On 12/9/2012 11:06 AM, Woodman wrote:
A power supply is not a simple project.


Thanks for ALL of the above informative, - very useful input on pitfalls of moding a battery charger. That's definitely a little more than I want to take on right now.

As a quick alternative, people have been moding computer power supplies. I'm sure these PS conversions have limitations as well. If you please, I'd like to here your thoughts on them as well.

I found this easy and quick way to use them for $14.
http://dangerousprototypes.com/docs/ATX_Breakout_Board


Woody




I like the name. dangerousprototypes.
The honesty is refreshing.

You steadfastly refuse to specify what you want.
There's a voltage tolerance spec linked
at that site.
If this meets your needs, go for it.

Be aware that, although the spec is written in stone,
the PS vendor is free to "interpret".
IF they decide that, for the intended application,
the 12V output will never be unloaded completely, they
can forgo a load resistor. They saved 3-cents. They don't
care that the open-circuit voltage goes to 18V under some
conditions that will never happen in a computer.

I predict what most people are gonna do.
They're gonna decide that 1.25Amps is less than they need.
They're gonna look at the sticker on the ATX supply and
see 12V 10Amps and 5V 30Amps.
They're gonna short out the polyfuses and be happy campers...
until whatever they're working on explodes.
I'm a smart and careful person. I've never dropped a screwdriver
or had a probe tip slip...pay no attention to that big burned divot
in the probe tip.

There are technical issues. The 9W optional load resistor is
there for a reason.
Typical ATX supply has one transformer with multiple outputs.
Regulation is based on one output, usually 5V. The switcher
puts only enough energy into the transformer to make the 5V right.
Problem is that if you want a lot of current from the 12V output,
there just isn't enough energy to support it. The fix is to
add load to the regulated 5V output until the energy put into
the transformer can support the 12V load current.
The ATX supply is designed as a system component and people
who design loads accept the limitations.

The vendor has determined that their system works as advertised
at 1.25Amps.

If you want a real bench supply, EBAY listings and ham-radio swapmeets
are full of 'em. If you know a ham, call him up and ask him to put
the word out. Most of us have way more power supplies than we can ever use.
Put a want on Craigslist.
Stick your hook in the water and try to snag something cheap.
If you're patient, they're out there.

Designing a power supply is deceptively simple. 99% of it is trivial.
That last 1% comes with a lot of smoked circuitry.
You don't want to learn at the expense of the iPAD you were trying to power
or the battery that blew up in your face when your regulator latched up.

You would not trust a power supply design to a freshly-minted engineer
unless you had a lot of free time to mentor him on this fast-track
learning experience.