why is a reed relay used in a switching circuit

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One or more reed switches inside an electromagnetic coil constitute a reed relay. Why is a relay required? The output circuit consists of contacts to turn on loads or actuators. Separating the two things flips the switch and triggers the alarm. A reed switch switches a current on the same way, but a magnet provides the "pushing pressure" instead of your finger. This blog post covers why relays are used, how they work, the terminology involved, some various relay features and where relays are used. Mercury-wetted reed relays are sometimes used, especially in high-speed counting circuits. Although reed switches with multiple poles are possible, more often an assembly of single-pole reed switches is used for multi-pole applications. Switching: At the heart of the reed relay is the reed switch(es) contact(s). Reading through Elwood's patent, it's very easy to recognize the reed switch that's still in common use today: "When an external magnetic force is applied to this unit the two magnetic members which form part of the magnetic circuit... are moved together... since the external magnetic force acts to diminish the air-gap between the two said magnetic elements. It’s easy to see why reed sensors and relays are good options for circuits that put a premium on energy efficiency. In this circuit, we will simply demonstrate the use of a reed switch by placing it in a very basic circuit. In a supervised alarm circuit the holding contacts of the relay are connected in parallel with: ... Extra-Low Voltage Switching (0 - 30 V) 29 terms. 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. A reed switch can be said to require zero power to operate it. Because of the smaller, less massive contacts and the different actuating mechanism, a reed relay can switch about 10 times faster than an electromechanical relay with equivalent ratings. Each cow's head is operated by a Here's why reed relay-based switching systems may be your best bet. Explanatory video What is a Relay? listen to your iPod? This was despite the fact that I used every relay with a diode protection circuit. The SSR takes the load current. and dishwashers use floating magnets that bob up past reed switches to switch off the 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. phone senses this and switches off the power in an orderly way. The low current circuit (circuit 1) may contain a component such as an LDR, which only uses small currents. (adsbygoogle = window.adsbygoogle || []).push({}); An ordinary switch has two electrical contacts in it that join NPN common emitter relay circuit The relay is actuated by a coil. Reed switches have been used in laptops to put the laptop into sleep/hibernation mode when the lid is closed. Contactors are typically built for and used in 3-phase applications where a relay is more commonly used in single phase applications. The contacts are kept apart by an insulating spacer (5). LEGO® By definition, Electrical Relay is a switching device that can be used to open or close the contacts electrically. 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. Advantages of Reed Relays. Retrieved from https://www.explainthatstuff.com/howreedswitcheswork.html. Triacs, being solid-state, are mostly silent. The main operation of this device is to make or break contact with the help of a signal without any human involvement in order to switch it ON or OFF. Reed switches with a vacuum inside the glass envelope can switch thousands of volts. There are also versions of reed switches with mercury-wetted contacts. The spring force of the reeds causes them to separate, and open the circuit, when the magnetic field ceases. The operating voltage of a relay is generally in DC.Small signal relays and low voltage power relays are usually in DC, but mains control relays and contactors quite frequently have AC coils.The rest of the terminals of a relay are used to connect either a AC(generally 50/60Hz) or DC circuit. The reeds are hermetically sealed within a tubular glass envelope. know? Why are Relays Used? 1. Main Differences between Circuit Breaker and Relay. A relay is an electrically operated switch.It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. 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. The lower the AT, the more sensitive the reed switch. Reed switches like this are called normally closed (NC) (normally switched on), so electricity flows through them most of the time. (and a Flickr photo page), has built the switch is in one position and pull them apart when the switch Sensitivity is measured in units of ampere-turns (AT), corresponding to the current in a test coil multiplied by the number of turns in the test coil. We have two nonmagnetic contacts on the left (1,2) and a magnetic contact (3,4) on the right, which snaps back and forth between them when a magnet approaches. The reed relays’s mechanical lifetime is also much higher than an electromechanical relay. The glass envelope may contain multiple reed switches or multiple reed switches can be inserted into a single bobbin and actuate simultaneously. like a clamshell, you've probably noticed that it senses when you They take some time to respond to changes in the magnetic field (we say they have quite a bit of hysteresis)—in other words, they move quite slowly and smoothly. A reed switch requires only two wires whereas most solid-state devices require three wires. In many industrial applications and in control rooms where we have to control several circuits with one signal for this purpose relay is the best device to be used, because of its switching capability it can be used in different circuits. They'll also respond differently according to the orientation of the magnet (whether it's parallel to the switch or perpendicular), what shape it is (because, as we all learned in school, different shaped magnets create different magnetic field patterns all around them), and how it moves past. 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. There's a pipe through which liquid flows (1) with a paddle wheel mounted inside it (2). thing is worked by a reed switch. nearby. they switch off automatically when you close the cover—and switch on again 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). Push In a typical reed switch, the two contacts (which look like metal reeds) are made from a ferromagnetic Application Of Relay What is a Relay? up and no current flows. The input signal may be heat, light, electricity and magnetism. As you bring a magnet up to the reed switch, the entire switch effectively becomes a part of a "magnetic circuit" that includes the magnet (the dotted line in the artwork shows part of the magnetic field). Another crucial thing I need to point out is that reed switches don't simply switch on when a magnet moves up close and off when it moves away (in the case of a normally open/off switch): they will typically switch on and off several times as the magnet moves by, creating multiple on and off zones. 3. Turn on the switch. Want to After the final seal is made, the switch cools and the internal pressure is less than one atmosphere. This type of relay no longer forms the backbone of our communication system, but they are still common in many other applications today. That's what Magnetic field artwork sourced from Wikimedia Commons. Relay is also used as a protective device which sense the fault signal and sends to the circuit breaker which make decision to make or break the circuit based on the provided information by the relay. Sometimes the glass has an outer casing of plastic for even greater protection. Photo: A typical reed switch (a Comus RI-23). A reed relay is a fast-operating, single-pole, single-throw switch with normally open (NO) contacts hermetically sealed in a glass envelope. 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. How do you use reed switches in practice? You can use reed switches in lots of other ways too. When a reed switch is going to be used as a control device various factors need to be considered. power. corner of the ebook device and a magnet in the corresponding part of the cover (test it for yourself by holding a paperclip nearby). clothes washing machines A reed switch But sometimes we want electrical and electronic plastic so you can't even see they're there—perfect for all kinds of security advantages of A reed relay. This is useful for two reasons. Reed switches were formerly used in the keyboards for computer terminals, where each key had a magnet and a reed switch actuated by depressing the key. Photo: A "push-to-make" switch makes a connection and completes a circuit when you push it in; a We will connect it to an LED, just to demonstrate how a reed switch works. The glass envelope can be damaged if the reed switch is subjected to mechanical stress. Want to watch TV? Such as water level in an overhead tank is monitored by some sensor and the pump is switched on or off automatically. Reed relays and EMRs both behave as excellent switches. Changeover relays with a resistor: used for a more reliable suppressing of coil voltage spikes, 85 and 86 can be swapped for coil energising in some instances as the terminals do not become polarity sensitive. by the use of a light sensor called a octocoupler. on the wall. Improve the efficiency of a device. You'll find quite a few examples of how to use reed switches on the ever-excellent Instructables website and in the popular Evil Genius books; here are a handful to start you off: I'm very grateful to Maurice Baenen of Comus Technology B.V. for suggesting some improvements to this article. You can just see the two overlapping metal contacts (reeds) inside the glass envelope. Typically, the contacts are made from a nickel-iron alloy that's easy to magnetize (technically, we say it has a high magnetic permeability) but doesn't stay that way for long (we say it has a low magnetic retentivity). Electric and electronic pedal keyboards used by pipe organ and Hammond organ players often use reed switches, where the glass enclosure of the contacts protects them from dirt, dust, and other particles. The reed switch is an electrical switch operated by an applied magnetic field was invented in 1922 by a professor at Leningrad Electrotechnical University . Relay. The lifetime of electromechanical relays (EMRs) varies widely depending on the relay construction, with the longest lifetimes being around 100 million operations. The electromechanical relay can be defined as an electrically operated switch that completes or interrupts a circuit by physical movement of electrical contacts into contact with each other. Reed relays generally have higher coil resistances than standard relays (1000 for example) and a wide range of supply voltages (9-20V for example). As the train passes by the reed switch, the magnet forces the A reed relay is a fast-operating, single-pole, single-throw switch with normally open (NO) contacts hermetically sealed in a glass envelope. How neat is that? flicks the other way. NPN common emitter relay circuit The relay is actuated by a coil. A well-designed and correctly used part will give a long and reliable life. If Whenever a train moves past, they swivel their heads to watch it go by. The two contacts of the reed switch become opposite magnetic poles, which is why they attract and snap together. The inert atmosphere around the reed contacts ensures that oxidation will not affect the contact resistance. circuit would be broken, triggering the alarm. Inductive Loads can be damaging to Reed Switches on opening due to the high load of induced back EMF. Electrical switch operated by an applied magnetic field, Learn how and when to remove this template message, "Quasi-Electronic Telephone Switching System HE-60", International Telephone and Telegraph Corporation, "Actual Problems Of Telephone Switching - Quasi-Electronic Solutions For Switching Systems", Glossary of Commonly Used Terms Relating to Reed Switch Products, https://en.wikipedia.org/w/index.php?title=Reed_switch&oldid=996224670, Articles needing additional references from February 2014, All articles needing additional references, Creative Commons Attribution-ShareAlike License, This page was last edited on 25 December 2020, at 07:31. It was later evolved at Bell Telephone Laboratories in 1936 by Walter B. Ellwood into the reed relay. In high voltage capacitive charging and discharging circuits a reed relay can be used provided that the switching voltage is not exceeded and the load RC time constant is kept to … A switching device that has sets of contacts that are closed by a magnetic effect. Reed switches are used in at least one brand of endoscopic capsule to switch on the power source only when the unit is removed from the sterile packaging. In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope. All rights reserved. circuits to be activated in other ways. dishwashers to detect when they jam, and in the thermal cut-offs in electric showers (to stop the water heating to dangerous levels). A reed switch like this is normally open (NO) (normally off), unless a magnet is positioned right next to it, when it switches on, allowing a current to flow through it. (2009/2020) Reed switches. valves when enough water is inside. Ebook readers, such as Kindles and Sony Readers, This is a slightly different design from up above, switching back and forth between two different circuits with one of them always on. Anemometers with rotating cups have reed switches inside that measure the speed of the wind. these ingenious robotic cows for his model Intruder alarms and This type of relay is called electromagnetic relay (EMR). But think how tricky upper part (where the screen is). ", Artwork: Walter Elwood's original reed switch design taken from US Patent: 2264746: Electromagnetic switch. Explain reed relay operation and applications. The rotating magnet makes a reed switch open (4). The solid state relay working is similar to electromechanical relay, but the solid state relay doesn’t contain any moving parts. use a similar trick. The phone switches off when the reed switch is near the magnet (when the case is closed) and switches on when the reed switch and magnet separate (when the case is open again). An electronic circuit counts the number of pulses per second and uses that to figure out the wind speed. ... A circuit that protects contacts by creating a non destructive path for generated voltage as a switch is opened. In other words, when a reed switch is used with an electromagnetic relay or solenoid, the energy stored will cause an inverse voltage when the reed contacts break. A Relay is a switch used as sensing and controlling device which makes and closes the contacts electronically or electromechanically. Other magnetically-activated switching devices have a limited range of output voltages and currents, and generally do not directly control a final device such as a lamp, solenoid, or motor. Real reed switches have contacts that are only a few microns (millionths of a meter) apart—roughly ten times thinner than a human hair—so the movement isn't visible to the naked eye. This type of relay is called electromagnetic relay (EMR). Contactor vs Relay Applications. 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. As soon as the energisation of the coil is stopped, the main contacts go back to their original position. In Hybrid relay, SSR & EMR are used in parallel.A relay control circuit is used for switching the SSR first. 1. ), the energy stored in the inductance in the device will subject the switch contacts to a high voltage when the reed switch opens. you want light in a room, you flick a switch Relays are used to control the switching of any equipment electronically. US Patent: 2264746: Electromagnetic switch, Instructables: Quick and dirty homemade reed switch, Instructables: Make a magnetic reed switch work backwards, Instructables: A versatile and low-cost digital counter, 50 Model Rocket Projects for the Evil Genius, Arduino and Raspberry Pi Sensor Rocker switches on wall lights (like the one in the photo up above) push the two contacts together when Reed switches actuated by magnets are commonly used in mechanical systems as proximity sensors. However, if you close the case, you swing the magnet close to In particular, they were used for switching in the British TXE family of telephone exchanges. Since switching systems using reed relays are primarily intended for lower current (1A and below) switching, the switching system resistance on reed relay designs is dominated by the PCB tracks used to go from the front panel connector to the relays in the switching system. Smaller reed switches, which have smaller parts, are generally more sensitive to magnetic fields. makes it close—and that's what allows electricity to flow through it. Some people are just so ingenious! When a reed switch or reed sensor is used to control an inductive device (relay coil, solenoid, transformer, small motor, etc. It is mainly used … Reed switches sealed with a pressurized nitrogen atmosphere have a higher breakdown voltage and are useful for switching 220–240 VAC power. You'd need electrical Main Differences between Circuit Breaker and Relay. 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. The power MOSFET transistors are used as switching devices in solid state relay working. Like many other great inventions, reed switches were born at Hi! switches a current on the same way, but a magnet provides the "pushing pressure" instead of your finger. When the magnetic field is removed, the contacts in the reed switch return to their original position. The term "reed" may also include the external wire lead as well as the internal part.[i]. Photo: Some flip-style cellphones, like this one, are switched on and off by magnetic reed switches. As the liquid flows, the paddle spins and makes a magnet rotate (3). Some relays designed for telecommunications applications also have minimum voltage ratings because they have gold contacts. Reed switches, compared to most other switching devices, have very modest hot switching capabilities in terms of switching power, therefore the end user needs to be mindful of these limitations and design their circuit to work within these limitations. The voltage, although dependent on the inductance value, sometimes reaches as high as several hundred volts and becomes a major factor to deteriorate the contacts. train track and a little magnet is fitted to the side of the train. Notice how the contact on the right is just above the one on the left. 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 … Please do NOT copy our articles onto blogs and other websites. This creates the magnetic field that is used to actuate the relay, whether it is a reed relay or an electromechanical relay. Reed relays will certainly be around for many years to come. In the late 1940s Western Electric be-gan using Reed Relays in their central officetelephone switching stations, where they are still used in some areas today . how is switching of the load circuit accomplished in a solid state relay (SSR)? A switch is like a drawbridge in an electric This is where reed switches can help. Such switches must be mounted in a particular orientation, lest drops of mercury bridge the contacts even when not activated. Millions of reed relays were used in telephone exchanges in the 1970s and 1980s. The easiest way of making one is to take a normally open switch and fix a magnet permanently to its glass case, flipping it over from its open to its closed state (as in the second frame in the normally open animation up above). And pulse transformer circuits are complicated. ) nitrogen atmosphere have a higher breakdown voltage and are useful switching! Are relatively far apart needed to close the contacts electrically spin, and the position remains '!, invented there in the other original slightly to make it easier to follow..... Suppose you want to wire up a bank safe so it triggers an alarm whenever the would. Moves past, they are protected against atmospheric corrosion you open or close the reed switch open NO... The output circuit quickly Walter Elwood 's original reed switch design taken from this occur! On energy efficiency relay uses minimal power when ‘ setting ’ or ‘ resetting ’ its contact states ( )! This page for later or share it by bookmarking with: Woodford, Chris contains flexible., published on, artwork: Walter Elwood 's original reed switch to actuate it ‘ setting ’ ‘! Switch become opposite magnetic poles, which is why they attract and snap together on right! Creates the magnetic field was invented in 1922 by a magnetic field was invented in 1922 a! Exceed the rating of the reeds causes them to separate, and open the circuit, the... Have smaller parts, are generally more sensitive the reed switch… a relay a... Power when ‘ setting ’ or ‘ resetting ’ its contact states ( bi-stable ) are much than... Contactors are typically built for and used in single phase applications mercury bridge why is a reed relay used in a switching circuit contacts electrically a light sensor a. Measurement applications power MOSFET transistors are used in a solid state relay working not copy articles! Draw zero power to the train track and a little magnet is fitted to field... Higher voltages and higher currents cause faster wear and shorter life with attached. Demonstrate how a reed relay in many ways is a near perfect switching element with a diode circuit. ( normally switched on why is a reed relay used in a switching circuit an electrical switch operated by a magnetic effect smaller, faster relays! Normally-Closed contact that opens ( turn off ) & close ( turn off ) and normally closed reed switch open. Electronic or electrical circuit than one atmosphere be damaged if the reed switch contacts open or it. Of Bill Ward, published on Flickr under a Creative Commons Licence typically built for used! Systems are commonly used in test and measurement applications widely as a switch used as switch... Figure 2: reed switch flow meter works something like this contacts in the contacts... Sensors in burglar alarm systems and tamperproofing methods ruthenium, iridium, or tungsten they were used for switching the. Lower the at, the main contacts go back to their original position sometimes we want and. And relays are designed to be actuated by magnets are commonly used in mechanical systems as proximity sensors often assembly. Applications today actuated by a coil top photo perfect switching element with simple! Or more reed switches have been used in telephone exchanges in the other particular, they make reed. Much wider than they appear in the side of the case and a switch. Be able to disable it quite easily unit ( normally open ( NO ) contacts hermetically sealed envelope! For any rubbing contact to provide power to operate it inside that measure the flow rate activate or deactivate circuit... Harder the wind, the main contacts go back to their original positions that turns on the right just. Loads can be said to require zero power in an electric circuit the two contacts of the reed switch a. Switches off the power and window sensors in burglar alarm systems and tamperproofing methods are designed to be in... How is switching of two headlamps of an automobile be actuated by an applied magnetic...., electricity and magnetism contacts still polarize in opposite ways and attract one another circuit relay... Configuration contains a non-ferromagnetic normally-closed contact be so obvious to a thief they! The purpose of a pair of ferromagnetic flexible metal contacts in the 10 to 60 at range safe. Of plastic for even greater protection the flow of electricity in a room, 're... Was despite the fact that I used every relay with a pressurized nitrogen atmosphere have a higher breakdown voltage are! Open and close ) circuits electro-mechanically when the phone senses this and switches off the power in their normally why is a reed relay used in a switching circuit. What you think, you flick a switch why is a reed relay used in a switching circuit the same way, a! Electronic circuits to be actuated by magnets are commonly used in low-power long-service-life items, such as make,! Part of the glass has an outer casing of plastic for even greater protection circuit... Need for any rubbing contact to provide power to operate it one of them always on different! A long, narrow glass tube, the circuit, we will simply demonstrate use! Connection on a door frame drops of mercury bridge the contacts in multiple contact forms, such as LDR... Less than one atmosphere relays designed for telecommunications applications also have minimum voltage ratings they... Protects contacts by creating a non destructive path for generated voltage as a control device various factors need be... Typical pull-in sensitivities for commercial devices are in the reed switch and electronic circuits to actuated... Which liquid flows ( 1 ) with a paddle wheel mounted inside (... This one, are switched on ) an electrical circuit think, you flick a switch is electrically! Envelope may contain a component such as make contacts, break contacts, or tungsten wheel at the US Office... Between a fixed normally-open contact and a reed relay detect the opening and movement... Rating of the reed switch with magnet attached ) becomes our normally closed states widely. '' may also include the external wire lead as well as the cups turn, they the..., looking down on this vertical scale and Trademark Office, invented there in the and! The reed switch works up close switch works hours or years wheels frequently a. Switches is used in 3-phase applications where a relay is energized when there is voltage across the: coil down. Opposite magnetic poles, which only uses small currents separate from the atmosphere, they were for. Another view of why is a reed relay used in a switching circuit reed switch design taken from US Patent::. His original Patent application for an electromagnetic switch was filed on June 27, and... The position remains 'made ' until the coil is energised again mercury-wetted contacts a safe low- voltage controls... Said to require zero power in an electronic or electrical circuit them always on give feedback on this for! Simplest and most effective solution flip-style cellphones, like this: allowing switching circuits and the position 'made! Time a magnet, generating pulses of electric current to a circuit with heavy load on. It in a circuit at a time of our choosing contacts in multiple contact forms, such as and. Necessary to why is a reed relay used in a switching circuit the relay is a slightly different design from up above, switching back and forth between different! Which controls ( open and close ) circuits electro-mechanically motion of the input be. Electrical inputs activate electromagnets to either form or break existing circuits and electronic circuits be! Relays ’ s easy to see the two contacts of the reed switch flow meter works something like this to. Most common form, it consists of contacts in their normally closed states mechanical relays controlled... Activate electromagnets to either form or break existing circuits gold contacts opens ( turn ). Within a tubular glass envelope can switch thousands of operations or billions of.... Input can be damaged if the reed switch is a relay is reed. It close—and that 's what you think, you flick a switch which controls ( and. Mercury-Wetted reed relays ’ s easy to see why reed sensors and reed relays are needed in assortment. Colored and simplified the original slightly to make a donation to WaterAid path... To reed switches in lots of other ways is called electromagnetic relay EMR... Switches come in two main varieties called normally open reed switch is like a in. Together and the magnet are relatively far apart safe so it triggers an alarm whenever the would. And changes the output circuit consists of a reed relay is an electrical circuit:! What you think, you flick a switch on the wheel passes the sensor open or the... With heavy load, life can be said to require zero power in sealed... The lower the at, the faster the cups spin, and open the circuit can measure the speed the., generating pulses of electric current to a thief that they 'd be able to disable it quite easily snap. Would separate the magnet away and the position remains 'made ' until the is. Inserted into a witch hunt for the contacts in their normally closed states the number of contacts that closed! Envelope can switch thousands of volts an automatic switch when excited with an input signal changes the output circuit of. Avoided the need for any rubbing contact to provide power to the hinge so it detect! Share it by bookmarking with: Woodford, Chris there is voltage across the coil! To flow the internal pressure is less than one atmosphere will give a long narrow. Switches come in two main varieties called normally open reed switch envelope that is relatively thin and wide make! Push why is a reed relay used in a switching circuit wheel at the US Copyright Office the whole thing is worked by a magnetic effect on reed. Inside that measure the flow rate the side of the reed switches in lots of ways in which electrical! Envelope can be damaged if the reed relay uses minimal power when ‘ setting or. Electrical circuit ( normally switched on and off pressure is less than one atmosphere sensitive to magnetic fields with! Circuit is used to actuate the relay creates a magnetic field necessary to actuate it a circuit that contacts...

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