Reed Relays

Reed Relay Technical Summary

Hermetic Sealing

Reed relays are hermetically sealed from dust, debris, and corrosion, making them ideal for use in harsh environments. This hermetic sealing also protects the contacts from contamination, which can extend the life of the relay.

Intrinsic Safety

Reed relays are intrinsically safe, meaning that they cannot generate sparks that could ignite flammable gases or vapors. This makes them ideal for use in hazardous locations, such as oil refineries and chemical plants.

Long Life

Reed relays have a very long life, with some relays capable of billions of switching operations. 

High Density

Up to eight reed switches can be integrated into a single relay, making them a very space-efficient solution. This high density is particularly beneficial in applications where space is limited.

Low Contact Resistance

Reed relays have a very low contact resistance, typically below 50 mΩ. This low contact resistance helps to reduce losses and improve efficiency.

High Insulation Resistance

Reed relays have a very high insulation resistance, typically greater than 10^15 Ω. This high insulation resistance helps to prevent leakage currents and protect sensitive circuitry.

High Voltage Switching

Reed relays can switch voltages up to 10,000 V. This high voltage switching capability makes them suitable for use in a variety of high-voltage applications.

High Current Switching

Reed relays can carry currents up to 5 A. This high current switching capability makes them suitable for use in a variety of high-power applications.

Fast Switching

Reed relays have fast switching times, typically ranging from 500 microseconds to 3 milliseconds. This fast switching speed makes them suitable for use in high-speed applications.

Low Signal Switching

Reed relays can switch very low signals, down to the nanovolt range. This low signal switching capability makes them suitable for use in sensitive applications.

High Frequency Switching

Reed relays can switch frequencies up to 7 GHz. This high frequency switching capability makes them suitable for use in a variety of RF applications.

Reed relays offer a wide range of unique switching advantages that make them a versatile and reliable component for use in a variety of applications. Their hermetic sealing, intrinsic safety, long life, high density, low contact resistance, high insulation resistance, high voltage switching capability, high current switching capability, fast switching speed, low signal switching capability, high frequency switching capability, automotive standard compliance, and compliance with various safety standards make them an ideal choice for a wide range of applications.

There are many different types of reed relays, but some of the most common are:

• Form A relays: These relays have one set of contacts that are normally open (NO). When the coil is energized, the contacts close.

• Form B relays: These relays have one set of contacts that are normally closed (NC). When the coil is energized, the contacts open.

• Form C relays: These relays have two sets of contacts, one NO and one NC. When the coil is energized, the NO contacts close and the NC contacts open.

• Latching relays: These relays have two coils, one for setting the contacts and one for resetting them. Once the contacts are set, they will remain in that state until the reset coil is energized.

• Mercury-wetted reed relays: These relays have contacts that are wetted with mercury, which helps to reduce contact resistance and increase switching speed.High-voltage reed relays: These relays can switch voltages up to 10,000 V.

• High-current reed relays: These relays can carry currents up to 5 A.

• Fast-switching reed relays: These relays have switching times of less than 500 microseconds.

• Low-signal reed relays: These relays can switch signals down to the nanovolt range.

• High-frequency reed relays: These relays can switch frequencies up to 7 GHz.

Reed switches, activated by magnets, are commonly employed as proximity sensors in various mechanical systems. They play a crucial role in burglar alarm systems, where door and window sensors detect intrusions. Additionally, reed switches have found applications in tamperproofing mechanisms, safeguarding against unauthorized access. They can also be activated through stainless steel with is less magnetic than other types.

Laptops utilize reed switches to trigger sleep or hibernation mode upon lid closure, conserving battery power. Speed sensors on bicycles often employ reed switches to detect the passage of a magnet attached to the wheel, accurately measuring speed.

In the realm of computing, reed switches were once prevalent in computer terminal keyboards, with each key paired with a magnet and a corresponding reed switch for keystroke detection.

Electric and electronic pedal keyboards used by pipe organ and Hammond organ players often incorporate reed switches, providing reliable operation and protecting the contacts from contaminants.

Underwater applications, such as diving equipment, also benefit from reed switches. They are employed to control flashlights or cameras, ensuring functionality while sealed to prevent water ingress under high pressure.

Brushless DC electric motors once utilized reed switches to determine rotor position relative to field poles. This enabled transistors to function as a commutator, eliminating the wear and electrical noise associated with traditional DC commutators. Motor design could also be inverted, placing permanent magnets on the rotor and switching the field through external, fixed coils. This eliminated the need for sliding contacts to deliver power to the rotor. Such motors found applications in low-power, long-service-life devices like computer cooling fans and disk drives. However, with the advent of inexpensive Hall effect sensors, reed switches were replaced, offering even longer service lifetimes.

Amp-Turn Rating

Ampere-turns (AT) is a unit of magnetomotive force (MMF), which is a measure of the magnetic field strength required to operate a reed relay. The lower the AT rating of a reed relay, the more sensitive it is to magnetic fields. 

The AT rating of a reed relay is determined by multiplying the number of turns in the coil by the current flowing through the coil. For example, a reed relay that operates with a coil that has 10 turns and is carrying 1 amp of current has an AT rating of 10 AT.

The AT rating of a reed relay is an important consideration when selecting a reed relay for a particular application. If the magnetic field strength is too weak, the reed relay will not operate. If the magnetic field strength is too strong, the reed relay may be damaged.

Here are some examples of how AT ratings are used in reed relay applications:

• Door and window sensors: Door and window sensors typically use reed relays with an AT rating of 20 AT or less. This is because the magnetic field strength from a permanent magnet is typically only a few AT.

• Security systems: Security systems may use reed relays with an AT rating of 50 AT or more. This is because the magnetic field strength from a security system's electromagnet may be stronger than the magnetic field strength from a permanent magnet.

• Medical devices: Medical devices may use reed relays with an AT rating of 100 AT or more. This is because the magnetic field strength from a medical device's solenoid may be very strong.

In general, it is best to select a reed relay with an AT rating that is slightly lower than the expected magnetic field strength. This will help to ensure that the reed relay operates reliably.

Here is a table that shows the approximate AT ratings for different types of reed relays:

Example ratings of a low cost reed contact:

- Max. Switching Voltage: 300 VDC

- Max. Contact Rating: 10W

- Max. Switching Current: 0.55 A

- Max. Operate time: 0.45 ms

- Bounce time: 0.25 ms

- Max. Release time: 0.35 ms

- Resonant Frequency: 5000 HZ

- Max. Operating Frequency: 400 HZ

- Pull in Value: 20-70 AT

- Min. Drop out Value:4 AT

- Max. Contact Capacitance:0.5 pF

- Electrical Life: 50mV-10μA-1x106