Battery Spot Welder from Microwave Transformer

Before starting: a microwave oven transformer (MOT) has a primary at 120/240VAC and a secondary at 2000V+ AC. The primary is fine; the secondary is lethal. Remove and discard the secondary winding completely before any work.

The rewound secondary for spot welding uses 2-4 turns of 4AWG welding cable, giving approximately 2-3V AC output at several hundred amps. A thyristor (SCR) switch triggered by an Arduino controls the weld pulse duration — typically 20-100ms for 18650 battery tabs.

For nickel tab welding (0.1-0.2mm), a single pulse of 40ms at full power on a 120VAC-fed MOT works well. The key variable is electrode pressure — insufficient pressure causes arcing and spatter; too much pressure quenches the weld. Copper electrodes with a pointed tip, 2-3kg of pressure, and consistent dwell time gives reliable welds.