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
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