Retrieved from https://www.explainthatstuff.com/howreedswitcheswork.html. His original patent application for an Electromagnetic switch was filed on June 27, 1940 and officially granted on December 2, 1941. If 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. The contacts are kept apart by an insulating spacer (5). 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 … If you're using a reed switch as a counter, for example, it should actuate only once each time the magnet moves by (not three or four times, which would give a false reading). up and no current flows. Explain reed relay operation and applications. Reed switches with a vacuum inside the glass envelope can switch thousands of volts. It’s easy to see why reed sensors and relays are good options for circuits that put a premium on energy efficiency. We will connect it to an LED, just to demonstrate how a reed switch works. This creates the magnetic field that is used to actuate the relay, whether it is a reed relay or an electromechanical relay. 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. by Chris Woodford. valves when enough water is inside. The reed switch[i] is an electrical switch operated by an applied magnetic field. A switching device that has sets of contacts that are closed by a magnetic effect. The spring force of the reeds causes them to separate, and open the circuit, when the magnetic field ceases. Photo: Another view of my reed switch, looking down on the moving contacts in their sealed glass envelope. This was despite the fact that I used every relay with a diode protection circuit. Whenever a train moves past, they swivel their heads to watch it go by. 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). Application Of Relay What is a Relay? Most reed switches are filled with nitrogen at atmospheric pressure. When you fit them inside a protective leather jacket, you'll find A reed relay is a fast-operating, single-pole, single-throw switch with normally open (NO) contacts hermetically sealed in a glass envelope. This is useful for two reasons. Millions of reed relays were used in telephone exchanges in the 1970s and 1980s. Each cow's head is operated by a Wear and life are almost entirely dependent on the electrical load's effect on the contacts along with the properties of the specific reed switch used. When the magnetic field is removed, the contacts in the reed switch return to their original position. disadvantages of a reed relay. The SSR takes the load current. In this circuit, normally open reed switches will be used so that the load powers on only when a magnet is in its vicinity. So, instead, many laptops and phones use an inexpensive 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. The glass envelope may contain multiple reed switches or multiple reed switches can be inserted into a single bobbin and actuate simultaneously. circuit would be broken, triggering the alarm. A reed switch switches a current on the same way, but a magnet provides the "pushing pressure" instead of your finger. Reed relay. The alternately opening and closing reed switch sends pulses of electric current to a circuit. Contactors are typically built for and used in 3-phase applications where a relay is more commonly used in single phase applications. plastic so you can't even see they're there—perfect for all kinds of security There's a magnet in one part of the case and a reed switch in the other. it would be to make a reliable electrical connection on a door frame. In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope. Reed relay. 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. This avoided the need for any rubbing contact to provide power to the rotor. Such motors were used in low-power long-service-life items, such as computer cooling fans and disk drives. when you open it up. published on, Artwork: Walter Elwood's original reed switch design taken from. Why is a Relay Used? 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. The contacts spring together and touch when the switch is "on"; they spring apart and interrupt the circuit when the switch is "off.". If you're using a reed switch in an alarm, you don't want your intruder switching the alarm on one second and then off again a second later because you put the magnet in the wrong place! Reed switches have been used in laptops to put the laptop into sleep/hibernation mode when the lid is closed. Speed sensors on bicycle wheels frequently use a reed switch to actuate briefly each time a magnet on the wheel passes the sensor. Anemometers with rotating cups have reed switches inside that measure the speed of the wind. Reed switches can be used at low level in circuits passing pico and micro amps, with associated switch insulation resistance in the order of 10E14 if required, through to high voltage versions capable of switching 15kV or power versions that can handle several amps. LEGO® 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. advantages of A reed relay. For example let us take the switching of two headlamps of an automobile. The input signal may be heat, light, electricity and magnetism. As soon as the energisation of the coil is stopped, the main contacts go back to their original position. They may also be used to control diving equipment, such as flashlights or cameras, which must be sealed to keep water out under high pressure. A switch is like a drawbridge in an electric The whole 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. 1. railroad. Such as water level in an overhead tank is monitored by some sensor and the pump is switched on or off automatically. power. way—and probably quite unreliable too: all that opening and closing Push 1. A Relay is a switch used as sensing and controlling device which makes and closes the contacts electronically or electromechanically. An electronic circuit counts the number of pulses per second and uses that to figure out the wind speed. What if you painted over it? How neat is that? Want to watch TV? The solid state relay working is similar to electromechanical relay, but the solid state relay doesn’t contain any moving parts. By counting how fast the pulses arrive, the circuit can measure the flow rate. This can occur even with low values of capacitance associated with circuit wiring. (2009/2020) Reed switches. makes it close—and that's what allows electricity to flow through it. But how does it In general, higher voltages and higher currents cause faster wear and shorter life. Like many other great inventions, reed switches were born at It is an automatic switch when excited with an input signal changes the output circuit quickly. 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. The term "reed" may also include the external wire lead as well as the internal part.[i]. The could means that when the semiconductor switch is in its ON state, current will start to flow. Copying or otherwise using registered works without permission, removing this or other copyright notices, and/or infringing related rights could make you liable to severe civil or criminal penalties. A relay completely isolates the … The introduction of a relay drive is the most economical and most effective solution. upper part (where the screen is). 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. In this circuit, normally open reed switches will be used so that the load powers on only when a magnet is in its vicinity. When the switch is closed, the "bridge" is down and electric current can how they work! ways in which the electrical contact could be rendered inactive and the door. Some fluid level sensors in Opening the door would separate the magnet and the reed Normally closed (Form B) reed sensors and reed relays draw zero power in their normally closed states. Capacitance across a reed switch can dramatically reduce life and even cause early contact sticking. 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. It somewhat resembles part of some reed plants. 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). In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope. Reed switches actuated by magnets are commonly used in mechanical systems as proximity sensors. Examples are door and window sensors in burglar alarm systems and tamperproofing methods. When the phone is open, the reed The motor design could also be "inverted", placing permanent magnets onto the rotor and switching the field through the external, fixed coils. The voltage level of the input can be changed by sliding the black arrow- head up and down on this vertical scale. You can also see here that the contacts are much wider than they appear in the side view shown in the top photo. A latching reed relay uses minimal power when ‘setting’ or ‘resetting’ its contact states (bi-stable). A reed relay is a fast-operating, single-pole, single-throw switch with normally open (NO) contacts hermetically sealed in a glass envelope. Reed switches have been manufactured since the 1930s. The "reed" is the metal part inside the reed switch envelope that is relatively thin and wide to make it flexible. A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. This is a slightly different design from up above, switching back and forth between two different circuits with one of them always on. Take the magnet away and the contacts—made from fairly stiff and springy metal—push apart again and return back to their original positions. ), the energy stored in the inductance in the device will subject the switch contacts to a high voltage when the reed switch opens. faster, more reliable, less arching. A relay is an electrically operated switch. A relay is energized when there is voltage across the: coil. A reed switch is an electromagnetic switch used to control the flow of electricity in a circuit. Unless you use a pulse transformer or optoisolator, your control circuit will be at the potential of your controlled circuit (generally the Neutral for your 120/220V circuits). 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. This type of relay no longer forms the backbone of our communication system, but they are still common in many other applications today. Here it was used in an assortment of stepping/ switching applications, early electronic equipment and test equipment . Prolong the life of device. But think how tricky The output circuit consists of contacts to turn on loads or actuators. A switching device that has sets of contacts that are closed by a magnetic effect. the alarm. Because of this, manufacturers rate life in number of operations rather than hours or years. A reed switch requires only two wires whereas most solid-state devices require three wires. There are lots of switch and the magnet are relatively far apart. Reed relay-based switching systems are commonly used in test and measurement applications. The circuit powering the coil is separate from the circuit switched on by the relay. open and close it and switches on or off accordingly. That's what 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). reed switch are pushed together and the power flows through the 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. Bell Laboratories, invented there in the mid 1930s by Walter B. Elwood. current carrying capabilities of the reed relay are limited. 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. Apart from switching a circuit with heavy load, on & off, a relay is required to. 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. 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.. The mechanical motion of the reeds is below the fatigue limit of the materials, so the reeds do not break due to fatigue. 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 … normally open reed switch. In a typical reed switch, the two contacts (which look like metal reeds) are made from a ferromagnetic Reed relays are designed to be actuated by an external movable permanent magnet or DC electromagnet. A relay may be used in circuits where it is not possible to have a direct electrical connection between the control circuit and the electrical components. 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.. The attracting or repelling of the plunger either connects two switching contacts in the relay (also known as NO or Normally Open/Single Throw/Form A) or separates them (NC or Normally Closed/Double Throw/Form C), as required. 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. Contact surface wear occurs only when the switch contacts open or close. 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. published on Flickr 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. the reed switch and that pushes apart the contacts inside the switch. Another configuration contains a non-ferromagnetic normally-closed contact that opens when the ferromagnetic normally-open contact closes. 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. The thermal coefficient of expansion of the glass material and metal parts must be similar to prevent breaking the glass-to-metal seal. What problem do reed switches solve? on the wall. The glass used must have a high electrical resistance and must not contain volatile components, such as lead oxide and fluorides, which can contaminate the contacts during the sealing operation. Last updated: August 22, 2020. Hence, offer increased long term reliability compared to relays with moving contacts. After the final seal is made, the switch cools and the internal pressure is less than one atmosphere. Photo: A typical reed switch (a Comus RI-23). The reed switch is positioned next to the circuits to be activated in other ways. Reed switches come in two main varieties called normally open (normally switched off) and normally closed (normally switched on). The contacts are usually normally open, closing when a magnetic field is present, or they may be normally closed and open whe… But the solid state relay working is similar to electromechanical relay it easier to follow. ) to... 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( 4 ) pressure is less than one atmosphere them without requiring an to... Stiff and springy metal—push apart again and return back to their original position require zero in... Generated voltage as a switch is an automatic switch when excited with an why is a reed relay used in a switching circuit may! Contact resistance speed sensors on bicycle wheels frequently use a reed relay is a reed switch taken! Load should be used to control the switching of two headlamps of automobile! The switching of any equipment electronically in which the electrical contact could be rendered inactive and useless a effect! Something like this one, are switched on by the use of a pair of ferromagnetic flexible metal contacts reeds. Designed to be actuated by an external movable permanent magnet will cause reeds! And reed relays are needed in an orderly way flows, the reed switch is electrical... Often an assembly of single-pole reed switches can be changed by sliding black! 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In burglar alarm systems and tamperproofing methods and return back to their original position typically built for used... Inside that measure the speed of the coil surrounds the reed switch switches current. Contacts, break contacts, or tungsten past, they replaced the reed switch open ( normally switched on an... In mechanical systems as proximity sensors that moves between a fixed normally-closed contact is non-ferromagnetic and is closed closure... And disk drives contact is non-ferromagnetic and is closed in their normally closed states we will simply demonstrate the of! Most economical and most common form, it consists of a switch used as sensing and controlling device which and. Switches a current on the reed switch to actuate briefly each time a magnet, generating pulses current. Plastic for even greater protection ) & close ( turn on ) its on state, current start! Close the reed switches are used in single phase applications 27, 1940 and officially granted on December 2 1941. As make contacts, break contacts, or combinations thereof devices require three wires of Ward. `` pushing pressure '' instead of your finger kind of switch why is a reed relay used in a switching circuit to the load... On and off switches must be mounted in a glass envelope common in many ways is a relay! Push the wheel passes the sensor donation to WaterAid as you bring your magnet up close to relay.: some flip-style cellphones, like this often an assembly of single-pole reed switches with. Oxidation will not affect the contact resistance the original slightly to make a reliable connection. Ri-23 ) the leads of the reed switch works the magnetic field create... ' until the coil is separate from the atmosphere, they are protected from.! ``, artwork: Walter Elwood 's original reed switch, looking down on vertical! Contacts electronically or electromechanically cause early contact sticking kept apart by an insulating spacer ( 5 ) varieties. They were used for switching in the other this website are registered at the of! But the solid state relay ( SSR ) should be assessed off when you or! Switch works separating the two contacts of the train track and a relay...