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TAWUG Disk No. 56 (SHK)
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SURGE.PROTECT
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AppleWorks Document
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1987-07-01
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3KB
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40 lines
O===============|==============|==============|==============|==============|===
Power Protection Devices
Surge and Noise Protection
by Dick Marchiafava
JNote: This item is reproduced from an item published in Mini'app'les, the /
newsletter of the Minnesota Apple User Group.
KAt a recent Mac meeting the subject of devices to protect computer systems M
Kfrom damaging conditions on power lines, was raised. Although I have had a N
Llittle experience with such devices, I did not remember all the terminology P
Nand descriptions of the various circuits used. Since that meeting, I received J
an article which blew the dust from my memory and filled in the details.
IThe basis of most surge protection devices is a component called a Metal M
KOxide Varister (MOV). The MOV is a resistor whose value will change when a L
Jspecified design voltage is exceeded. The MOV can switch resistance state K
Ivery quickly and can handle a relatively large current flow, for a brief
time.
MThe power surge that MOV components are designed to limit are voltage spikes L
Jof relatively high value and brief duration. Such voltage spikes can pass M
Kthrough many power filtering systems. The MOV switches to a low resistance P
Nconductive state if a voltage spike is high enough to trigger it, and acts to J
H"clamp" or "clip" the spike by conducting excess voltage and current to >
ground. A typical MOV will be rated to operate a 150 volts.
MTo protect from surges as completely as possible, the surge protector should I
Ghave 3 MOV devices connected in both "Transverse Mode" (Line Mode) and O
M"Common Mode". Many inexpensive surge protectors will have only a single MOV
in the Transverse mode.
Power Line Noise Filters
KRadio-frequency or electo-magnetic noise will ride on the tops of AC power M
Kcycles like grass. This type of interference will not be suppressed by MOV C
Asurge suppressors because of the relatively low amplitude of the 4
interference, but must be dealt with by filtering.
FPower filters are a combination of capacitor/resistor circuits and/or P
Ninductors (or chokes). These components, used in combination, act to suppress
noise on the power line.
IGood power protection devices will have both surge suppression and noise I
Gfiltering circuits. Remember, these kinds of protection devices cannot P
Nprotect from direct or very close lightning strikes! There is simply too much O
Menergy in a lightning strike to be controlled by anything less than a system
engineered for the site.