Magnetic Contacts 101
By Tab Hauser
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 twenty 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 most autos produced in North
America with Anti lock brakes (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 that 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withdrawn the reed blades will open causing the circuit to open and the alarm to be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.
One type of magnet that has entered the burglar alarm business has been the Neodymium
type magnets. These magnets have the advantage of being smaller, yet offer a similar gap to
their larger ALNICO V types. The advantage today for smaller magnets is seen in the tight
fitting windows. Smaller magnets can at times be inserted into the tracks or frames of
windows with out any drilling. Tane’s mini or “donut magnet” can be either screwed into a
frame or stuck on with the adhesive it comes with. The sole disadvantage is that the
magnets can be more brittle. Installers using bare magnets should show more care when
installing these types so they do not chip them.
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. A contact should have high quality and flame resistant plastic. The better the
plastic the more the case will endure any shrinkage of any who drilled when weather
changes.
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 they
have the installers opinions. Remember, we need your opinion so tell us what works and
what does not. Please email me at tabh@hascorelays.com. Tane is always happy to see
what is new and needed out there.
[Tab Hauser is President of Tane Alarm Products, a contact manufacturer since 1983. He
haswritten in the past on topics such as relays and reed switches.Tab may bereached via
email attabh@hascorelays.comorwww.tanealarm.com516-328-9292 Hishobbies include
travel, photography and boating