Described is a technology by which magnetic flux is used to provide backup power. A transformer has a line power source controllably coupled to a first input winding, and secondary power source controllably coupled to a second input winding. A controller monitors the line power and switches to the secondary power source if the line power voltage drops too low, or uses the secondary power source to augment the line power source if the line power current gets too high. Also described is incrementally transitioning from the secondary power source back to the line power source.