Methods and apparatus to detect voltage conditions of power supplies are disclosed. An example power supply monitor to detect fault conditions in a power supply includes a capacitive element communicatively coupled to the power supply, the capacitive element being configured to change state between a collapsed state and an open state in response to the power supply having a first voltage associated with a first fault condition of the power supply; a detector communicatively coupled to the capacitive element to detect a voltage spike generated from the state change of the capacitive element; and a signal generator to generate a fault signal in response to the voltage spike to indicate the power supply being in the first fault condition.