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101
 
Magnetic Contacts 101
By Tab Hauser
En Español: Magnetic
Contacts 101 
 Tab
Hauser is President of Tane Alarm Products, a contact manufacturer
since 1984. He has written in the past on topics such as relays
and reed switches. This article has appeared in 4 different countries
over the last few years. Tab may be reached at (800) 852-5050.
Magnetic contacts are the first line of defense in the burglar
alarm industry. They are inexpensive, very reliable, and in most
cases out of sight. While PIR's and other interior detectors are
an excellent back up, it is the contact that immediately triggers
the alarm system when the door or window is open. Contacts should
be used on all entry sights because a PIR will pick up the intruder
after they are inside.
Over the last fifteen years Tane has been asked many questions
regarding reed switches and magnetic flux that pertains to the
alarm industry. Installers ask us all the time how reeds and magnets
work and why they work. Some of their other concerns deal with
contacts and magnets on steel surfaces.
It is not my intention to have this article be considered the
law on the subject. However, it is my intention to help those
newcomers in the industry and anybody who may not be up to date
on reeds and magnets.
A reed switch is the heart of a magnetic contact. It is nothing
more than a simple switch that turns on and off with a magnet.
Presently we sell our reeds in every car produced in North America
with ABS. The reed is in the brake fluid box and a magnet in on
a float measuring brake fluid for ABS systems to work. (Reed switches
are also turned on and off with an electromagnetic coil. This
is called a reed relay. Reed relays are frequently found in the
panels and PIR's you work with). The best contacts have reed switches
that use a de-activated rhodium over gold material in the center
of the switch. Many of today's top contact manufacturing companies
use this reed style.
A reed switch in a closed loop system works on a simple process.
When you put the magnet near the reed the two blades inside the
glass will close at the same time. When the magnet is withdrawn
the reed blades will open causing the circuit to open and the
alarm to be triggered. In an open loop the opposite occurs. The
reed is open with the magnet near it and closes with the magnet
moved away. A supervised system may require a single pole double
throw (SPDT) contact. This is a three wire contact, when the magnet
is moved away the current, or lack of, will switch to the other
pole.
A reed switch is a highly reliable device because the blades
are sealed in glass. The best quality reeds are sealed in an inert
gas. This means that the outside environment will have no effect
on the actual contacts, such as contact resistance build up as
sometimes found on mechanical contacts. By sealing in an inert
gas contact resistance between the blades is kept to a bare minimum.
A big myth with the magnetic contacts is if the reed is closed
for years and not opened the reed will magnetize shut. This is
highly unlikely. In fact we say it is impossible if you are using
a high quality reed switch in your contact. A reed will 'lock
up' if that circuit experiences a tremendous electrical surge
in the system. A magnetic contact on the average will only carry
one amp at DC100 volts. On a rare occasion a surge or lightning
strike will shoot current through the contacts many times the
rated capacity. This will create an arcing and welding of the
contact. One former manufacturer made a so-called lightning switch.
Their principle at the time was that lighting on that contact
forced the reed to remain open when hit by lighting. Its biggest
problem was its high cost and replacement needs. Tane at this
time is experimenting with high voltage contacts to counter the
surge problem. Another way a reed could lock up is when no current
or very little current and voltage (dry circuit) flows through
it. This lock up is called 'soft sticking' and does not usually
occur in better grades of the bare reed switch.
Another question asked to Tane is the speed of a reed switch.
It is not likely that one can ever open a door fast enough to
jump a contact. (You can jump a contact with a magnet if you know
where that contact is, as well as the correct polarity. This circumstance
is rare and involves high security bias contacts). A reed operates
in microseconds and will not miss.
Finally, reed switches are not affected by weather too easily.
Today's reeds are specified from -45 to +150 degrees. A contact
cannot freeze closed. Just ask your fellow installers from Alaska
if any of their Tane contacts ever froze closed.
Another myth regarding magnetic contacts involves magnets. A
good ALNICO V magnet used in the industry will never permanently
lose its magnetic flux. A magnet of this type will only lose some
of its power under certain conditions. Flux loss can be associated
with the 'he-man' installer whacking the magnet into a tight hole
using a hammer. (Magnets hate violence). You can also weaken a
magnet by taking it from a very hot environment to a very cold
one quickly. This type of problem never happens because most of
us do not bake and freeze magnets. Installers tell me they freeze
or roast in their trucks depending on the season. Do not lose
sleep on your magnets losing power here. They adapt well when
conditions are gradual.
Another myth about magnets is that when you place them on steel
they may permanently lose their power in time. On the contrary,
magnets and steel feed off each other. An ALNICO V magnet will
only lose part of its magnetic flux giving a smaller gap. Depending
on the thickness of the steel an installer may find a contact
with a 1" gap on a wood or aluminum surface go down to a
1/2" gap. This loss in magnetic flux will not get any worse.
Contact manufacturers will tell you, in the event of steel surfaces
use spacers for surface mount or use a true ¾" press
fit that leaves a little air gap between the reed or magnet.
When installing contacts remember the best gaps come when you
mount the reed and magnet in parallel or head to head. You can
also align the reed side and magnet in an "L" position.
Always avoid putting a magnet and reed in a "T." The
reason a "T" position mounting does not work well is
because all magnets and reeds have a small neutral spot so if
you set it this way and there is any shift in the surface of your
installation you are more likely to get a false alarm.
One other point manufacturers request is care. A magnetic contact
uses a glass reed switch in the center of the contact. Please
make sure holes are not too tight for recess. You are more likely
to get a bad contact by pushing a recess into a tight hole than
you are by drilling carefully.
All contact manufactures are out to make the best product they
can. Some of the best ways to advance this part of the alarm industry
is to make sure your contact manufacturer knows what you want.
At times a manufacturer does not know what is good or bad unless
we have the installers opinions. Remember, we need your opinion
so tell us what works and what does not.
For a description on how reed switches work, please go to http://www.hascorelays.com/reed_switches.asp |
