An externally powered temperature calibration device includes a system that provides a warning of high temperatures within the device after the device has been disconnected from the external power. The warning system includes a capacitor that provides power to a light-emitting diode ("LED") after the calibration device has been disconnected from the external power. A temperature sensor monitors the temperature of an internal component. An output signal from the sensor is used to control a switch that connects the capacitor to one of several resistors having different resistances. The switch therefore controls the discharge rate of the capacitor based on the sensed temperature at the time the calibration device was disconnected from the external power. As a result, the period during which the capacitor powers the LED can be commensurate with the time required for the internal component to cool from its initial temperature.