Power sources, backup power circuits, power source control circuits, data storage devices, and methods relating to controlling application of power to a node are disclosed. An example power source includes an input, backup power source, and a backup power source control circuit. The input is configured to be coupled to a primary power source and further configured to couple the primary power source to the output when the input is coupled to the primary power source. The backup power source control circuit is configured to control a current path from the backup power source to the output based at least in part on a voltage applied to the input.