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Subject: Re: Devices for transient/surge protection

Date: 07/29/00 at 8:30 PM
Posted by: Ray
E-mail: rerogers@gte.net
Message Posted:

In Reply to: Devices for transient/surge protection posted by toysl on 07/27/00 at 11:25 PM:

Not much traffic here so I will give you brief rundown;
as far as I recall.
1)gas discharge tubes: These are high voltage (>90V) that
are usually used in situations where the stess is severe and
requires significant power absorption. Lighting or perhaps
protecting a radar reciever against an adjacent radar
transmitter come to mind. The protected circuitry must
still withstand the breakdown voltage. I don't remember
the time to breakdown but it must be under a microsecond.
2) Zener: These are diodes with well defined breakdown
voltages used to protect systems whose operating voltage
is relativly close to voltage that damages them. They are
very fast and probably limited by lead inductance. They
protect like a diode in one direction and breakdown in the
other direction. Wattage vary but go up to at least 10W.
Pay some attention to the junction temperature (transient
thermal impedance) because you can blow the semiconductor
with a lot less power than the wattage rating if it is
packed into a short enough period of time.

3) TVS, MOV: I don't know about TVS but the MOV are
Metal-Oxide semicondutor. They are fast and used on long
lines (close to the protected equipment) to protect against
line transients. The ones I have seen are moderate wattage
and should be very fast. I believe they are bidirectional.
typically higher voltage devices (>20V). They are also used
in automotive modules to protect against inductive spikes.
4) You missed the plastic self reseting fuses. They used
to have lousy tolerances but I think that has gotten better
lately. They are thermally activated and so are relatively
slow.
5) For real heavy duty applications there are hydraulic
circuit breakers that will trip in the microsecond range.

You must always evaluate: Wattage required, response time,
and precision required. Then choose the solution matching.
Both underprotection and overdesign look silly. Be aware
that house and industry power lines have 4kV 100uSec pulses
on a monthly basis, the frequency and voltage follows a line
on a log-log plot. The IEEE has a standard based upon some
earlier GE work. I doubt if you can find the GE app notes
anymore so you will probably have to buy the IEEE standard
if you want power line protection. (My favorite comment
from the GE app note was that "A voltage monitor is the
best possible protection"; meaning that the research team
would go to sites where they knew there were failures and
be unable to capture them). There are instruments that will
moniter power line transients if you have particular problems.

As always, understand your problem and then any solution
you implement. In electronics design ignorance is a sin
that is punishable in this life.


Corrections or questions welcome.
Ray


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