Date   : Thu, 24 Oct 2002 00:26:05 +0100
From   : "Richard Gellman" <r.gellman@...>
Subject: Re: BBC motherboard from a PC power supply

>> IIRC, the BBC Micro requires 4 isolated supplies, 3 of +5v + 0v, and one
>> of -5v (this is a purple wire in my Model B).
>> The -5v line and at least one 5v/0v pair can be obtained from the main
>power
>> connector of the PSU. The other two you might have to gain from the drive
>> power connectors.
>
>Er, that's rubbish :-)  The supplies aren't isolated at all.  It's a simple
>single +5V at about 2A and -5V at only a few mA, and that's only required
>for the sound and the serial interface.  The three +5V/0V cable pairs are
>commoned at the PSU output.  The reason Acorn had them built like that was
>just to avoid voltage drops across the board.  In fact, the three +5V
>supplies are also interconnected on the BBC circuit board.

Ahh...

Well, according to the circuit diagrams, the Master 128 power suppy
definitely uses isolated supplies
(referred to as 0VA, 5VA, 0VB, etc). I assumed the Model B did the same.

>Oh, and by the way, this technique of using multiple cables coming from the
>same source is a common one.  In fact, PC power supplies do it for the
>motherboard and dirve power leads -- they're all commoned.

Ick... How poorly thought out... (pc power supplies, not ur response heh) ;)

If thats the case with the Model B's supply, then just go ahead and use
whatever supplies on the PC PSU you can get hold of :)

-- Richard Gellman