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From: grege@gold.gvg.tek.com (Greg Ebert)
Newsgroups: sci.electronics
Subject: Re: Uninterruptible power supplies
Message-ID: <1737@gold.gvg.tek.com>
Date: 30 Nov 90 21:46:46 GMT
References: <9011052132.AA07561@ferrari.nmc.ed.ray.com>
Organization: Grass Valley Group, Grass Valley, CA
Lines: 33


I know of 2 types of UPS's (Uninterruptible Power Sources).

Standby - As the name implies, it kicks-in when the power drops below spec.
	The only problem is that it takes a finite amount of time to
	determine if a failure has in fact occurred, and then it has to
	start-up and switch from the line to the inverter. I have seen
	many UPS's specifying 1/2 cycle switchover (8.3msec).
	For very sensitive loads, this may be unacceptable.

Continuous - Always running. The AC line constantly recharges (if necessary)
	the battery pack, and the inverter drives the load. Although the
	switchover time is zero, thus no glitching, you are always burning
	some kWh in the inverter/charging circuit due to circuit losses.
	The other thing that might be unsettling is that a failure in the
	UPS can disrupt power to the load even though the AC line is OK.
	[This happened here a few months ago. Not very funny].

Power ratings are MAXIMUM, and usually apply to resistive (ie unity power
factor) loads; you will need to derate for inductive loads and probably
most power supplies. Trying to squeeze a few extra watts might cause the
beast to shut down or 'fold back' (ie, lower Vout to reduce Pout).

My $0.02: Get a standby UPS for a PC. Price is usually $0.80/watt, or 
$350-$400 for 450 W.

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