Reed switch
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Reed switches (or reed contacts , historically also Herkon ) are contact tongues made of an iron - nickel alloy that are (hermetically) melted into the glass tube and operated by a magnetic field .
The term "reed" (English for little tube, reed, North German reed ) refers to the thin-walled glass tube in which the contact wires are melted. "Herkon" stands for "hermetically sealed contact". Reed switches are contained in reed sensors or reed relays . The ferromagnetic circuits move towards one another when an external magnetic field is applied. This technology makes it possible to produce reliable, hermetically sealed switching elements with a small size for - compared to conventional relays and contacts - fast switching processes.
The main components of a reed contact are the contact wires (paddles) made of a nickel-iron alloy (Ni approx. 48%) with the outer soldering surface (approx. 2–6 µm tin or gold) and inner contact surfaces made of precious metal. A glass tube fixes and protects it and contains the protective gas filling (nitrogen / hydrogen) or a vacuum for high-voltage switches.
history
The reed switch has its origins in the USA and was developed there by Bell Labs in late 1930. From 1940 onwards there were already the first industrial applications for reed sensors and reed relays, mainly in simple magnetically triggered switching functions and the first models of test devices. In the late 1940s, it was Western Electric company that introduced reed switches into telephone systems. Even today's designs still take advantage of the reed switch in such applications. During this time there was a comings and goings of manufacturers.
In the 1980s, these switches were called Geko contacts in the GDR, derived from a component with a protected contact.
Most manufacturers have managed to achieve a very high level of reliability with modern production machines. The worldwide demand for reed switches has now grown to around 1 billion pieces per year: The field of application is the entire spectrum of electrical engineering and electronics such as the automotive market , alarm systems , test and measuring equipment market , household appliances, medical technology , industrial applications .
features
Due to the materials used and the hermetically sealed design, reed switches can be used in almost all environmental conditions. However, there are a few points to consider that affect reliability. The glass feedthrough of the connecting wires, which is responsible for the tightness, is sensitive to breakage when subjected to bending loads and, due to the different expansion coefficients , sensitive to thermal shock when soldering close to the glass. The contact material ( rhodium or ruthenium ) is applied by sputtering or electroplating and requires high purity. Foreign particles, even in very small concentrations, are the source of unreliability. Such noble metal contacts are not suitable for high switching capacities.
In the course of time, the length has been reduced from the original 50 mm to 5 mm. In addition to miniaturization, this enabled new applications to be opened up, particularly in high-frequency technology and through higher switching speeds.
Some characteristic values that can be achieved with reed contacts are listed below:
- Switching up to 10 kV
- Switching currents up to 5 A.
- Switching voltages down to 10 nV and currents down to 1 fA
- Use up to 7 GHz
- Insulation resistance across the open contact up to 10 15 Ω
- Contact resistance in the closed state typ. 50 mΩ
- Opener, closer and bistable switching function possible
- Closing time approx. 100 to 300 µs
- Operating temperature between -55 ° C and +200 ° C
- Insensitive to moisture, vacuum, oil, grease and many aggressive substances
- Shock resistance up to 200 g
- Can be used for vibrations from 50 Hz to 2 kHz at 30 g
- Long service life: with switching voltages below 5 V (arc limit), switching cycles well in excess of 10 9 can be achieved
Layout and function
A reed switch consists of two ferromagnetic switching tongues (usually nickel / iron alloy), which are hermetically sealed and melted in a glass tube. In the case of changeover or break contacts, the end of one of the switching reeds is non-magnetic. The switch tongues overlap and have a small distance of a few micrometers to approx. 1 mm from one another. If an axial magnetic field acts on the switch, the two paddles move towards each other - the switch closes. The contact area of the two reeds is coated with a very hard metal, usually rhodium or ruthenium, but also tungsten and iridium . These are applied either galvanically or by sputtering . The contact surfaces are important for the very long service life and good contact of a reed switch. Before it is melted down , the air present is replaced by nitrogen or an inert gas mixture with a high nitrogen content. Reed contacts are evacuated for increased switching voltages (kV range).
If the magnetic field generated by permanent magnets or coils is stronger than the spring action of the paddles, the two contacts close. The field to be fallen below for opening is much smaller.
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The procedure described applies to the 1Form A switch (short NO for normally open ), normally open contact or ON switch (short SPST for single pole single throw ). There are also multiple switches such as 2Form A (2 NO contacts ), 3Form A etc.
If the switch is closed in the rest position, it is referred to as a 1Form B function, also known as NC contact (NC for normally closed ). This can only be achieved with a passive ferromagnetic paddle. Without a field, the tongue rests against a non-magnetic contact.
The 1Form C switch, also known as a changeover contact (SPDT for short for single pole double throw ), is used for switching . In the rest position and without an applied magnetic field, the normally closed contact is connected to the tongue. With the magnetic field, the contact changes from the normally open to the normally open contact. Normal and working contacts are unmoved contacts. All three paddles are ferromagnetically conductive, only the contact area of the break contact (break contact) is provided with a non-magnetic plate. As a result, the path of the field lines to the normally closed contact is longer than to the normally open contact and the tongue migrates through the field to the normally open contact (which is closer magnetically). This is technologically necessary in order to be able to use the same, thermally adapted material (nickel-iron) for all glass penetrations.
Form and strength of the actuation magnetic field
If reed switches are used as position sensors (door contact, level, limit switch), permanent magnets are used for actuation. To ensure precise switching, the field must be aligned axially, i.e. in the direction of the switch tongues.
If the field is exactly across the reeds, the contact opens. This is used, for example, to maintain exact switching positions during reference runs of positioning drives: the contact initially closes when approaching, but opens when the tongues and magnet are in a T-shape.
Reed contacts can also be manufactured with a bistable function (“ latching ”). With these it is possible to change the switching status with a magnet or a coil. The sensor remains in the previous position until the external magnetic field reverses. This is achieved by a pre-magnetization that is just sufficient to hold the contacts, but not to attract them. If the external field removes the bias, the contact drops. If both fields add up, it attracts.
Reed switches can be operated, for example, by the following actions:
- Magnet moves towards or away from the reed switch
- Reed switch or magnet in rotating motion
- Ring magnet is pushed over the reed switch
- the magnetic field is interrupted or shielded by an iron sheet
Reed contacts can be used to make current sensors by surrounding them with a few turns of thick wire. Examples are the monitoring of the function of a warning lamp or the brake light.
The distinctive hysteresis characteristic is characteristic of such sensors and generally of reed contact applications, i.e. in this case the pick-up current is significantly greater than the holding and release current.
Interference
Ferromagnetic materials can interfere with the magnetic field required for switching. It is therefore recommended to use non-magnetic materials such as austenitic steel or brass for assembly . A distance of at least a few millimeters should be maintained from magnetizable components (e.g. made of ferrous materials).
Application examples
Reed switches, reed sensors and reed relays are produced for many different industries, such as: B. for mechanical engineering, automation technology, safety technology, automotive industry, aviation, agriculture, test & measurement technology, medicine, telecommunications, household appliances and marine.
Reed switches, combined with a permanent magnet on a float, are used as level sensors (float switches). Proximity switches are used to monitor doors, flaps and locks and to determine their position. Motion and acceleration sensors are further possible applications of the combination with permanent magnets.
In the marine they are used for the anchor position, controlling the bilge pump, the fuel level, the rudder end position, current monitoring, the toilet control or the oil level.
Medicine: In implantable and other devices, it is often important to use switches that are hidden behind surfaces. Devices such as electrosurgical generators use high-voltage relays to regulate the power supply for operative cauterization of the vessels. Similar devices use RF energy combined with saline solution to seal the vessels. High-frequency relays are a suitable solution for this.
The high-resistance switching of signals is important for many applications, for example in data acquisition systems, oscilloscopes, circuit board testers or semiconductor testers.
Security technology: Windows and doors for which the opening status should be electronically monitored for control purposes, e.g. B. for access controls, alarm systems and. Like. Other examples are fire extinguishers, seat belts and the like. v. m.
Further applications are coaxial high-frequency relays , current sensors and hidden contacts that can only be operated with magnets.
Parameters for reed components
Pickup and shutdown sensitivity
Tightening sensitivity (AW on , PI) specifies the closing point of the switch. With permanent magnets, the switch-on point is measured as the switch-on distance in mm or the magnetic flux density is specified . A measuring coil can be used to determine the ampere turns (AW, but mostly referred to as AT, ampere turns) up to the tightening point. To do this, the current in a coil wound around the contact is increased to the switch-on point and multiplied by the number of turns. The maximum value is given. Even if the paddles are of the best glow quality, a residual remanence must be taken into account. In order to create defined conditions before the measurement, a so-called saturation pulse is applied to the coil. The specification generally applies to 20 ° C.
The switch-off sensitivity (AW from , DO) determines the switch-off point of the reed switch and is determined in the same way as AW an .
Hysteresis
The switching hysteresis in% is the ratio between the pull-in and cut-out magnetic field strength in ampere-turns (AW for short)
Hysteresis = AW on / AW from × 100%
The hysteresis depends on design features such as coating thickness, paddle overlap, paddle properties, paddle length, melt zone, paddle spacing.
Static contact resistance
The static contact resistance is the direct current resistance generated by the paddle and the contact surface. The nickel / iron material with a specific resistance of 8… 10 · 10 −8 Ω · m has the greatest influence here . Compared to that for copper of 1.7 · 10 −8 Ω · m, this is relatively high. Typical for a reed switch are approx. 70 mΩ, whereas the contact point is only approx. 10… 25 mΩ. In reed relays, nickel / iron is often used as connection pins, these guide the magnetic flux and provide the spring force. However, they contribute approx. 25 ... 50 mΩ to the resistance.
Dynamic contact resistance
When measuring the dynamic contact resistance (DCR), one determines the behavior of the reed switch, especially at the contact point, especially with regard to contamination.
For the test, the contact is switched with a frequency between 50 Hz and 200 Hz. A measuring voltage of 0.5 V and a current of approx. 50 mA are sufficient to locate potential problems. The measurement result can be displayed either with an oscilloscope or by digitizing the signal. The voltage of 0.5 V should not be exceeded in order not to "break through" any dirt films on the paddles. These can result from unclean cuts in the manufacturing process. For the smallest measurement signals, this film of dirt would then be an interruption that is only broken down by the higher test voltage, but does not visualize the problem as such.
Switching voltage
The maximum permissible voltage that the contact is able to switch is specified. Switching voltages above the arc limit can cause material migration on the contact surface. This normally happens from 5 V. These flashovers are the cause of the shortening of the life of a reed switch. Nevertheless, good reed switches are able to switch voltages between 5 and 12 V many tens of millions of times; Of course, the switching current also plays a decisive role there.
Switches with a pressurized atmosphere in the glass tube can switch voltages up to a maximum of 500 V, since the spark that arises is extinguished when opening. Any additional switching requirements are met by vacuum switches; voltages of up to 10,000 V can be achieved here.
With a switching voltage of 5 V, there is no arcing and therefore no material migration; life expectancies can also be achieved here over 10 9 switching cycles.
The lowest voltage in the range of 10 nV can be switched if care is taken to ensure low thermal voltages during construction. This large working area is a particular advantage of the reed switch.
Switching current
The switching current is the maximum permissible current in amperes when closing the reed switch. The higher the current, the greater the switching arc when closing ( bouncing !) And opening. Opening under power in particular determines the service life of the switch. Closing at a high current can lead to the contacts sticking (welding). Capacities (the load capacity) of the connected circuit are also important for a long service life. The first 50 ns are of crucial importance. This is where the potentially destructive spark arises. With a high capacity, combined with a correspondingly high voltage and / or current level, the resulting spark can destroy the contact in the long term and thus greatly reduce the service life. With relatively high switching signals, it is advisable to limit the current in the first 50 ns. A permanent influence on the reed switch can occur at 50 V and 50 pF.
Transport stream
The transport current in amps specifies the maximum permissible current via contacts that are already closed. Since the contacts are already closed, a significantly higher current is permissible than during the switching process, because a switching arc only occurs when closing and opening. A closed reed switch can transport very high currents; However, a short pulse length is important in order to avoid overheating.
Compared to other relays, reed relays have the advantage of very low leakage currents - minimal currents in the range of femto-amps (10 −15 A) can therefore also be processed.
Switching capacity
The switching capacity in watts is the product of current and voltage at the moment the switch is closed. If a switch is specified with a switching voltage of 200 V, 0.5 A and 10 W, the power of 10 W must not be exceeded. With a switching voltage of 200 V, the switching current must therefore not exceed 50 mA. If 0.5 A is switched, the switching voltage must be limited to 20 V.
Isolation voltage
The insulation voltage determines the point shortly before the breakdown of the isolating distance of a reed switch and is higher than the switching voltage. With larger evacuated reed switches up to 50 mm, insulation voltages of up to 15,000 V are not uncommon. Smaller models around 20 mm withstand 4,000 V, while 15 mm switches (with light gas pressure) have isolation voltages of 250 to 600 V.
Insulation resistance
The insulation resistance is measured across the open switch. A typical value for reed switches is 1 · 10 14 Ω. This good insulation causes only the smallest leakage currents from femto- to picoampere. Test devices that require a high-resistance switch between several inputs can thus be implemented.
Dielectric absorption
The dielectric absorption describes the charges remaining in the insulator and has a major influence on the handling of currents below 1 nA. These are delayed currents and voltages at the open contacts or against the surface of the contact, which occur in the order of seconds.
Closing time
Closing time is the time it takes to close until after the bouncing stops. Except for mercury-wetted reed switches, a rebound can be observed, which is determined by the switch-specific elasticities and masses. One or two bouncing events in a time window of 50… 100 µs are to be expected. Most reed switches have a closing time of 100… 500 µs.
Opening time
The opening time is the time until the switch opens after the magnetic field no longer acts on the switch. If the voltage of the relay coil is reduced below the drop or drop voltage, the contact paddles open in a time of about 20 ... 50 µs.
A distinction must be made between the fall time when a diode is connected anti-parallel to the coil (catching diode to suppress its switch-off self-induction voltage) - the time increases to approx. 300 µs. If a 12… 24 V Zener diode (cathode to cathode) is connected in series with this diode (the voltage pulse is limited to the Zener voltage), opening times well below 100 µs are achieved.
Resonance frequency
At the resonance frequency (see natural oscillation ) of the open tongue, the reed switch can close unintentionally due to external vibrations of sufficient amplitude. An approx. 15 mm long reed contact has z. B. a resonance frequency of 5500 Hz. Such resonance vibrations also pose a threat to the mechanical stability of the reed switch: they can damage the seal (glass bushing) of the switch up to total failure.
Contact capacity
The contact capacity is the capacity between the opened contacts. The values are in the range of 0.1 ... 0.3 pF. The low contact capacitance is a special feature of reed contacts compared to other relays and enables them to be used at high frequencies and / or for switching high-resistance AC voltage signals with low crosstalk (high attenuation when the contact is open).
literature
- Vladimir Gurevich: Electric Relays: Principles and Applications . CRC Press Inc, 2005, ISBN 978-0-8493-4188-5 .
Web links
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Individual evidence
- ↑ Rudolf Scheidig: Herkon relay 80, a relay series with hermetically sealed contacts for printed circuits . In: SEL messages . 7, No. 1, 1959, pp. 6-8.
- ^ Karl W. Steinbuch: Taschenbuch der Nachrichtenverarbeitung , 1st edition, Springer-Verlag OHG, Karlsruhe, Germany 1962, pp. 307, 431, 435-436.
- ↑ Hilmar Beautiful Meyer: Quasi-Electronic Telephone Switching System HE-60 . In: International Telephone and Telegraph Corporation (ITT) (Ed.): Electrical Communication . 39, No. 2, Standard Elektrik Lorenz AG, Stuttgart, Germany, 1964, pp. 171, 244-259 [245-246, 251, 254-257].
- ↑ Hoeckley Oden: Actual problem Of Telephone Switching - Quasi-Electronic Solutions For Switching Systems . In: Telecommunication Society of Austria (Ed.): The Telecommunication Journal of Australia . 14, No. 5/6, October 1964, pp. 342-355 [350, 355]. "The dry reed switch manufactured by SEL is sold under the registered name" Herkon "(hermetically sealed contact)."
- ^ Friedrich - table books electrical engineering . VEB Fachbuchverlag Leipzig, 20th edition, 1985, p. 255
- ↑ 7 GHz HF REED RELAY. (PDF; 1.3 MB) MEDER electronic, accessed on August 19, 2013 (With a few minor adjustments, the manufacturer estimates that 10 GHz can also be achieved.).
- ↑ Recommendations for the installation of manufacturers of window contact switches such as the Honeywell block reed contact 030002.17 or flat reed contact 030001.17
- ↑ Data sheet from Comus for the RI-26 series