RE: C64 power supply replacement
Date: 2004-01-18 21:50:31

> -----Original Message-----
> From:
> []On Behalf Of ext Ryan Underwood
> Sent: 18 January, 2004 21:16
> To:
> Subject: Re: C64 power supply replacement
> > > I had a design proposed to me by a friend; advantage is that it is
> > > easily built from Radio Shack parts but I would like some 
> > > opinion on it.
> > > 
> > > - Start with 120V / 12.5VAC transformer
> > Why not 9V AC secundary?
> Because I need parts I can easily get (read: at Radio Shack...) ;)
I believe you will easily find a 120 to 9 V xformer. Or just use the one
from the original C64 PSU.

> > > - Use voltage divider to derive 9VAC from secondary and send that
> > Do you mean, a two-resistor voltage divider? I hope not.
> Yeah, such as:
You can't use such a voltage divider for power applications, for 2 reasons:
the utilization factor is abysmal, you would end up dissipating more (perhaps 
much more) power on the resistors in the divider, than the power 
consumed by the computer. Voltage dividers are used for signal reduction, 
such as the loudness regulation in amplifiers. The second, practically
unsormountable disadvantage of such circuit, is that Z1 and Z2 (R1 and R2
in case of purely resistive impedances, which is what you'll have in practice)
will have to vary as the current consumption varies! If you actually check
the very nice web page you mentioned, you will see that by varying ZL, Z1
and Z2 will have to vary, too. Otherwise Vout will not be constant.

> > >   straight to the C64 (it needs no regulation, correct?)
> > > - perhaps a bridge rectifier on secondary?
> > > - Send 9VAC through a 7805 regulator set to 5V
> > > - Filter ripple from 5V with a RC pair
> > Actually, an RC pair would consume way too much energy. Same as the 
> > voltage divider you mentioned previously.
> > > - Zener protection on regulator output to prevent short 
> > > failure damaging C64
> > Not necessary! The 7805 NEVER shorts. It's an incredibly
> > well-protected and well-protecting IC.
> I didn't know that.  At least it seems like every other 
> regulator in the
> world likes to pretend it is a wire at some times...
> BTW, I thought your design was simple and elegant, but I just took
> another angle at it (from the cost reduction standpoint).
 don't want to take credit for it - it's standard application circuit
for the 7805, with some details tweaked. My only addition was the
9V AC line. My only goal was to highlight that it can be done easily,
and this is how.
As for the 7805: it was done right long time ago, and now you have many
similar devices, which I could have included just as simply in the schematic.
However, I thought most members of the list are familiar with the 7805 rather 
than some obscure Microchip or Maxim device.

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