Abstract: A power supply switch circuit includes a transistor for switchably connecting portions of a current path between a standby power supply and a power supply controller, with a capacitor as a temporary current source for the controller once the path portions are disconnected. The transistor is coupled to a switch which causes the transistor to connect and disconnect the portions of the current path. The capacitor is coupled to the transistor for discharging current to the power supply controller for a predetermined time after the switch signals the transistor to disconnect the path portions. A power-down indicator senses this signal by the switch to the transistor and in response transmits an alert signal to the instrument logic warning that a break in current is imminent. This warning permits the logic to save relevant information while power is provided through the capacitor.