24 Jun
2013
24 Jun
'13
1:01 p.m.
Since I support the CBM crowd, I've had more than a few people send me
comments over the past few years, asking when I might consider offering
a replacement power supply for the home computing machines (VIC-20, 64,
128, +4)
Initially, I resisted for liability reasons and certification costs.
I know the problem.
My workaround is simply to publish the design, to be built at the
constructor's risk (do be sure the design _can_ be safe...). Certainly if
I wanted a supply for any old Commodore stuff, I would _want_ to make it
myself, that way I know it's been built correctly (you've not see the
horrors I've seen in modern amins-powered stuff...)
But, I decided earlier this year to put off those questions for a bit
and just see what might be technically and economically possible.
I've chased a few ultimately fruitless options, and I thought I'd see if
some fellow soul on this list might have some ideas.
Obviously, the first thought was a complete custom supply. My interest
was never in power design, so I am ill-equipped (and uncertain of my
abilities) to design such an item myself. Thus, I started looking for a
manufacturer that could design one.
Absolute requirements:
5VDC at 4.3A.
9VAC at 9VA unregulated
To this I added my requirements:
The 5VDC be a switching PSU output
Why? Efficieincy, or is there some other reason?
ISO C13 input power jack (so I can support many areas
without building
custom power cords
115/230 switchable design (or two very similar designs that can handle
the 2 voltages)
My cost under USD$30.00 per unit.
Actually, I tend ot do tthe design and if you don't like the price,
that's your hard luck. Building down ot a price is what has casuedd me a
lot of probles. Certainly if I was making such a supply it would const
considerablly more (some years ago I mand an unregualted 6V PSU for a
client. Byt the time I had finished, the components had come to over
\pounds 100.00. That did nto cover my time in making it either.
Transformer,s cases, etc are not cheap.
On the 9VAC front, I had little luck finding a
suitable 9VAC supply that
is 45VA or 63VA to support a 5VDC at 5A draw while leaving enough for the
5A 5VDC output. As well, I am not sure I am equipped to design a
switching 5V PSU from a 9VAC source.
What I would do is start with a 100VA transformewr with 2 separate 9V
secodnary windigns. That is a standard transfomer. Get one with a pair
of 115V primaries and use the standard selector swich circuit to connect
it for 115V or 230V mains.
One of the seondaries is used fro the 9V AC output as-is.
The other one is rectified, smoothed, and fed to a simple switching
regulator to bring it down to 5V.
Incidentally, this is basically the circuit of the C128 power brick (at
least the one I have).
With the old power bricks getting more brittle and prone to failure each
day, I feel some sense of urgency to find some solution for folks.
For liability reasons and because 5VDC is the more important voltage in
the units, I myself lean towards a small box that turns 5VDC into 5VDC
and 9VAC, but designing a voltage doubler/sine wave generator/isolation
I think you are going about it the wrong way. Given that that 9V AC can
be used for almost anyting, I think you want a reasoanbly sinusoildal
wave. And to produce that from 5V DC is not trivial. It can be done, but
heck, you've got AC mains around. Just use a transformer.
Anoyehr way, which I like less becuase it involves a mains-side SMPUS is
to have a 50VA 9V transofmrer (if there are 2 secondary windings, wire
them in parallel) to give the 9V AC output and a sepoarate off-the-shelf
5V PSU. THe mains input selector becomse more complciated
(series/parallel switchign for the transformer and a shorting switch for
2 pins on the SMPUS, which means a 3 pole swithc, which is not so easy to
find) but it avoids you having to make the regulator. On the other hand,
low-votlage input switchign regulators are a lot easier to design than
amins-input SMPSUs, so it sould be psosible to make one.
-tony