Abstract: A dual-input, automatic-switching power supply comprises a primary power input, a secondary power input, and a power output. Included are first and second DPST mechanical relays and a third DPDT mechanical relay. A first relay input is connected to the primary power input and a corresponding first relay output is connected to a third relay input. A second relay input is connected to the secondary power input and a corresponding second relay output is connected to a different third relay input. The third relay output is connected to the power output. The relays in combination provide a relay moving contact gap spacing, source to source, of at least three millimeters. A first under voltage control module maintains the first and third relays in states causing the primary power input to be connected to the power output as long as voltage from the primary power input remains above a preestablished primary voltage dropout level.