On 10/4/07, Ralph Shumaker <[EMAIL PROTECTED]> wrote:
>
> Could a computer be run directly from a car battery (with appropriate
> voltage drops)? I thought maybe it could except for the negative
> voltages. Why does a UPS have to supply AC to the PC when the PC just
> converts it *back* to DC? I like the idea of having the computer
> running directly from a car battery on a battery charger set for a
> trickle charge. If the computer is the only thing running off the
> battery, then it should be able to stay up for several hours of outage
> before it runs out of spark. And it should survive longer if it doesn't
> have to convert to AC and back to DC again, right? A voltage monitor
> could signal the PC to shut down. A Myth machine would never miss a
> recording because of a mere power outage (assuming the necessary cable
> equipment had sufficient UPS also).
"Appropriate voltage drops" implies a switching DC:DC converter if you
want to have reasonable efficiency. In an abstract way, this is still
converting DC to AC to DC, but the AC part takes place at a much
higher frequency than the 60Hz of the standard main line. 20 kHz or
higher, to keep the size and weight of the transformers down.
Standard computer power supplies convert the nominal 120V 60Hz to
about 300V DC and then use a switching converter at many kHz to
produce the 5V DC and 12V DC and whatever else is needed. There is no
reason (except for lower efficiency in the switching circuits) that
the supply could not be designed to run from nominal 12.6V DC, a
6-cell lead-acid battery. Note that transistor switches lose about
0.5V which is a significant part of 12.6, but negligible with respect
to 300.
If you wanted to run off 4 car batteries in series, you can buy
off-the-shelf computer power supplies that have nominal 48V DC input.
The phone company runs everything from 48V so there is a large market
out there.
Oh, yes, keep the computer hardware away from the acid fumes from the
batteries. Or use sealed gel-cells.
Probably more than you wanted to know for the first installment.
carl
--
carl lowenstein marine physical lab u.c. san diego
[EMAIL PROTECTED]
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