Reed relay
A reed relay is a relay which operates with a reed switch (also called reed contact ).
Reed switch contacts are in vacuum or protective gas in a glass flask fused contact tongues, which form both the contact spring and the armature. The name comes from the reed woodwind instruments, engl. reed that resembles the vibrating reeds. The reeds are made with noble metal ferromagnetic material (eg soft iron). The contact actuation takes place by a magnetic field acting from the outside, which is electrically generated in a solenoid coil associated with the sensors or switching contacts of a placed in the vicinity of permanent magnets or the reed relays. By the magnetic field, the two reeds attract and close thus. Once the magnetic field falls off or a certain field strength is exceeded in the relay, the contact due to the spring action reopens.
Construction
A reed relay consists of a coil, which is usually wound from enamelled copper wire. The reed switch is in the middle of the coil.
With thick wire coil from the current sensors can be realized, for example, for monitoring the operation of a warning lamp.
Often reed relays are magnetically shielded to the outside, so that they can be tightly packed without affecting each other. Towards the switching contact can be in an electrical shield, which also forms the outer conductor of a coaxial cable for high frequency applications.
Properties
Reed relays are extremely durable due to the hermetically sealed contacts very reliable and with low switching powers. Reed relays generally require a relatively low control power and can be driven not only by transistors often directly with TTL or CMOS outputs. Due to the hermetically sealed switching contacts and the contact layer is corrosion protected reed relays are very particularly suitable for switching small signals up to femto and NV.
Low thermal stresses of the materials are important for switching low voltages: Reed relays can be specified with thermoelectric voltages of <1 uV / K.
Reed relays may be made for switching frequencies above 6 GHz, by being manufactured as part of a matched impedance coaxial line with a defined ( for example, 50 ohms). The insertion loss can be < 0.2 dB. The standing wave ratio ( VSWR) is <1.2.
Bounce-free switching and higher contact lifetimes at higher load are with mercury-wetted contacts (now no longer allowed in the EU without special permission ) reachable.
Properties (see also Reed switch):
- Particularly for switching small signals up to femto and nano volts through contact materials such as gold, iridium or palladium
- Hermetically sealed switching contacts, therefore very robust against water, air, vacuum, oil, gasoline, dust, etc.; explosion-proof and certified designs
- No mechanical wear on the movement points, mechanical life up to several billion (109 ) switching cycles
- Low contact resistance (typically 50 milliohms ), low capacitance and high isolation
- Low control performance with minimal size
- Short shift and bounce times
- For voltages up to 10,000 volts
- Comparatively low switching capacities
- Sensitive to capacitive loads, and the self-inductance of inductive loads
Reed relays are normally open, normally closed, change-over and bistable available.
History
Was patented in 1936 by the reed contact WB Elwood, in the Bell Laboratories worked at that time. However, it was not yet possible to produce reed contacts to a greater extent, since the materials technology still could not provide suitable materials for the contact tongues. The first available on a larger scale reed contacts were made until the late 1950s. The main application area was mainly in the 1960s to 1980s, the telecommunications in the exchanges.
Types
The contacts of reed relays are protected by their design against corrosion and oxygen. Therefore, they are extremely reliable and are ( again, for example in 10 years ) suitable operation for small switching power, and often quite rare. Reed relays may be designed for very high switching voltages (up to approximately 10 kV) in comparison to similarly sized conventional electromagnetic relay by increasing the internal pressure of the fill gas.
Reed relays are also executed in the DIL and SMD construction, that is, in the form of conventional microelectronics housing designs for PCB mounting.
Reed contacts for detecting the position of the reed contact is actuated by a permanent magnet; when approaching the magnet, the switch contacts are drawn to each other and close a circuit. Such contacts are often encapsulated robust housed in cuboidal or cylindrical cases and with molded connection cables. The actuating magnets are often designed to fit or they are integrated in machine parts.
Closed contacts and other features
In the basic function of a reed contact which contact is closed by an axial magnetic field. However, there are also so-called -switching ( change-over switch ), or open reed - contacts, which have an inverted switching behavior. In these, there is the open reed of non-ferromagnetic material. With such contacts can also build -switching reed relay. The production costs of this reed switch contacts are relatively high. Moreover such contacts are not suited for switching higher currents, since the contact force is not supported by the magnetic field. The closing and bounce higher.
Alternatively, the operation of a reed relay with an additional transistor circuit to invert. The circuit scale and the power consumption, however, being increased as a permanent bias current flows through the transistor circuit.
Another way to invert the switching function is to be attached to the glass body of the reed switch has a smaller permanent magnet. The polarity of the external field must be reversed on this. To compensate the magnetic fields of both magnets and the contact opens. Without external field the reed contact through the integrated magnet is closed. In this way, other functions can be implemented:
- Reed relay with self- hold function: the field of integrated permanent magnet is not enough to close the reed contact, but it can keep him closed. In this manner, a field- reinforcing pulse of current through the relay coil to the ( permanent) suit and a field- compensating current pulse to fall.
- Reed relays with enhanced sensitivity suit: the field of integrated permanent magnet amplifies the external field, however, that the drop is only so far ensured without box. These relays require a prescribed polarity of the control voltage.
Applications
Reed relays are often used there for switching low voltages and currents, where it depends on reliability and lowest contact resistance ( eg for signal switching in measuring instruments ). Another application is high-frequency relays - here, the contacts can be operated as an inner conductor of a coaxial line arrangement.
Reed contacts are used in conjunction with permanent magnets are widely used for door contacts or for limit position (eg on pneumatic cylinders and in general in automation technology ). Reed contacts can also be switched by walls throughout, such as in diving lamps or level controllers.
Due to the enclosed design reed contacts are also permitted in hazardous areas, such as those found in mines or in the chemical industry. For the same reason, they are suitable for use at high altitudes, where otherwise the contact distance due to the higher striking distance in the thinner atmosphere would not be enough.
A special application is the use in medical implants. A reed switch provides an easy- to-implement method for transmitting information dar. Cardiac pacemakers may thus, for example, be switched into a programming mode, the implant works while with a fixed clock. At this point it must be noted that a medical examination may result in magnetic resonance imaging to the fact that this contact is closed, which is undesirable in this case.
In DC motors ( reed contact ) engines can be used instead of commutators. Some bike computers determine the speed and cadence via reed contacts on the front wheel or the crank - in this application for measuring rotational speed but also very common contactless Hall sensors are used.
With an attached outdoor thick winding provided reed relays used as a current sensor and can be used eg for failure monitoring of light bulbs or brake light control for vehicles.
Often can be replaced with also, MAGNETIC Hall sensors today reed relay and reed contacts - in combination with electronic switches such as field effect transistors can thus achieve functionally similar switch characteristics, but which operate without mechanical contact and therefore are less mechanically sensitive ( no break - and impact sensitivity, no special gas filling). Furthermore, Hall sensors can be designed for much higher switching frequencies than the mechanical moving reed contact tongues in order to be able to detect, for example higher speeds directly. The disadvantage of Hall sensors is that they require auxiliary power ( operating voltage). Therefore, reed contacts are often used even today.
Reed relays have low operating times. The time period from the command to suit to close the contact may be in some designs below 0.5 milliseconds.