Commodore 64 Power Supply

Roy J. Tellason rtellason at
Wed Jul 2 11:36:23 CDT 2008

On Wednesday 02 July 2008 02:32, Seth Morabito wrote:
> On Mon, Jun 30, 2008 at 1:48 PM, Philip Belben <philip at> 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.
> 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
> authorities!  (i.e., Chapter 6 of "The Art of Electronics" and Chapter
> 17 of "The ARRL Handbook"!)

Which chapter depends on what edition of the handbook.  :-)

If you're talking about a full-wave (4-diode) bridge across a 9V winding you 
can get the same result by using two diodes with an 18V winding.  For the 
other,  where what you want is only 9VAC,  you can use half the winding,  
just one end and the center tap and not use the other end.

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