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- UPS with DC Output
January 9, 2006, 3:13 pm
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generated by a power brick from mains. This PC needs to run unattended,
and I need to be sure that it will power up cleanly after a power cut.
I think I need a UPS therefore. It doesn't need a lot of battery
capacity since its only purpose is to allow the PC to shut down cleanly
when the power fails.
The "normal" solution to this problem would be to add a UPS with an
RS232 output to the PC and some software which will shut down the PC
when the UPS tells it that the power will soon go off, and start it up
again when the power returns. However, looking at this solution more
closely, it seems dumb. I now have a power supply to generate low
voltage DC from mains and charge a battery. I then have an inverter to
take the battery voltage and convert it back to mains. Then I have
another power supply to convert the mains back to 16V DC. Why can't I
combine all these things into one box, which just converts the mains to
DC and uses it to recharge a 16V battery? This should be cheaper,
smaller, more efficient and more reliable than the "normal" solution,
it seems to me. Actually it's very like what happens in a lap-top.
It will still need the RS232 output and the software to shut down and
start up the PC, of course.
Can I buy a box that does this? Does anyone make this?
Is there any other better way of solving this problem?
Thanks - Rowan
Re: UPS with DC Output
While the # of parts needed to do what you want would be
less and thus (all other things equal) total cost less, all
other things are not equal. Your needs are unique and thus
such a specialty device would have a low customer base,
driving up costs substantially to a large % of production
and marketing costs per unit. That would far offset the
cost of the parts used and it would cost multiple times as
much to buy it.
The benefit of the ready-made ups in your situation is
having the integral software that shuts down the system, and
possibly reports the status of the UPS in some cases. Since
whatever you choose will be insufficient for a lengthly
power outtage (several hours opposed to a few dozen
minutes), it may not matter much if it's not of the utmost
efficiency, as this is going to be a temporary loss of
power, not long term. if you had longer term requirements
then I suggest a large array of cells in a perpetual power
arrangement where the AC power only serves to charge them
and there is no "UPS" involvement, it is always running with
no shutoff mechanism and when AC power comes back on it was
still running, merely the battery charging resumes.
We don't know your specific needs though, what downtime
would cost you for example or the total budget for the
solution. We also don't know the acceptible input voltage
range for your device. This alone could be very useful
information- Suppose your device can accept up to 27V, if
that were true then if you plugged a self-developed relay
circuit into the UPS, when the UPS AC kicked on it would
only have to supply enough power to energize the relay, a
very light load so little loss... actually I'm not even sure
if the UPS would function with such a light load, you would
have to try it and see.
With the UPS ac output energizing the relay, that relay
could simply complete the circuit to the system so it was
being supplied by an UPS that used 2 x ~ 12V (actually
approaching 13.8V in some instances) batteries. That is
quite efficient and when the UPS senses battery voltage is
dropping too low (same as when battery was directly powering
a system via AC output), it would shut off the AC output
which then de-energizes the relay, breaking the circuit. Of
course the UPS software had already powered-off the system
if the power outtage were long enough. The key to this
would be that your UPS is one that continually powers the AC
output, an online type rather than one that stays "off"
untill power goes out. That in itself is an inefficiency,
so any way you look at it, the complete solution is never as
efficient as a custom-design that would be quite costly.
On the other hand if the UPS was a simplier type that stayed
off, you could just power it with the current adapter and
use a relay to switch between the power sources rather than
only switching one on or off. Since a power brick has more
limited power reserve than a full sized supply, it might be
necessary to suppliment this with a few bulk capacitors to
stabilize it till the switchover were complete.