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    When idling, the diodes are having approx. 40V in the non-conducting direction, hence causing the left end of the capacitor to float.

    What does float mean in this context?

    Does “left end” refer to the orientation of the capacitor in the diagram, or is there some de jure or de facto orientation for capacitors?

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      He refers to the “left end” of the cap as it’s drawn in the schematic. There is no convention around this, otherwise.

      We say a node in a circuit is “floating” when it does not have an electrical path to any other part of the circuit. Since the left end of the cap is only connected to the two diodes, and both diodes are in their non-conducting state and effectively acting as open circuits, that counts as floating.