A DIY spot welder works by pushing a massive burst of electrical current through two overlapping pieces of metal for a fraction of a second, generating enough heat at the contact point to fuse them together. The most common approach uses a salvaged microwave oven transformer, rewound to produce very high current at low voltage. With basic tools and about $20-50 in materials, you can build one capable of welding thin steel sheet, nickel strips for battery packs, and similar light-gauge work.
How Spot Welding Actually Works
Spot welding relies on a simple physics principle: heat equals the square of the current multiplied by the resistance of the metal and the time current flows. In formula terms, Q = I²Rt. This means doubling the current quadruples the heat. That’s why spot welders prioritize extremely high amperage (hundreds or even thousands of amps) at very low voltage, typically under 5 volts. The electrical resistance at the contact point between two metal sheets converts that current into intense, localized heat, melting a small “nugget” that fuses the pieces together.
Three variables control the quality of your weld: current, time, and clamping pressure. Industrial spot welders for 1/16-inch steel use around 10,000 amps, a quarter-second pulse, and 600 pounds of electrode force. A DIY build won’t hit those numbers, but it doesn’t need to. For thin nickel strips or light sheet metal, 200-400 amps for 50 to 250 milliseconds is enough to get a solid weld.
Sourcing a Microwave Oven Transformer
The heart of a homemade spot welder is a microwave oven transformer, or MOT. These are free if you have an old microwave, or cheap at appliance recyclers and thrift stores. Look for a transformer from a microwave rated at 700 watts or higher. The transformer steps household voltage up to around 2,000 volts to power the magnetron, but you’re going to reverse that relationship by replacing the high-voltage secondary winding with a few turns of very thick wire, producing low voltage at extremely high current instead.
Before you disassemble anything, unplug the microwave and discharge the high-voltage capacitor inside. This capacitor can hold a lethal charge even when unplugged. Use an insulated screwdriver to short across its terminals before touching anything else. Once safe, unbolt the transformer from the microwave chassis.
Removing the Secondary Winding
The transformer has two coils wrapped around an iron core. The primary winding (thicker wire, fewer turns) connects to wall power. The secondary winding (thinner wire, many turns) is the high-voltage output you need to remove. You’ll also see a set of thin magnetic shunts between the two windings on some models. Remove those shunts as well, since they limit current flow and you want maximum output.
Cutting out the secondary is the most physical part of the build. Use a hacksaw, angle grinder, or chisel to cut through the secondary winding on one side of the core, then pry and pull the wire out. Be careful not to damage the primary winding or nick the insulation on it. If the primary gets damaged, the transformer is useless. Take your time. Some builders find it easier to use a drill to bore out the winding material in sections.
Winding the New Secondary
Your new secondary winding needs to carry hundreds of amps, so it requires a very large cross-sectional area of copper. There are two common approaches.
The simplest option is a single loop of thick welding cable (2 AWG or thicker) passed through the transformer window two or three turns. Two turns will give you roughly 2-3 volts of output. This is the easiest to build but produces less current than other methods.
For better performance, use layered copper strip. One well-documented approach uses 4 to 5 turns of copper strip (0.5mm thick, 25mm wide) stacked 5 or 6 layers deep. Five layers of 0.5mm strip gives a cross-sectional area of about 62.5 square millimeters, good for roughly 200 amps. Six layers would push that closer to 212 amps. Before committing to your final winding, make a test winding with regular insulated wire to check how many turns you need to get about 5 volts at the output. Most MOTs end up around 4 to 5 volts with 3 to 5 turns.
Insulate between each layer of copper using Kapton tape or high-temperature electrical tape. The turns need to fit through the transformer window, so measure carefully. If the window is tight, fewer layers of thicker copper may be necessary. The goal is maximum copper cross-section that physically fits.
Building the Electrodes
The electrodes are the two tips that clamp the workpiece and deliver current. Copper is essential here because it conducts electricity far better than steel and resists fusing to the workpiece. Copper-chromium alloy is what industrial welders use (classified as RWMA Class II), but for a DIY build, solid copper rod works well. Use 1/4-inch (6mm) diameter copper rod, available at most hardware stores.
Sharpen the tips to a blunt point or a flat face about 3-5mm across. A smaller tip concentrates current into a tighter spot, which produces a stronger weld on thin material. A larger tip spreads the current and is better for slightly thicker stock. You’ll likely need to re-dress the tips periodically with a file as they wear and develop pitting.
Attach the electrodes to the ends of your secondary winding cables using heavy-duty copper lugs and bolts. Every connection in the high-current path needs to be tight and clean. Loose connections create resistance, which wastes energy as heat in the wrong place.
The Clamping Mechanism
You need a way to press the electrodes firmly against both sides of the workpiece. Clamping pressure matters more than most beginners expect. Too little force causes arcing, electrode sticking, and weak or cracked welds. Too much force on thin material can deform it.
The most common DIY design uses a lever arm, like a pair of heavy-duty pliers, tongs, or a hinged wooden or metal arm with the electrodes mounted at the tip. A simple approach is bolting the transformer to a plywood base and building a pivoting arm from wood or steel flat bar, with a spring or handle to apply pressure. For welding nickel strips onto batteries, you only need a few pounds of force. For thicker sheet metal, you’ll want something that can apply 20-50 pounds or more at the electrode tips.
Adding a Timer Circuit
Controlling how long current flows is critical. A weld pulse that’s too short won’t fuse the metal. Too long and you’ll burn through thin material or overheat the workpiece. For most DIY tasks like battery tab welding, weld times between 50 and 120 milliseconds work well. Thicker material may need pulses up to 250 milliseconds.
The simplest control method is a momentary push-button switch on the primary (wall voltage) side of the transformer. You tap the button briefly to fire a pulse. This is imprecise but functional for learning. A better option is a timer circuit using a microcontroller (like an Arduino) or a dedicated 555 timer circuit that triggers a relay or solid-state relay on the primary side. This lets you dial in pulse duration in millisecond increments, which makes your welds far more consistent.
A solid-state relay rated for at least 25 amps at 240 volts (or your local mains voltage) is the preferred switching method, since mechanical relays wear out quickly when switching high-current loads repeatedly. Wire the timer to trigger the relay, which connects and disconnects mains power to the transformer primary for the exact duration you set.
Testing and Adjusting
Start with scrap material identical to what you plan to weld. For battery pack nickel strip (typically 0.1-0.15mm thick), begin with short pulses around 50 milliseconds and increase until you get a weld that holds when you try to peel the pieces apart. A good spot weld on thin nickel strip should tear the base material before the weld nugget fails.
For thin steel sheet metal (around 0.5-1mm), you’ll need longer pulses and higher clamping force. If you see sparks flying from the edges of the electrode, your clamping pressure is too low or your electrode tips are dirty. If the electrode sticks to the workpiece, the current is too high or the pulse is too long. Blackened or heavily discolored metal around the weld point usually means excessive heat, so shorten the pulse.
Keep welds spaced at least a few seconds apart to let the transformer cool. MOTs are not designed for continuous duty, and the primary winding will overheat if you fire rapid pulses repeatedly. If the transformer gets too hot to touch, stop and let it cool for several minutes.
Safety Considerations
The secondary output of your welder is low voltage (under 5 volts) and not a shock hazard. The primary side, however, runs on full mains voltage and can kill you. Insulate all mains-voltage wiring thoroughly, use proper connectors, and never work on the circuit while it’s plugged in. Enclose the primary wiring so no exposed terminals are accessible during use.
Wear safety glasses when welding. Small bits of molten metal can eject from the weld zone, especially if clamping pressure is insufficient. Work on a non-flammable surface and keep the area clear of anything that could ignite from a stray spark.