On Thu, 13 Oct 2011 11:45:48 -0700, (Dave Platt)
wrote:

In article ,
Adam Sampson wrote:

Because that's how linear regulators work -- they behave like a sort of
intelligent resistor. If you regulate 28V down to 12V with a linear
regulator, the regulator acts like the right value of resistor to get
rid of 16V as heat.

You can do better with more modern regulators, but you need a switching
regulator rather than a linear one. National's "Simple Switcher"
regulators are good for this sort of thing; you pretty much just add an
external capacitor and inductor.

Murata now has some interesting little switching regulators which
"mimic" a 7805 - they have the same pinout, same current capacity, and
are built on a small PC board which is roughly the size and shape of a
TO-220.

Although I didn't say so explicitly, the two devices I mentioned are
also TO-220 "drop-in" switchers. It is certainly an emerging market
area.