Subject: Re: [linux-audio-dev] OT: Electronic advice for PC.
From: Paul Winkler (pw_lists_AT_slinkp.com)
Date: Wed May 01 2002 - 19:39:56 EEST
On Wed, May 01, 2002 at 08:01:41AM -0700, Patrick Shirkey wrote:
> Another question I have on this is what is the best way to connect the inverter to the board? Do I have to wire it into the power supply or can I get doubles of all the connectors that the power supply has and connect them directly? Is there another part that I will need apart from the voltage regulator?
The voltage regulator is a part of a typical power supply unit.
It's used to help ensure a clean, consistent maximum DC voltage level.
If you use an inverter, then you're generating 120 or 220V AC *from*
a DC battery source, so that you can then use a conventional
AC PSU in your computer. Seems daft, since the PSU then
converts the result back into DC of various voltages...
but that may be the most economical and expedient solution.
Just so you understand why I suggested a regulator:
I thought you were going tobe building your own power supply.
Maybe you'll still go that route, who knows.
Quick power supply description:
Typical power supply looks like this:
AC -> transformer -> rectifier -> filter -> regulator -> device
The transformer steps the AC from 120 or 220 volts (or whatever) down
to the ballpark of what you want, e.g. a 10:1 transformer
would give you 12V if the input is 120V. But it's still AC:
a sine wave which is positive sometimes, negative sometimes.
The rectifier takes the negative half of the wave and inverts it,
so you have a series of positive half-cycles. Still not very smooth.
The filter (usually based on some big fat capacitors) reduces a lot
of that ripple.
All fine and dandy, you've now got DC. But if the input is supposed
to be 120V AC and the circuit is designed to produce an output of 12V DC,
what happens if the input is actually 127V AC?
Then the output will actually be 12.7V, not what you want.
The regulator fixes that; it won't put out anything more than
its specified voltage. A simple regulator is a zener diode combined
with a resistor. You get consistent voltage across the zener, and
any excess appears across the resistor. As long as the input is
not so high as to fry any of the parts, the output of a 12V regulator
will be a nice, consistent 12V DC.
If you need several DC voltages, from your supply, use a transformer
with multiple taps that provide different AC voltages, and then
connect a separate rectifier->filter->regulator chain to each.
Now the bad news: This is the simplest kind of AC to DC power supply.
Computers actually use what's called a "switching supply" which
does funny things that I don't know about.
But the goal is the same: take AC of a
given voltage and produce DC of one or more different voltages.
Switching supplies are more complex, but can be smaller and lighter,
and they're cheap to mass-produce so the size / weight advantage
pays off.
> I get the impression that I am building my own power supply box in the process. Does anyone know of a product that is designed for this purpose already? In other words a power supply for a PC that runs on batteries?
I wish I did! It would also mean you could connect an external power supply
to your PC and get rid of that obnoxious fan.
Basically you're imagining a device that can take a single, 24-volt
battery, or two 12V batteries,
and produces well-regulated +12V, +5V, -5V, -12V, and +3.3V from it.
All you should really need to do that is some resistors, diodes, and
regulators. No fooling. But I'd be leery of trying it myself
because I don't know what kind of load the motherboard places on
those various supplies and I can't really afford to fry a motherboard
or two in experimentation. :)
It gets more complicated when you add essential things like a low-voltage
warning indicator, voltage spike protectors, etc. ..
It's completely possible that somebody's already marketing something
like this...
but I don't know of any. They may exist somewhere...
maybe browse around digikey.com? All kinds of weird stuff there...
and there are some DIY things on the web, search google!
I just found this:
http://www.google.com/search?hl=en&q=PC+battery+power+supply
This one's more complex than I described because it gets +12V and
-12V out of a single 12V car battery. That's way beyond my
knowledge of such things...
Anyway, back to inverters: The whole point of an inverter is that
it produces AC from a DC source like a battery. So you can forget
all of the above: buy a standard PSU, connect it to the inverter output,
plug the battery in to the inverter input, and you're done.
If you suspect that some current is being wasted in the inverter,
you'd be right. There is no 100% efficient power converting device
of any kind.
> Paul, you're right when you say that I may need professional help which is why I have posted to LAD :)
It ain't professional unless you pay for it. ;-)
One more note about current: you need big honkin' batteries to provide
the kind of current used by modern high-speed desktop cpus.
See e.g. http://www.compute-aid.com/atxspec.html
where it says a P4 needs the 5V supply to be capable of 20A.
20A is a LOT. 5 * 20 = 100 watts.
Are you sure you don't want to use notebook components? :)
--"Welcome to Muppet Labs, where the future is made - today!"
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