Subject: Re: [linux-audio-dev] OT: Electronic advice for PC.
From: Lamar Owen (lamar.owen_AT_wgcr.org)
Date: Wed May 01 2002 - 19:02:09 EEST
On Wednesday 01 May 2002 06:35 pm, Patrick Shirkey wrote:
> Reading the specs for the SFX power supply design guide it seems that the
> minimum for a 120watt power supply is 1.5 amps and the max is 19.2
Looking at the specifcation at
http://www.formfactors.org/developer/specs/microatx/microatxspecs.htm gives
me much more of an idea of what you're dealing with.
While it seems like a win to float a battery across the DC outputs of an SFX
supply, this is a very bad idea for a number of reasons:
1.) Multiple voltages: you need a separate battery for each of 5V, 3.3V, 12V,
and -12V. Some of these voltages are difficult to derive within the required
tolerance via battery.
2.) Regulation of those voltages: worst speced voltage is the -12V rail at +/-
10%. Only NiCd cells are anywhere close to capable of +/- 10% over their
discharge curve. A lead-acid 12V battery, for instance, is typically at
12.6V at full charge, and 10.0 or slightly less fully discharged. This is
over 20% swing -- and won't cut the mustard for directly powering sensitive
CPU's and the like.
The spec mentions an SFX12V supply with a 12 volt nominal input voltage. This
would be the way to go, in my book.
I would set such a system up with a couple or three Power Wheels toy car
batteries (these things stand deep cycle!) and the SFX12V supply. Barring
the SFX12V supply availability, you can use the standard 120V AC supply and
most any inverter of at least 200W rating (to handle the surge current on
initial startup). Since this is an audio application, I would VERY HIGHLY
recommend the SFX12V supply instead, as a typical AC inverter throws alot of
audible hash off as RFI/EMI. You can get true sinwave inverters, but they're
not cheap.
The Power Wheels batteries have a recommended charger, and they're about $25
each in the States at Wal-mart. Don't know about Korea. They are sealed
lead acid, not too heavy, very rugged, fused, and come with a pigtail. The
special connectors can be cut off easily enough and replaced with more
readily available Molex 20A connectors.
The charger is a wall wart -- mount the Power Wheels batteries in the case,
run the pigtails out the back, and put a pigtail on the SFX12V supply with
mating connector. You can then run on one battery while the other charges,
or somesuch. Sounds like an interesting project....
> Most of the time it will only be running the CPU, RAM, 1xHDD, PCI soundcard
> and a usb numpad. Ocassionally you could throw in a remote sensor for a
> keyboard/trackball (has it's own batteries anyway). If a cdrom/dvd was
> being used most people would have access to a power supply (need it for the
> monitor at least) and they definitely won't be running around with those
> attached.
None of the devices mentioned can be run directly off battery due to the
regulation requirements. Notebooks contain special DC-DC converters to
change the varying DC output of the battery to a regulated output.
Oh, and contrary to popular belief, a voltage regulator can indeed step up the
voltage if it is a switching regulator. Step-up switchers are harder to
design properly, but they do work (using the flyback principle -- the same
thing used by your television to generate the 25KV for the second anode of
the CRT).
-- Lamar Owen WGCR Internet Radio 1 Peter 4:11
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