What Is a Reed Relay?
A reed relay is an electromagnetic switch that is activated by passing an electric current through a drive coil.
It consists of one or more reed switches surrounded by electromagnetic coils and a small relay that opens and closes by electromagnetic force. The magnetic field generated in the coil mechanically actuates the contact point, so that current leakage when the switch is not operating is very small compared to semiconductor switches.
One of the advantages is the independence of the input and output sides, with no polarity, reducing errors during installation and providing resistance to pressure, high insulation, dust, and external gases.
Applications of Reed Relays
Reed relays are used in a wide variety of electrical equipment. While semiconductor relays currently dominate the relay market, reed relays are often preferred for specific use cases where semiconductor relays may not be suitable.
Reed relays are particularly well-suited for he environments:
- Electrical equipment that is greatly affected by the weak leakage current that flows when semiconductor relays are open.
- Electrical equipment that must operate in an environment where there are substances that can contaminate semiconductors.
- Circuits used for relays with extremely high voltages or in high-voltage environments.
Specific examples of use are as follows:
- Storage batteries for electric vehicles, solar batteries
Reed relays are used when the system voltage is high and stable operation is required even at voltages exceeding 1,500 VDC. - Medical equipment
Highly reliable switches are required for applications such as electrocautery control, bed position detection circuits, high-voltage charging circuits for AEDs, and detection of surgical instruments left inside the body.
Reed Relay Principle
Reed Relays consist of a reed switch and a coil. The reed switch consists of a glass tube sealed with an inert gas. It contains two leads with a gap between them. Inside the reed switch, there are reeds made of a magnetic material that respond to the influence of a magnet. The points where the reeds make contact with each other when driven are fitted with a high current-carrying metal.
When current flows through the coil surrounding the reed switch, it generates a magnetic field. This magnetic field causes the two reeds inside the reed switch to come into contact, allowing electricity to flow through the contacts and perform the relay function. When a Reed Relay is driven, a magnetic field is generated by the coil.
This may affect surrounding electronic components, so care must be taken in the mounting position and conditions of use. In addition, the circuit in which reed relays are employed should be designed properly, including the incorporation of spark-elimination circuits and the installation of protection circuits to handle temporarily high currents and for reverse currents.
Characteristics of Reed Relays
Reed relays have many features compared to other relays:
- Compared to semiconductor switches, mechanical switches have a very small leakage current when the contacts are open.
- No polarity specification for both input and output, reducing operational errors.
- Generally high withstand voltage and high insulation.
- Compared to general electromagnetic relays, the contacts are hermetically sealed to minimize the effects of dust and organic gases.
- Compact, lightweight, and long life.
- High-speed operation and recovery time of approx. 1/10.
Other Information on Reed Relays
1. Life of Reed Relay
Relays have electrical and mechanical life spans.
Electrical Life
This is the life in a resistive load test in which a load of rated voltage is applied to the coil to open and close the reed switch. Electrical life depends on the volume of the load, the variety of the load, the frequency of opening and closing, and temperature conditions.
Mechanical Life
It refers to the life span in a no-load test where no load is applied. Relays are mechanical parts, and opening and closing them causes fatigue and wear of the parts themselves. Relay life also differs depending on the environment in which the relay is used, such as temperature conditions and cases in which a load greater than the coil’s rated voltage is applied.
2. Precautions for Using Reed Relays
Important precautions in the use of reed relays are cleaning and magnetic interference.
Cleaning
Since the reed portion of the reed relay coil is sealed in a glass tube, cleaning will not degrade the characteristics of the reed portion. However, when cleaning the product, a special cleaner must always be used.
Magnetic Interference
Magnetic fields generated by relay coils have an effect on the external environment. Especially in the case of close mounting, relays may not operate properly due to magnetic interference between relays. Therefore, relays that are not magnetically shielded must be mounted with a minimum distance of 15 mm between relays.
Furthermore, it is necessary to refrain from using relays in locations where objects that generate strong magnetic fields, such as transformers or permanent magnets, are present, as they may cause malfunctions.