using Two Microwave Power Transformers — a cautious how-to

⚠️ Important disclaimer (please read)

Disclaimer: This DIY involves very high-voltage transformers and wiring and could fatally electrocute you or someone else, start fires, or cause serious injury or property damage. This project is for entertainment/educational purposes only and does not endorse anyone attempting it. Do it at your own risk. If you are not experienced with mains electricity and transformer safety, stop now and buy a commercial spot welder or ask an experienced professional.

What this blog will (and won’t) do

What you’ll get here:

• A conceptual explanation of how to use two microwave power transformers (MOTs) to create a DIY spot welder.

• A general materials and tools list.

• Safety guidance and recommended protective measures.

• Alternatives that are much safer and often cheaper in the long run.

What I will not provide:

• Exact wiring diagrams or step-by-step instructions for modifying MOTs or wiring mains terminals in a way that could cause electrocution.

• Specific measurements, voltages, or instructions to bypass safety features on appliances.

• Anything that would knowingly enable unsafe modification of high-voltage equipment.

The working spot welder:

The concept:

A spot welder delivers a short, very large current pulse through two metal surfaces at the contact point, heating them locally enough to fuse. Hobbyists sometimes repurpose microwave oven transformers (MOTs) because MOTs are inexpensive and can deliver large currents when their secondary windings are modified. Using two MOTs (or two secondaries) allows more current and sometimes a layout that yields the low-voltage, high-current pulses needed for spot welding.

Key conceptual points:

• Spot welding needs big current, short time. The welding action comes from current; control of pulse duration is crucial.

• MOTs are mains-connected devices. They contain high-voltage components and can be lethal if handled incorrectly.

• Enclosures and insulation matter. The mechanical structure must protect both the operator and the electronics.

• Controls and timing matter. Without a proper timing/control circuit you risk overheating, poor welds, or catastrophic failure.

General materials:

• Two microwave oven transformers (MOTs) removed from decommissioned microwaves. (Check local laws and proper disposal - and only use transformers in good physical condition.)

• Spot Welder Pulse Encoder and a secondary transformer to power the welder module: Ex: Spot Welder Dual Pulse Module 

• Sturdy copper welding cable and heavy copper electrodes (for the contact points). Ex: Spot Welding Pen, Welding Cable 4 gauge.

• Welding Panel Connector. Ex: Connector Plug and Socket Set, Male /Female Panel Connector

• Heavy-duty pushbutton or footswitch with proper mains rating and safety interlock. Ex: Heavy Duty Footswitch

• A safe, fire-resistant enclosure to mount components.

• Personal protective equipment: welding helmet or eye/face protection rated for welding, leather gloves, long sleeves, hearing protection, and a fire extinguisher rated for electrical fires.

• Tools: appropriate insulated hand tools, multimeter (for low-voltage tests only), and a qualified isolation transformer or residual current device (RCD/GFCI) in the incoming supply if available.

Note: This list is intentionally general. Choosing incorrect wire sizes, connectors, or insulation is dangerous.

Build outline:

1. Assess the MOTs safely - do not power or probe a transformer without proper training. MOTs can have high secondary voltage and capacitors inside associated microwaves may still hold charge.

2. Decide the welding topology - people commonly use two MOT cores in parallel. I connected 2 MOTs in series to increase the voltage as secondary windings are already pretty low (high current). The MOT secondary windings were removed and replaced with heavy gauge wire in few turns to increase current. This step typically requires specialist skills — do not attempt unless you are trained in transformer work. 

3. Mechanical assembly - mount transformers on a non-conductive, ventilated enclosure. Include insulating barriers and secure and insulate all live parts inside the enclosure.

4. Control and timing - a proper spot welder uses a controlled pulse (milliseconds). Use a purpose-built, properly rated control circuit or a commercially made welding timer/controller. Do not attempt to use improvisational timing methods that could leave mains live or cause uncontrolled current flow.

5. Enclosures, fusing and protection - fit properly rated fuses or breakers and, if possible, an RCD/GFCI upstream. Provide ventilation to prevent overheating and use thermal cutouts to avoid fires.

6. Testing procedures (safe, low-risk checks) - before any attempt to weld, verify mechanical integrity, check that no exposed conductors are reachable, and test control logic using low-voltage dummy loads where safe and appropriate. If any step requires exposing mains wiring for measurement, stop and consult a professional.

Safety best practices (don’t skip these)

• If you doubt your skills, stop. Consult an electrician or buy a certified spot welder.

• Work with a buddy who knows electrical safety and can disconnect power in an emergency.

• Disconnect power and discharge capacitors before touching transformer internals. Remember: microwaves and MOTs can retain dangerous charges.

• Never operate near flammable materials. Sparks and hot metal can ignite dust, solvents, and vapors.

• Use proper PPE. Welding produces bright arcs, ultraviolet light, and hot splatter.

• Fuse and breaker protection must be correctly rated and installed upstream of the unit.

• Enclose live parts so accidental contact is impossible during operation.

• Grounding: any exposed conductive casework should be properly grounded by a qualified person.

• Local regulation: check local electrical codes - modifying mains equipment may violate codes or void warranties and insurance.

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Why a commercial unit is often the smarter choice

• Commercial spot welders are engineered for safe current delivery, timing, insulation, and mechanical repeatability.

• If you will weld battery tabs, sheet metal, or jewelry regularly, a low-cost, new spot welder is often cheaper, safer, and more reliable than a risky DIY build.

• Buying used commercial welders from surplus or welding suppliers can be cost-effective.

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Safer alternatives to modifying MOTs

• Buy a capacitor discharge spot welder kit made for battery tab welding. These operate at safer voltages and are tailored for the job.

• Use a professional battery tab spot welder if you’re working with cells - these include matched timing and safety features.

• For occasional small welds, consider soldering (if the application permits) or mechanical fasteners.

• If your goal is learning, take a class or work with a maker space that has an existing, operator-approved spot welder and supervision.