Summary： This article is a recap of every basic that an engineer needs to know about reed relay construction, circuit diagrams, working, advantages, disadvantages, and applications.
The first idea about reed relays came into existence around 1922 when a professor who worked at Leningrad Electrotechnical University proposed it. His idea was about a contract that was controlled using magnetic forces to switch the influence of the magnetic field. In 1936, Walter Ellwood completed the process by inventing the modern-day reed switch while he was working at Graham Bell Telephone Group Laboratories. The relay found significance in the area of telephones, where they are used in the automation of call exchanges. Although modern phones have moved away from reed switches, the technology is currently used in relays and sensors. This article focuses on reed relays, which are basic information that an engineer or a technician should be equipped with.
These are relays that borrow their existence from the reed switches. Reed switches form the core component of the technology behind the reed relay. Reed switch is made up of two contacts that are made of nickel-iron material and then plated with another material that ensures that the device has an increased life span. The contacts of the reed switches overlap each other so that anytime they close, they can make contact. The spacing between the two contacts is about 0.05mm and 1.0mm, where the bigger the spacing, the bigger the switch withstands the potential difference. If the gap is small, expect quick switching speeds.
Figure 1: Reed Switch Internal Assembly
The reed relay construction is based on a reed switch, electromagnetic coil, and blades, as shown in Figure 2.
Figure 2: Reed Relay Construction
From the construction diagram above, a reed relay is a sealed glass enclosure that has two reed contacts and two reed blades that are constructed of magnetically sensitive materials, such as ferromagnetic materials. The external glass enclosure is filled with completely dry inert gas, which helps protect the device against oxidation. An electromagnetic coil is wound around the glass enclosure to make the reed switch perform like an armature. The reed switch blades end up being connected to the circuit output and input, and the other side of the blade is fixed with two contacts of the relay.
A few steps define the working of this type of relay. It has an electromagnetic coil that produces current immediately after there is an application of the voltage. The current produced by the coil will generate an effect of the magnetic field, which plays the role of shutting the reed switch contacts. On the other hand, when the voltage is removed from the system, the coil does not generate the current that produces the magnetic field, and hence, the contacts open.
Circuit Diagram of Reed Relays
Figure 3: Circuit Diagram of Reed Relay
Figure 3 shows the reed relay circuit diagram. The circuit is made up of a switch that has magnetic contacts that move any time there is the application of the magnetic influence from an external excitation from the coil surrounding the relay glass enclosure.
Redd relays have the benefit of very quick switching speeds as compared to other relays, but they have a lower switching current and voltage due to contact thickness.
When no voltage is availed in the circuit above, the reed switch goes open, and the connected light-emitting diode remains in OFF condition. In the second electric circuit of the reed relay, providing a DC voltage to the relay coil generates a magnetic field that triggers the closing of the circuit by occasioning the flow of current from the connected battery to power the LED.
Below is a discussion of the types of reed relays that are available in the market.
5V Reed Relay
It is operated like a switch controlled electronically. It can switch voltages of up to 200VDC or 200VAC at a current of 500mA. The contact’s movement is dependent on the magnetic force. The coil is rated at a voltage of 5VDC and requires 10mA to work. They have a quick activation due to the absence of the mechanical armature.
High-Power Reed Relay
They are present in ratings of 1000VDC switching voltages. They are used in high voltage applications like automotive, solar, electrical vehicle battery management systems, medical, and automated testing equipment.
DIL Reed Relay
DIL stands for dual in line. This relay has a housing of 7.5mm in height and a base size of 19mm by 10mm. They have the benefit of great contact arrangements and special pin arrangements. Due to their miniature size, they have been used to replace other types of relays.
Fast Switching Reed Relays
These are relays that can switch very fast in the factor range of 5 to 10. They have lighter and simpler movable parts, which allow them to change faster than EMRs. The operating speed has been found to range between 200 and 500 microseconds.
Micro-Reed Relays
They are very miniature and very fast in terms of conduction as compared to EMRs. They have a provision for lower insertion loss and offer a very superior hot-switching action. They are good to switch up to 10W at 0.5A.
Difference between Reed Relay and EMR
Table 1: Comparison between Reed Relay and EMR
The advantages of reeds relay include
· simple, inexpensive, with a reliable operation that allows flexibility in the design.
· Very high performance
· Easy to customize
· Very quick switching speed
· Easily produced as SIL and DIL packages
· They are durable
The disadvantages include
· Easily damaged glass cover
· There can be arcing, especially between the contacts.
· Other magnetic fields can accidentally activate the relay.
· Used in RF switching and microwave applications
· They find use in low leakage current applications such as the photomultiplier sensors
· Used for selecting fixed inductors or capacitor values in antenna tuners.
