[time-nuts] Where does 28V come from?

[Majdi S. Abbas]

On Mon, Jul 20, 2009 at 11:59:46PM -0700, Hal Murray wrote:
> Lead acid batteries are close to 2V per cell.  For cars/trucks, they come 
> conviently packaged in 6V and 12V units.  The phone company works off 48V.
> 
> But where does 28V come from?

	I'm not sure, but some ideas come to mind:

	28V is a common supply voltage for some RF transistors, particularly
amplifiers.

	28VDC is also common on aircraft -- including their battery strings.

	That, or it's related the Phone Company (tm) -- while their strings
are at 48V, the buses are typically floated on rectifiers at 54-56V which
you could divide to around 28V easily.

	Alternatively, maybe it's just easy to regulate down to 24, or 
+/- 12V, accounting for voltage drop across the regulators.

	--msa