CG> Well, Alex, powering the PC from a battery is not the same
CG> as using a UPS.= I'm sorry I didn't understand what you
CG> were proposing. A UPS puts out 115= AC from an inverter
CG> using an internal battery. I powered our local voice=
CG> repeater from a battery for emergency power, and you could
CG> do something= like that.
I know. :-)
CG> Use a six volt battery and run
CG> the power from the battery to= the +5v power supply through
CG> two 10 amp diodes to drop the voltage from= full charge
CG> voltage of 6.9 volts to 5.5 volts (the drop through each
CG> diode= is .7 amps). Or use a 10 amp 115 volt AC relay
CG> powered across the mains and= when the AC power drops out,
CG> the relay switches the battery in. It may= drop your
CG> computer in the meantime, so the diode approach is better in
CG> that= respect.
I recommend against the diode approach. It may work well for a repeater, but
it is dangerous for a computer. First, the overvoltage tolerance on the +5V
line of a motherboard is very tight, and you will start to blow up gates if you
run even slightly high, say as much as +5.5V. Especially if there is a voltage
regulator module on the motherboard to drop the +5V source to +3.3V or +2.9V or
whatever is required for a fast CPU, overvoltage will be deadly.
Secondly, by powering the motherboard from the regulated output of a battery
and floating the battery with a simple charger, the machine will not be spiked
and should stay running.
CG> You could try a large capacitor across the power supply.
I strongly recommend against this. Most motherboards use tantalum capacitors
for bypass, and they will not survive the result spike that will result from
the capacitor being abruptly changed from a sink to a source. In fact, I have
had tantalum capacitors on motherboards go up in flame because of this.
CG> You= need + 12 v of about 1 amp, which you
CG> can get from a 12 volt battery, and= -12 volts low current
CG> which you can get from a DC-DC power supply or= another
CG> battery.
As I said, almost nothing in a computer uses -12V. It was originally needed by
DRAM chips for bias, but DRAM has not worked this was for a decade. Now, the
only thing commonly tied to -12V is an RS-232 port that would probably work as
well in practice without it.
CG> Of course, you need to be concerned how to charge
CG> the= batteries, and hopefully, the entire combination will
CG> cost less than UPS,= if cost is the primary consideration.
CG> I can get a basic UPS for in the low= 100's, so the cost
CG> differential is different here.
Yes, I think we've all agreed on this by now.
-- Mike