From: mb4008@cehp11 (Morgan J Bullard)
Subject: Re: Uninterruptible Power Supply
Organization: University of Illinois at Urbana
Lines: 47

alung@megatest.com (Aaron Lung) writes:

>In article <1qk724INN474@hp-col.col.hp.com> cab@col.hp.com (Chris Best) writes:
>>I'm no expert at UPS's, but you said something that made it sound like
>>you didn't realize something.  On a typical UPS (well, on ours, anyway),
>>there is NO switchover from AC to DC.  All the protected equipment is
>>ALWAYS running from the batteries (via an inverter), with the usual 
>>condition of also having them on charge.  If the power fails, big deal - 
>>the computers never see it (until the batteries start to droop, but 
>>there's something like 60 car-sized batteries in that cabinet, so it 
>>takes a while).
>>
>>If you were gonna run the guts on straight DC instead of an inverter,
>>why not do it all the time?  Then there'd be no switchover to screw
>>things up, and no having to sense the failure fast.  Just keep the DC
>>on charge when the power is on, and it'll be there in zero time when
>>you "need" it.
>>

>Actually, it's a bit more complicated than that...I sounds to me,
>your UPS takes in AC, rectifies it to DC to charge the batteries, and
>then takes the battery DC and chops it to AC again, feeding your
>equipment.  This approach is the easiest and cleanest way to
>switchover from the mains to battery once your power kicks out since,
>as you mentioned, nothing will know about what happened down the line.

>Another way to do the UPS scheme is to use the mains until you
>lose power, and then kick in the battery backup with it's inverter to
>replace the lost power.  The problem here is the switchover time and
>you've got to resync the AC in no time flat.
That's a standby unit not a UPS, otherwise there would be no interuption.
Many standby units are labled as UPS's though.  
 			Morgan Bullard mb4008@coewl.cen.uiuc.edu
				   or  mjbb@uxa.cso.uiuc.edu


>Unfortunately, most everything is built around the assumption that
>AC is available, so the UPS guys have to provide and AC output to
>be usable...ya sorta have to make it work with what there already.

>Similar story with our telephone system.  It was first invented back
>in the 1800's.  We're still using the same damn system (media) as they
>did back then.  If I have a phone from back then, I can assure you
>it'll work on today's phone system.  It costs too much to overhaul
>everyone to a new system, so they make it work with what is out there.

>.
