A mathematical model for the type of transformer/rectifier unit used in aircraft power-supply systems is described. The model is suitable for investigating both the effects of variations in the transformer parameters on the unit performance and the interaction of the unit with other items of plant in the supply system. The experimental unit on which the model was validated exhibits a marked imbalance between the two diode bridges fed from the transformer secondary windings, and this is satisfactorily predicted by the model. Introducing even a moderate harmonic content into the supply voltage to the transfer increases considerably the imbalance, which under full-load conditions could result in overloading and possible failure of the diodes of one of the bridges.