upper part (where the screen is). Sometimes the glass has an outer casing of plastic for even greater protection. The reed switch[i] is an electrical switch operated by an applied magnetic field. dishwashers to detect when they jam, and in the thermal cut-offs in electric showers (to stop the water heating to dangerous levels). In general, higher voltages and higher currents cause faster wear and shorter life. Photo: Some flip-style cellphones, like this one, are switched on and off by magnetic reed switches. The term "reed" may also include the external wire lead as well as the internal part.[i]. Where switch-off time is particularly critical, controlled avalanche MOSFETs might be appropriate. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. Photo: A "push-to-make" switch makes a connection and completes a circuit when you push it in; a spring makes it pop back out again when you take your finger away. Explain reed relay operation and applications. Explanatory video What is a Relay? In a typical reed switch, the two contacts (which look like metal reeds) are made from a ferromagnetic In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope. A Relay is a switch used as sensing and controlling device which makes and closes the contacts electronically or electromechanically. The reed switch is positioned next to the Examples are door and window sensors in burglar alarm systems and tamperproofing methods. This creates the magnetic field that is used to actuate the relay, whether it is a reed relay or an electromechanical relay. Transistor switch driving a relay Download for Macintosh or for PC.. A transistor switch is used to allow a 12 volt relay to be operated (turned on and off) by a small input voltage, E in.. phone. Why are Relays Used? on the wall. when you open it up. or a cellphone that flips open This is really important when it comes to practical applications: you need to make sure you use the right magnet and that it moves in just the right way to actuate your reed switch. enthusiast Bill Ward, who runs the superb Photo: A typical reed switch (a Comus RI-23). began to findapplication widely as a sensor and a Reed Relay . But sometimes we want electrical and electronic If the current stops altogether or flows all the time, you know the fluid has stopped moving, possibly indicating a jam or a blockage. A switching device that has sets of contacts that are closed by a magnetic effect. In particular, they were used for switching in the British TXE family of telephone exchanges. A contactor joins 2 poles together, without a common circuit between them, while a relay has a common contact that connects to a neutral position. it can detect the opening and closing movement? This is useful for two reasons. Main Differences between Circuit Breaker and Relay. There are many types of relay and each relay has its own application, a standard, and generally used relay is made up of electromagnets which in general used as a switch. 3. There are hundreds of other, less obvious applications for reed switches. Millions of reed relays were used in telephone exchanges in the 1970s and 1980s. As soon as the energisation of the coil is stopped, the main contacts go back to their original position. You can get reed switches built inside little pieces of Reed switches have small leakage currents compared to solid state devices; this may be useful, for example, in medical devices requiring protection of a patient from tiny leakage currents. A common example of a reed switch application is to detect the opening of a door or windows, when used as a proximity switch for a security alarm. The motor design could also be "inverted", placing permanent magnets onto the rotor and switching the field through the external, fixed coils. The two contacts of the reed switch become opposite magnetic poles, which is why they attract and snap together. Such motors were used in low-power long-service-life items, such as computer cooling fans and disk drives. In production, a metal reed is inserted in each end of a glass tube and the ends of the tube are heated so that they seal around a shank portion on the reeds. There's a magnet in one part of the case and a reed switch in the other. The contacts on the This creates the magnetic field that is used to actuate the relay, whether it is a reed relay or an electromechanical relay. (Please note that I've colored and simplified the original slightly to make it easier to follow.). Please rate or give feedback on this page and I will make a donation to WaterAid. Some fluid level sensors in A well-designed and correctly used part will give a long and reliable life. Reed switches are sometimes also fitted to the spinning arms of whereas, a relay is an electrical switch that control (switch on & off) a high voltage circuit using a low voltage source. The could means that when the semiconductor switch is in its ON state, current will start to flow. the switch is in one position and pull them apart when the switch The circuit powering the coil is separate from the circuit switched on by the relay.  This allowed switching transistors to act as a commutator, but without the contact problems, wear and electrical noise of a traditional DC commutator. Such as water level in an overhead tank is monitored by some sensor and the pump is switched on or off automatically. Triacs, being solid-state, are mostly silent. When a reed switch is going to be used as a control device various factors need to be considered. Dictionary says that relay means the act of passing something from one thing to another, the same meaning can be applied to this device because the signal received from one side of the device controls the switching … Reed relays are small and fast acting; they require a much lower level of power to actuate them than other traditional types of relay and as a result they find many uses in various forms of electronic circuits. As the liquid flows, the paddle spins and makes a magnet rotate (3). Magnetic field artwork sourced from, Photo: LEGO® cows operated by a reed switch. Inductive Loads can be damaging to Reed Switches on opening due to the high load of induced back EMF. Capacitance across a reed switch can dramatically reduce life and even cause early contact sticking. A reed relay is a fast-operating, single-pole, single-throw switch with normally open (NO) contacts hermetically sealed in a glass envelope. The power MOSFET transistors are used as switching devices in solid state relay working. And wouldn't it be so obvious You can also get reed switches that work the opposite way: the two contacts are normally snapped together and when you bring a magnet up to the switch, spring apart. Bell Laboratories, invented there in the mid 1930s by Walter B. Elwood. Projects for the Evil Genius, US Patent 2,264,746: Electromagnetic switch, US Patent 4,038,620: Magnetic reed switch, US Patent 3,348,175: Normally closed reed switch, Introduction to Reed Switches, Magnets and Magnetic Fields. Brickpile blog material (that means something as easy to magnetize as iron), coated with a hardwearing metal such as rhodium or ruthenium (to give them a long life as they switch on and off), and sealed inside a thin glass envelope filled with an unreactive gas (typically nitrogen) to keep them free of dust and dirt. Sealed in a long, narrow glass tube, the contacts are protected from corrosion. When … The normally-closed contact is non-ferromagnetic and is closed by the flexible reed's spring force. A latching reed relay uses minimal power … They are made from two or more ferrous reeds encased within a small glass tube-like envelope, which become magnetised and move together or separate when a magnetic field is moved towards the switch. Reed switches can be used to directly switch a variety of loads ranging from nanovolts to kilovolts, femtoamperes to amperes, and DC to radio frequency. Relays are electric switches that use electromagnetism to convert small electrical stimuli into larger currents. It somewhat resembles part of some reed plants. Some kind of switch wired to the hinge so Why is a Relay Used? The advantage of using a relay is that a small current (circuit 1) can be used to switch on and off a circuit with a large current (circuit 2). This type of relay is called electromagnetic relay (EMR). by Chris Woodford. way—and probably quite unreliable too: all that opening and closing But how does it The relay is an electromechanical switch used as a protecting device and also as a controlling device for various circuits, equipments, and electrical networks in a power system. plastic so you can't even see they're there—perfect for all kinds of security The inert atmosphere around the reed contacts ensures that oxidation will not affect the contact resistance. Generally both contacts move (not just one) and they make a flat, parallel area of contact with one another (rather than simply touching at a point), because that helps to extend the life and reliability of the switch. Reed relays will certainly be around for many years to come. Take the magnet away and the contacts—made from fairly stiff and springy metal—push apart again and return back to their original positions. Although most reed switches have two ferromagnetic contacts, some have one contact that's ferromagnetic and one that's non-magnetic, while some (like the original Elwood reed switch illustrated at the bottom of this article) have three. Most of the electrical switches we encounter are ones we control ourselves. Explain reed relay operation and applications. Speed sensors on bicycle wheels frequently use a reed switch to actuate briefly each time a magnet on the wheel passes the sensor. published on, Artwork: Walter Elwood's original reed switch design taken from. It has a normally closed reed switch in When you fit them inside a protective leather jacket, you'll find use a similar trick. This blog post covers why relays are used, how they work, the terminology involved, some various relay features and where relays are used. 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