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.

I recently installed one in an old MFJ packet-terminal node controller
(TNC). THe original TNC design uses a 7805, with a small clip-on
heatsink, and a thoroughly-incompetent attempt to couple the heatsink
to the case for additional heat transfer (they gooped the heatsink
surface with silicone compound, but did *not* actually fasten the
heatsink to the case!). Even with the clip-on heatsink, the 7805 was
running quite hot... not too hot to touch, but warmer than I like.

The Murata part dropped right in, requires no heatsink, works nicely,
and it's barely warm to the touch.

As with any decision to substitute a switching regulator for a linear
regulator, you'll need to consider the output noise - it might be too
much for an audio amplifier to tolerate. Or, one or more additional
stages of filtering may be required.

These Murata parts are about $4.50 each in onesies, through Mouser.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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