Reed switch

Reed switches - and reed contacts - are melted in the glass tube contact blades made ​​of an iron - nickel alloy, which is magnetically actuated.

These hermetically sealed switches are actuated by a magnetic field. The name " Reed " (English for tube, reed, North German Reet ) means the thin-walled glass tube, in which the contact wires are melted down. Reed switches are contained in Reed sensors or reed relays. The ferromagnetic reeds move towards each other in an externally applied magnetic field. This technique allows reliable, hermetically sealed switch elements with low size - to make quick gear changes - compared with conventional relays and contacts.

The main components of a reed contact, the contact wires (paddle ) of a nickel- iron - alloy ( about 48% Ni ) to the outer solder surface (about 2-6 microns tin or gold) and inner contact surfaces of the noble metal. A glass tube fixed and protects and contains the protective gas filling (nitrogen / hydrogen ) or a vacuum at high voltage switches.

History

The reed switch has its origin in the U.S., where it was developed by Bell Labs end of 1930. From 1940 there were already first industrial applications for reed sensors and reed relays, mainly in simple magnetically triggered switching functions and the first models of test equipment. In the late 1940s it was the Western Electric Company, which introduced reed switch in telephone systems. Even today's designs take advantage of the reed switch in such applications still. During this time there was a coming and going of manufacturers.

In the 1980s, these switches in the GDR were called Geko contact, derived from component with protected contact.

Most manufacturers have managed with modern production machinery to achieve a very high reliability. The global demand for reed switches has now grown to approximately 1 billion units per year: Used for the full range of electrical engineering and electronics such as automobile market, alarm systems, test and measurement market, household appliances, medical, industrial applications.

Features

Due to the materials and hermetically sealed design used Reed switches can be used in almost all environmental conditions. Nevertheless, some points to consider that affect the reliability. The person responsible for the tightness of glass bushing of the connecting wires is fragile under flexural load and due to the different expansion coefficients sensitive to thermal shock during soldering near the glass. The application of the contact material ( rhodium or ruthenium ) by sputtering or electroplating and requires high purity. Foreign particles already in the smallest concentration, the source of unreliability are. Such precious metal contacts are not suitable for high switching capacities.

Over time the original length of 50 mm to 5 mm could be reduced. This enables new applications could be developed in addition to the miniaturization, especially in the high frequency technology and higher switching speeds. The following are some characteristic values ​​are listed, which can be reached with reed contacts:

• Switch to 10 kV
• Switching currents up to 5 A
• Switching voltages down to 10 nV and flows down to 1 fA
• Use up to 7 GHz
• Insulation resistance across the open contact to 1015 Ω
• Contact resistance in the closed state typ 50 milliohms
• NC, and bistable switching function possible
• Closing time is about 100 to 300 microseconds
• Operating temperature between -55 ° C and 200 ° C
• Insensitivity to humidity, vacuum, oil, grease and many aggressive substances
• Shock resistance up to 200 g
• Can be used with vibrations of 50 Hz to 2 kHz at 30 g
• Long life: with switching voltages below 5 V (arc - border) operations are well over 109 also reached

Design and function

A reed switch is comprised of two ferromagnetic reeds (typically nickel / iron alloy), which are hermetically sealed in a sealed glass tube. For openers changers or the end of the reeds is non-magnetic. The switch tongues overlap each other and have a short distance from a few micrometers to about 1 mm. Does an axial magnetic field on the switch, the two paddles move toward one another - the switch closes. The contact area of ​​the two switch tongues is coated with a very hard metal, usually rhodium or ruthenium, as well as tungsten and iridium. Are plotted this either by electroplating or by sputtering. The contact surfaces are important for the very long life and a good contact is a reed switch. Before melting the existing air is replaced by nitrogen or an inert gas mixture having a high nitrogen content. For higher switching voltages ( kV range ) Reed contacts are evacuated.

Is the magnetic field generated by permanent magnets or coils stronger than the spring action of the paddles close the two contacts. The opening to be border area is much smaller.

The procedure described applies to the 1Form A switch ( short NO for normally open ) NO or power switch ( SPST short for single pole single throw ). There are also multiple switches as 2Form A ( 2 NO ), 3Form A, etc.

If the switch is normally closed, one speaks of 1Form B function, also known as normally closed ( NC for short normally closed). This can only be achieved with a passive ferromagnetic paddle. The tongue is in no field on a non-magnetic contact.

To switch serves the 1Form C switch, also known as changer ( short SPTC for single pole double throw ). At rest and without applied magnetic field of the normally closed contact is connected to the tongue. With magnetic field of the contact from the rest of the contact changes. Rest and work contacts are unmoved contacts. All three paddles are ferromagnetic conductive, only the contact area of ​​the normally closed contact ( NC contact ) is provided with a non-magnetic plates. This leaves the path of the field lines to the idle contact is longer than the normally open contact and the tongue moves through the field to the (magnetic closer ) make contact. This is technologically necessary to use the same, thermally matched materials ( nickel-iron ) for all glass seals can.

Shape and strength of the actuating magnetic field

If reed switch used as position sensors (door contact, level, limit switches ) use for the operation of permanent magnets. To ensure precise shifting, the field must axially, so be aligned in the direction of the reeds.

If the field is exactly perpendicular to the reeds, the contact opens. This is used for example for maintaining precise switching positions with reference runs of positioning drives: the contact closes when approaching at first, but opens when tongues and magnet are in a T- shape to each other.

Reed contacts can also be made with bistable function ( "latching "). In these, there is the possibility of using a magnet or a coil to change the switching condition. The sensor remains in the previous position to the reversal of the polarity of the external magnetic field. This is achieved by a bias which is just sufficient to hold the contacts, but not for tightening. Cancels the bias on the external field, the contact drops. Add to both fields, he attracts.

Reed switches can be operated for example by the following actions:

• Magnet moves away to the reed switch toward or away from him
• Reed switch or magnet in a rotating motion
• Ring magnet is pushed over the reed switches
• The magnetic field is interrupted or blocked by an iron plate

Current sensors with reed contacts can be made by surrounding with a few turns of thick wire. Examples include the monitoring of the function of a warning lamp or brake light. Characteristic of such sensors and more generally for the reed -contact applications, the pronounced hysteresis characteristic, that is, in this case, the inrush current is substantially greater than the support and the waste stream.

Application Examples

Reed switches, reed sensors and reed relays are manufactured for many different industries, such as mechanical engineering, automation engineering, safety engineering, automotive, aviation, agriculture, test & measurement, medical, telecommunications, home appliances and navy.

Reed switch, combined with a permanent magnet in a float, can be used as a level sensor ( Float switch ). Proximity switches are used to monitor doors, lids and closures as well as for positioning. Movement and acceleration sensors are other possible applications of the combination with permanent magnets.

In the Navy, the anchor position, controlling the bilge pump, the fuel level, the rudder - final position, current monitoring, the toilet or control the oil level be mentioned.

Medicine: in implantable and other devices, it is often important that switches are used, which are hidden behind surfaces. Devices such as electro- surgical generators put a high voltage relay to control the power supply for the operational cauterizing the vessels. Similar devices use RF energy combined with saline to occlude the vessels. For this, high frequency relay, a suitable solution.

For many applications, where a high-impedance switching signals importance, for example in data acquisition systems, oscilloscopes, circuit board test equipment or semiconductor testers.

Safety Technology: Fire protection and fire doors in public and government buildings, hospitals, hotels and other buildings must be legally controlled electronically. Once they are opened, an alarm must be triggered. Other examples are fire extinguishers, window sensors, seat belts, and more.

Other applications include coaxial RF relays, current sensors and hidden, only with solenoid -operated contacts.

Parameters Reed Switch Products

Pull-in and Cut-off

Pull- ( Pull- PI) specifies the closing point of the switch. In permanent magnets to measure the switch- in Power-up distance in mm or is the magnetic flux density at. With a measuring coil can be the ampere-turns define (AW ) to the suit. For this, the current is increased in a coil wound around the coil until the switch-on contact and multiplying by the number of turns. It is the maximum value is specified. Even with optimum annealing quality of the paddles a residual remanence remains to be considered. To create defined before the measurement conditions applied to the coil with a so-called saturation pulse. The statement usually applies for 20 ° C.

The Cut-off ( Awab, DO) determines the switch-off of the reed switch and is determined analogously as Pull-in.

Hysteresis

The switching hysteresis in% is the ratio between pick-up and shut- magnetic field strength in ampere-turns (short- AW)

The hysteresis depends on design features such as coating thickness, overlap paddle, paddle nature, paddle length, melt-down, distance paddles.

Static contact resistance

The static contact resistance is the DC resistance, generated by the paddle and the contact surface. The most influence here has the nickel / iron material with a resistivity forward ... 10:10 8 -8 Ω · m. Compared with the copper of 1.7 x 10-8 Ω · m which is relatively high. Typical of a reed switches are about 70 milliohms, the proportion of the contact point, however, is only about 10 ... 25 milliohms. For Reed relays are often used nickel / iron as terminal pins, these lead to magnetic flux and provide the spring force. However, they contribute to the resistance of about 25 milliohms at ... 50.

Dynamic contact resistance

In measuring the dynamic contact resistance (DCR), the behavior of the reed switch is determined specifically for the particular contact point in respect of contaminants.

For testing the contact with a frequency between 50 Hz and 200 Hz is shifted. A measurement voltage of 0.5 V and current of 50 mA is sufficient to locate potential problems. Ads can be the result of measurement with either an oscilloscope or by digitizing the signal. The voltage of 0.5 V should not be exceeded in order to possibly not " beat " any dirt movies on the paddles to. This can be caused by unclean cuts in the manufacturing process. For minimum measured signals of these dirty film would be an interruption, which is only penetrated by the higher test voltage, but not visualize the problem as such.

Switching voltage

Specifies the maximum allowable voltage of the contact switch capable. Switching voltages on the boundary arc can cause material hikes on the contact surface. This usually occurs over 5 V. These same flashovers are the cause of the shortening of the lifetime of a reed switch. Nevertheless, good reed switches are capable of switching voltages 5-12 V many 10 million times; Of course there also the switching power plays a crucial role.

Switch with pressure atmosphere in the glass tube can switch voltages up to 500 V, because when you open the resulting sparks will be deleted. Any additional switching requirements are achieved by vacuum switch; Here are voltages up to 10 000 V feasible.

Under a switching voltage of 5 V there is no arcing and thus no material migration, here are life expectations also about 109 operations reachable.

Lowest voltage in the range of 10 LV can be switched, if care is taken in the construction to low thermal stresses. This large work area is a particular advantage of the reed switch.

Switching current

The switching current is the maximum allowable current at the closing of the reed switch in amperes. The higher the current, the greater the switching arc during closure ( Prellen! ) and Open. Especially under the opening current determines the life of the switch. Close at high current can cause the contacts for bonding ( welding ). Also capacity (load capacity) of the connected circuit are important for long life. Here are the first 50 ns of crucial importance. Here is the possibly destructive spark. At a high capacity combined with a suitably high voltage and / or current levels, the resulting spark can destroy the contact over the long term and thus greatly reduce the lifetime. It is recommended that at relatively high switching signals, to limit the current in the first 50 ns. Already at 50 V and 50 pF may result in a constant effect on the reed switch.

Transport stream

The transport stream in amperes specifies the maximum allowable current contacts are already closed. Since the contacts are already closed, a significantly higher current than the switching operation is allowed, since a switching arc arises only when closing and opening. A closed reed switch can carry very high currents; important is to have a low pulse width so as to avoid overheating.

Reed relays have over other relay the advantage of very low leakage currents - minimum flows in the range of femto (10-15 A) can therefore also be processed.

Switching capacity

The switching power in watts is the product of current and voltage at the moment of closing of the switch. A switch with a switching voltage 200 V, 0.5 A and 10W indicates the power of 10 W and may not be exceeded. At an operating voltage of 200 V, the switching current can not exceed 50 mA so. 0.5 A switched, the switching voltage must be limited to 20 volts.

Insulation voltage

The isolation voltage determines the point just before the breakdown of the separation distance of a reed switch and is higher than the switching voltage. For larger evacuated reed switches to 50 mm insulation voltages up to 15 000 volts nothing unusual. Smaller models to 20 mm resist still 4000 V, for 15 - mm - switches have ( with a slight gas pressure) isolation voltages of 250 to 600V.

Insulation resistance

The insulation resistance is measured by the open switch. A typical value for reed switch is 1.1014 Ω. This good insulation causes only the smallest leakage current of femto-to picoAmpere. Testing equipment, which has to be switched to high impedance between multiple inputs so can be realized.

Dielectric Absorption

The dielectric absorption describes the remaining charges in the insulator and has a great impact on the handling of currents less than 1 nA. Most are delayed effects that take effect in the order of seconds.

Closing time

Locking time is the time required to close up after the end of chattering. Except for mercury-wetted reed switches observed a harmonic oscillation effect which is determined by the switch -specific attenuation effects. One to two bounce events in the time window of 50 ... 100 microseconds is expected. Most reed switches have a closing time of 100 ... 500 microseconds.

Opening time

The opening time is the time until the switch opens after the magnetic field is no longer applied to the switch. If you reduce the voltage to the relay coil below the garbage or decrease voltage, open the contact paddle in a time of about 20 ... 50 microseconds.

Distinction must be made of it the fall time when a diode is connected in anti-parallel to the coil ( catch diode to suppress their shut- self-induction voltage) - the time increases to about 300 microseconds. Is in series with the diode 12 ... a 24 V zener diode ( cathode at the cathode ) is connected ( the voltage pulse is limited to the zener voltage ) to reach opening times significantly less than 100 microseconds.

Resonance frequency

Resonance frequency is the frequency ( natural vibration ) in which the reed switch closes inadvertently by external vibrations. In addition, such resonances are a danger to the mechanical stability of the reed switch; they can damage the melting of the switch to total failure.

Contact capacity

The contact capacitance is the capacitance between the open contacts. The values ​​are in the range of 0.1 ... 0.3 pF. The low contact capacity is a particular feature of reed contacts to other relays and allows use at high frequencies and / or for switching high-impedance AC signals with low crosstalk ( high damping with open contact ).