Commodore 64 Power Supply

[Tony Duell]

> 
> On Mon, Jun 30, 2008 at 1:48 PM, Philip Belben <philip at axeside.co.uk> wrote:
> > Roy J. Tellason wrote:
> >> The 9VAC has to be a separate transformer winding from the one that
> >> provides the +5VDC eventually,  because of how it's used...
> >
> > There are plenty of transformers around with two 9V secondaries.  I'd reckon
> > on using on of at least 20VA rating even if you have a switching regulator.
> 
> Oddly enough, whereas two 9V secondary windings seem common in the UK,
> I'm having a VERY difficult time finding one in the US.  I suppose
> it's got something to do with the fact that UK/European transformers
> are stepping down from 230V mains, where the North American ones are
> stepping down from 110-120V mains.


That shouldn't make any difference (you can wind the primary winding for 
any input voltage, and have any combination of secondary windings that 
you like). In fact many (although not all) transofrmers sold in the UK 
have a pair of 115V primary windings which you connect in series for 230V 
or parallel for 115V input. 

Note, of course, that when connecting windings together, the polarity 
_does_ matter. It is AC, but you need to get the phase correct. Otherwise 
you may end up effectively short-circuiting things and blowing fuses or 
worse.

> 
> I have been able to find transformers with a 115 V primary winding and
> two 18 V secondary windings with center taps.  I assume I can use the
> center taps, but honestly I'm not sure.  Time to consult some higher

You can, but you'd then only be using half of each secondary winding 
(you'd use the tap and one end, of course). You could use the whole of 
one winding to provide the pwoer for your 5V regulator, by using a biphase 
(2 diode) rather than a bridge rectifier.

Can you get a trapsformer with 2 9V centre-tapped windings? You could 
simply ignore the taps then.

-tony