Any device which receives its power from an alternating current (AC) system operates best on a steady diet of clean, pure AC power with a good sine waveform and with voltage band frequently held within close limits. This is especially true for a digital computer because the information and control signals are extremely weak when compared to usual power handling devices. Transients and other types of unwanted signals are produced by the normal operation of these high-powered processes and can spill over into the computer, using the 60-Hertz supply lines as the highway.
"Filtering" in the usual electronics sense can be helpful but is frequently inadequate to prevent some of the unwanted power line "noise" from being troublesome. Devices which include filtering up to 30 MHz or thereabouts and which provide other types of protection have been developed for use with computers to isolate the sensitive equipment from stray disturbances which would interfere with the proper operation. These are the Line Conditioners which can be bought from the suppliers of computer equipment. Some of the types of protection which they provide are described below.
Noise Filtration is the process of screening out the high-frequency electromagnetic interference (EMI) coming in over the power line. These annoyances may be continuous or they can be intermittent but they can disrupt the operation of the computer because they have frequency components and signal strengths comparable to those which the computer uses in its normal operations.
This type of filtering has been used with electronic devices for many years. The degree of sophistication used depends upon the individual situation.
Some solid-state devices used in the computer circuits have almost no tolerance to overvoltages, even momentary ones. Spike Elimination circuitry is provided in the line conditioner to help control them. Semiconductor devices are used to absorb the energy in the spike.
Surge Clamping works on slower, longer-lasting surges of power. These surge clamps are capable of shunting aside thousands of watts by Metal Oxide Varistors. A clamping voltage of about 250 Volts at a current of 100 Amperes is provided by some commercial units. The clamp operates within about one-nanosecond or one billionth of a second.
Overcurrent Protection is provided by either a fast-acting fuse or fast-acting circuit breaker. This limits the amount of current to the computer during abnormal conditions on the line or the possibility of a breakdown somewhere in the computer system.
