MOT

A microwave oven transformer, or MOT, is a step-up transformer that converts mains voltage (typically 120V or 240V AC) to the high voltage needed to operate a microwave oven's magnetron. The primary winding sits on the mains side; the secondary winding delivers 2000V to 2500V AC at modest current. This high voltage ionizes electrons in the magnetron, which then generate the 2.45 GHz microwave radiation that heats food.

MOTs are distinctive because they are heavily insulated and potted in epoxy resin or oil to withstand the voltage difference between primary and secondary windings. The transformer core is typically a laminated iron stack, and the whole assembly is compact and robust. Unlike general-purpose transformers, MOTs are not designed to handle variable loads or continuous low-power operation; they are engineered specifically for the duty cycle and impedance of microwave cooking.

Variants and practical use

MOT design varies slightly by region and oven manufacturer, but the voltage output is standardized within a narrow band. Some units include a center tap on the secondary winding, which allows the transformer to supply both the high-voltage anode and the filament heater of the magnetron. Others use separate filament transformers. Industrial or commercial microwave units may use larger MOTs with higher current capacity to handle longer cook cycles and higher duty factors.

MOTs are recognizable in dismantled ovens as heavy blocks, typically weighing 1.5 kg to 3 kg, with a single input cable and two or three output terminals. The terminal marked with a ground symbol is critical: it must be grounded to the oven chassis for safety. Failure modes include insulation breakdown (which results in leakage current to the frame), open primary windings (loss of output), and shorted secondary turns (reduced output voltage and excessive current).

The name reflects the transformer's function and application rather than any special electrical principle. In scrap recovery and DIY electronics communities, MOTs are sought after as salvage components, though their reuse outside microwave service requires careful attention to isolation, insulation testing, and protection against short-circuit currents.