A first resistance (125) within a power supply (110) can be connected to a power supply output (120). A variable resistance (130) can be serially coupled with the first resistance, thereby providing a first intermediate voltage at a point (115) disposed between the first resistance and the variable resistance. The variable resistance can be at a maximum in the absence of an expansion device (140) and at less than maximum in the presence of an expansion device. The first intermediate voltage and a second intermediate voltage (145) can be introduced to a comparator (150). The comparator can provide an output signal (170) when the first intermediate voltage exceeds the second intermediate voltage. The comparator output signal can be used to confirm the power supply capacity to power an external device (180) and to enable one or more external device functions.