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
