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Magnetic Contacts 101


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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 When the magnet is withdrawn the reed blades will open causing the circuit to open and the alarm to be triggered.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/hasco-reeds.html


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