A fly-back circuit includes a voltage source, a transformer comprising a primary winding electrically coupled to the voltage source and a secondary winding electrically coupled to a load, a diode connected between the secondary winding and the load, a switch electrically coupled to the primary winding, a controller operable to open and close the switch to control energy transfer from the primary winding to the secondary winding; and a voltage monitor connected to a node between the secondary winding and the diode. The voltage monitor includes a processor that generates a variable threshold tracking voltage (Vthtracking), generates an output signal that is a toggling output when voltage at the node is above Vthtracking and a constant output when the voltage source is operating below Vthtracking and increases Vthtracking when the output signal is toggling until the output signal is not toggling.