What is the correct way to terminate/wire these unused transformer pins? Do they get tied to GND?

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    \$\begingroup\$ Gosh no, that would cause the 750311307 to stop working. You could add another schottky rectifier and cap and use the voltage 1-2 generates as another (lower voltage) supply source. Traditionally this "aux winding" is used to power other parts of the circuitry. \$\endgroup\$ – rdtsc Sep 23 '19 at 21:01
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    \$\begingroup\$ What if I have no use for it? \$\endgroup\$ – circuit_noob533 Sep 23 '19 at 21:05
  • \$\begingroup\$ I'm wondering if the transformer would run a bit cooler with both primaries in parallel. Anyone got a reason not to do that? \$\endgroup\$ – Transistor Sep 23 '19 at 21:37
  • \$\begingroup\$ @Transistor would that not change the winding ratio? \$\endgroup\$ – circuit_noob533 Sep 23 '19 at 22:55
  • \$\begingroup\$ @circuit_noob533 No it would not change the ratio. It would be like having the same number of turns with thicker wire. Transformers with multiple configurations have wiring diagrams that instruct you to wire them that way for configurations where the extra turns (via putting coils in series) are not needed. \$\endgroup\$ – DKNguyen Sep 24 '19 at 2:58

You don't do anything with them, they are fine if they are floating and left open. If you're worried about a charge developing on the coil (which probably won't happen), you could tie one end to ground. Tying both ends to ground would result in a short and a considerable loss in energy.

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