IBM SG24-4576-00 - Manual (Page 67)

1.11 Uninterruptible Power Supply (UPS)
Digital computers require a clean source of direct current (DC). It is the
computer
′
s power supply which takes an alternating current (AC) from the input
line and transforms it into clean DC voltages. However, problems on the input
AC signal can often lead to DC voltages that are less than satisfactory for the
digital circuits to operate properly.
There are five main types of AC line problems that can cause trouble for a
computer system:
1. Brownouts
Brownouts are extended periods of low voltages often caused by unusually
high demands for power such as that caused heavy machinery, air
conditioning units, laser printers, coffee machines, and other high current
electrical devices.
2. Surges
Surges are extended periods of high voltages and can be caused, for
example, by some of the previously mentioned high current devices being
turned off.
3. Spikes
Spikes are short duration, high voltages often due to a lightning strike, static,
or faulty connections on the power line.
4. Noise
Noise is generally electromagnetic interference (EMI) or radio frequency
interference (RFI) induced on the power line and can be caused by poor
grounding.
5. Blackouts
Blackouts occur when the AC voltage levels are too low for the power supply
of the computer to transform them into DC voltages for the digital circuits. At
this point, the computer ceases to function. There are any number of causes
for these to occur: a power failure, a short circuit, downed power lines, and
human error, to name but a few.
A UPS is an external device which connects the AC input line to the computer
′
s
power supply. It contains several components which can alleviate most AC line
problems. These are:
• Surge suppressors which protect against any large spikes on the input line
• Voltage regulators which ensure that the output voltage lies within an
acceptable range for the computer input
• Noise filters which take out any EMI/RFI noise on the input line
• Batteries which can provide an instantaneous power source in the case of a
power failure and also help to filter the input line
The blackout is often considered to be the most common type of failure.
However, when monitoring the power line, users are often surprised to find that
it is brownouts which occur far more frequently. It is also the brownouts that can
cause the most damage since they are usually unobserved and unexpected. The
UPS is critical here because it filters the input line providing a clean, stable input
to the computer
′
s power supply.
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