I have a circuit with a 7805 regulator that was set-up with a 12.6V CT
transformer and two diodes on the input side. This gave it an input voltage
of 8.9V (or, 6.3V x 1.414 = 8.9).

The 12.6V transformer output is rated with the primary at 230V. Now I am
considering using it on 277VAC. This should bring the input to the 7805 up
to about 120% of the 8.9V, 0r about 10.7V.

At 8.9V no heatsink was required with a .5A load. How can I pre-figure how
much power will be dissipated when on the 277V primary, so I can select a
heatsink, or perhaps decide that the whole idea is either OK or NG???

I checked the National datasheet for the 7805, and still came up short.
Is it as simple as saying that I will have 5.7V dropout at .5A, thus will
have 2.85W to deal with?

TIA,
Chris

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