Abstract: An automatic uninterrupted direct current power source switch utilizes a voltage controlled latching relay for switching over between direct current voltage sources, such as batteries, which supply power to a common load, with virtually no interruption in load current being experienced during the switchover. The power source switch is intended to be used with two independent direct current voltage sources in conjunction with a recording device which receives its power through either one or the other of the voltage sources, depending on the state of the latching relay. The latching relay will change state when the voltage powering the recording device reaches an adjustable trigger level which is below the nominal working voltage, provided that there is a second direct current voltage source connected to the power source switch and the second direct current voltage source has a terminal voltage near the nominal working voltage.