Uninterruptible power supplies
| Date | 01 April 1990 |
| DOI | https://doi.org/10.1108/eb044994 |
| Published date | 01 April 1990 |
| Pages | 287-291 |
| Author | Howard Falk |
| Subject Matter | Information & knowledge management,Library & information science |
Hardware Corner
Uninterruptible power
supplies
Howard Falk
135
Watchung
Avenue,
Montclair,
NJ
07043,
USA
If you have ever been caught
by
a power blackout or brownout
— caused by lightning, storm
damage,
or simply a blunder by
a repairman — and have completely lost work which was
keyboarded but not yet stored on disk, you know how frustrat-
ing and infuriating that experience can
be.
There is little com-
fort, at such a moment, in realizing that your work habits
should really include more frequent disk storage commands.
Nor does it help to reflect that memory resident routines are
available which automatically store keyboarded material to
disk at frequent intervals.
An Uninterruptible Power Supply (UPS) offers
a
more ele-
gant way to avoid loss of material when a power failure oc-
curs,
and the UPS solves another problem as well. Perhaps
you didn't realize that if a power failure just happens to occur
while your disk is in the midst of read or write access activity,
there is a chance that a 'head crash' may occur, causing physi-
cal damage to the disk. Disk-stored files may be completely
lost, and the disk itself may have to be replaced.
What a UPS does
The function of an uninterruptible power supply (UPS) is to
provide electric power to computer equipment when the
power line is operating normally, and to continue to provide
electric power should the power line fail.
When the power line is operating normally, the UPS uses
the available power to keep its batteries charged. The battery
power operates a UPS device called an inverter, which turns
the direct current (DC) power from the battery into the alter-
nating current (AC) power needed by the computer equip-
ment. When the power
line
fails, the
UPS
continues supplying
power to the computer equipment directly from UPS bat-
teries,
and those batteries begin to discharge.
Standby power supplies
Like a UPS, a Standby Power Supply (SPS) provides an alter-
native power source (with power from a battery) when the
power line fails. However, the SPS power source goes into
operation only after the power
line
fails, and the SPS requires
some time to take over from the power
line,
leaving the com-
puter without power for a short interval.
Generally, the delay before an SPS begins to supply stand-
by power is not a serious one. Even if normal power to a
computer is completely cut off, the power to the circuits and
mechanisms inside the computer will not immediately go to
zero.
However, the chances of damage and data loss are
greater with an SPS than with a UPS.
The shorter the time needed for the SPS to come up with
the needed backup power, after a power failure occurs, the
less the chance of damage or data loss will be. A relatively
slow SPS may take 5 or more thousandths of a second to
supply backup power. Faster SPS units still take close to one
thousandth of
a
second to provide backup power.
A
useful feature for SPS units is the ability to accept a shut-
off signal from the computer equipment. Then, when the
emergency is over and the power line comes back into normal
operation, the computer can automatically shut off
the
SPS.
Battery power
After a power line failure, the time it will take to exhaust the
UPS battery depends on how fast the electricity
is
drawn from
it, and that depends on the power in volt-amperes (VA) re-
quired by the computer equipment fed by the UPS supply.
It is sometimes difficult to tell, from manufacturers' de-
scriptions, whether an emergency supply is a true UPS or is
actually an SPS to which the manufacturer applies the name
'UPS'.
If the unit includes automatic shutoff capability it is
almost certainly an SPS.
Backup time
Backup time is the interval, after a power line failure occurs,
during which the UPS supply can continue to keep computer
equipment running normally. The maximum available
backup time depends mainly on the watts required by the
computer equipment, and on the fully-charged capacity of the
batteries used in the UPS supply.
This means that a
UPS
supply with small capacity can pro-
vide only a short backup time, while a larger-capacity UPS
can provide a longer backup time. To put it in another way, if
the UPS has to supply power for only a short interval (while
the computer equipment is going through an orderly shut-
down) then a
small,
relatively inexpensive UPS supply can be
used.
No UPS, at least none that is affordable, is capable of run-
ning the computer for
an
indefinite time after
a
power failure.
The most feasible strategy for use of a UPS is to allow the
computer system or network to gracefully accept the power
failure, without loss of data or damage to disks, and then to
have the system users turn to other tasks until power is re-
stored.
In some computer installations, such as those which de-
liver patient
care
in hospitals, it may
be
essential that the com-
puter continue to operate for a period of minutes after the
power line fails, until an auxiliary electrical supply (such as a
gasoline-powered generator) comes on.
The Electronic Library, Vol. 8, No. 4, August 1990 287
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