Reed Relays| Operation and Types of Reed Relays

Reed relays are a reliable, fast switching solution which offer many advantages over conventional electromechanical or clapper relays. These relays feature few moving parts and provide true isolation when switched. Their speed of operation lies in the low-millisecond range, making them suitable for control systems that activate solenoids, motor starters, and other electromechanical devices. 

Where constant contact resistance is paramount, mercury-wetted contacts maintain their resistance to within ± 0.002 ohms throughout their life span – this allows these switches to be used for controlling a microvolt signal or for switching a 250 watt load up to 100 times per second. 

Mechanical relay contacts sometimes bounce when they make contact and can cause chatter in the output circuit, however this is avoided thanks to the mercury which maintains contact between the two surfaces during any bounce. 

Reed relays consist of magnetically actuated reed switches with actuating solenoids or coils. As such they are vastly different from other kinds of relays. 

Operation/Working Principle of Reed Relays

The reed members of a Reed Relay contain ferromagnetic portions which are magnetized by an external permanent magnet or an electromagnetic coil. This creates a magnetic field that causes the internal overlapping ends of the ferromagnetic elements to assume opposite polarity, thus closing the contacts. Removing the magnetizing force will cause the contacts to open again.

The switch can be activated by permanent magnets or an electromagnetic coil, which causes the contacts to close when a sufficient magnetic field is applied across the air gap.

Normally, a permanent magnet is used to bias the switch and achieve a normally closed operation. The magnet is placed close enough to the device so that its magnetic field can close the switch. To open it again, a coil with current running through it is introduced which creates an opposing magnetic field that overrides the first one. This combination of coil and magnet provides a latching action.

Types of Reed Relays

The term “reed relay” encompasses three different kinds of relays: dry-reed, ferreed, and mercury-wetted-contact relays. All of these types of relays are hermetically sealed and use thin, flat blades to serve as conductor contacts, springs, and magnetic armatures. 

1. Dry-reed relay

A dry-reed relay consists of two opposing reeds that are sealed into a narrow glass tube; their free ends overlap at the contact area which is usually plated with gold or rhodium or treated in order to achieve low contact resistance. The tube itself is usually made out of glass and is surrounded by an electromagnetic coil. Relay packages that incorporate two or more dry-reed switches are common, allowing for multiple switching arrangements. Some of them also feature permanent magnets for magnetic biasing, which gives them the ability to achieve normally closed contacts.

2. Ferreed relay

The ferreed relay is a type of reed relay with the addition of one or more magnetic components. The magnetization of these components can be altered by sending current pulses to their associated coils. When in the first state, the magnetic members create a field that is strong enough to close the contacts. However, when in the second state, this field becomes too weak to keep them closed. An operating pulse sent through the coils will cause it to transition into its first state and a release pulse will return it to its second state; both of these processes require only brief periods of time for magnetizing the magnetic members. 

The unique attribute of the ferreed relay is its incredibly quick response. For instance, it can be activated in as little as 5 microseconds. Furthermore, this relay stays in either setting without requiring a current to maintain it.

3. Mercury-wetted-contact relay

A mercury-wetted relay is a special type of reed relay. It consists of a glass-encapsulated reed with one end immersed in a pool of mercury. The other end is able to move between two sets of stationary contacts. When actuated, the mercury flows up the reed by capillary action and wets both the contact surface at the moving end and the contact surfaces of the stationary contacts, thus maintaining a mercury-to-mercury contact in its closed position. This type of relay is usually actuated by an electromagnetic coil surrounding the capsule.

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