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From: wtm@neoucom.UUCP
Newsgroups: comp.sys.att
Subject: Re: Power supplies & 3B1's
Message-ID: <774@neoucom.UUCP>
Date: Tue, 24-Nov-87 09:28:38 EST
Article-I.D.: neoucom.774
Posted: Tue Nov 24 09:28:38 1987
Date-Received: Sun, 29-Nov-87 04:22:05 EST
References: <942@woton.UUCP> <1805@ukecc.engr.uky.edu> <773@neoucom.UUCP>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 101
Summary: addendum to surge supressor article


I suppose I should have been more factual in the previous article.

A good quality protection device that is a passive device will
probably set you back $50 to $60.  Such a device will almost
certainly contain MOV (metal oxide varistors).  An MOV is
essentially a non-linear resistor.  MOVs are made of silicon
carbide or a similar material (please, no flaming, I don't have a
physics degree).  The MOV material exhibits a resistance that
varries inversely to something like voltage raised to the power of
eight.  In other words, when the voltage reaches a threshold, the
current conducted rises sharply.  Normally, MOVs are designed to
conduct the current of voltage spikes to ground or some other
inoccuous place that can absorb electrical charge.

An alternative to MOVs is a gas discharge tube.  Gas discharge
tubes function much like overgrown neon pilot lamps.  Power is
dissipated by ionizing the gas.  The ionized gas is also
conductive, thus permitting excess charge to be drianed away.  Gas
tubes are faster acting than MOVs and will stop working correctly
after they have triggered several hundred times.

MOVs also have a limited lifetime.  A good quality surge supressor
will be designed to feed a small current all the time (sub
threshold) through the MOV to detect if the MOV is working.  There
should be a pilot light on the surge protector to alert the user if
something is wrong.  Some cheap "surge protectors" have a pilot
light that is nothing more than an "ac is on" indicator, saying
nothing about the actual quality of protection provided.

A decent quality surge protector can be made from either MOV
devices or gas tubes.  In fact, some employ both types of
protection.  MOVs do have the advantage that it is easier to build
a circuit which monitors their protective effectiveness.

Some surge protectors also provide RF filtering.  The RF filtering
is of dubious utility, as computing equipment is required to meet
FCC type acceptance anyway.  An RF filter might be useful if your
computer is near a big source of RF energy, such as a radio
transmitter.  The FCC doesn't care much about a computer's EMC
characteristics when externally illuminated by RF.

When purchasing a surge protector, checking the incoming ac circuit
should (well, actually must) be part of your job.  You should
assure that the wiring is properly done so that the voltage
delivered to the computer is steady.  That is to say, have an
electricaian check the circuit to be sure that all the terminals on
the outlets, etc. are well tightened down.  A good solid ground is
essential when using a surge protector.  A defective ground may
actually case some protectors to deliver dangerous (to the
equipment) spikes when the attached equipment is switched off!

If you have inadequate voltage, and can no further improve the feed
from the fuse box to the computer, your only recourse is to install
a voltage correcting transformer.  Depending on the design of the
transformer, it may provide an additional layer of protection
against common-mode noise (that is the case where some sort of
interferrence is the same on both ac leads, but has a sizable
voltage with respect to ground).  There are two types of
transformers.  Tap changing transformers, and Ferro-resonant
regulators.  The former provides a more pure sine wave output,
while the latter is less efficient and has harmonic distortion, but
tighter voltage regulation.  Both types are relatively expensive.
You should talk to a power consultant to make the choice best for
you.  Voltage regualting transformers have been discussed recently
in sci.electronics.  Refer to the articles there for more details.

The last line of defense is an uninterruptible power supply (UPS).
A personal computer sized UPS contains a battery and an electronic
circuit to synthesize ac when the voltage from the power company
fails.  There are many grades of UPS available.  Some UPS more
accurately reproduce a sine wave from the battery supply, while
others simply put out a square wave.  In the case of the 3b1, the
square wave type would suffice, since the 3b1 ins't sensitive to
wave shape.  Some UPS-like devices are more accurately labeled
"standby power supplies".  A UPS notation implies that the
switch-over to backup power is essentially instantaneous when main
power fails.  A standby power supply might require 20 mS or longer
to switch over.  A quick standby power supply would be adequate for
the 3b1.  The 3b1 seems to be able to endure absence of power up to
100 mS, perhaps longer.  A UPS is better, but of course, it costs
more.  For me, a 200 watt UPS has been adequate for a 2 meg / 67
meg HDU 3b1.

You get what you pay for is a general rule, but some caution is
necessary with surge protectors et al.  The surge protectors are
often high mark-up items, and the dealers play on the fears of the
computer owner to ensure a sale.  I don't think it is a good idea
to buy your surge protector from one of those companies that sends
out glossy catalogs every 15 minutes.  There, about 50% of the
purchase price goes to the cost of the catalogs and to excessive
profit.  A case in point is that Inmac sells an autodialing
"computer room environmental monitor" for $300.  Radio Shack sells
the identical item as an autodialing "home security monitor" for
$150.  Shop around.

Sorry if this article is too electronically related for this group,
but having decent ac power to run our machines is a part of life.

Bill
(...[cbosgd|mandrill]!neoucom!impulse!wtm)