LATCHING & FORM B REED RELAYS & SENSORSIntroductionPurpose› Explore the technology involved in the use and operation ofForm B and Latching Reed Sensors & Relays
Objectives› Define key terms of Form B and Latching sensors and relays› Describe the structure and function of a Form B and Latching
Reed Relay› Describe the structure and function of a Form B and Latching
Reed Sensor

Key Terms
Contact - Contact blades making up a Reed Switch or

Electromechanical Relay

Contact, Form A - A single pole single throw (SPST) normallyopen (N.O.) switchContact, Form B - A single pole single throw (SPST) normallyclosed (N.C.) switch
Contact, Form E - A latching contact that can exist in either theN.O. or N.C. state controlled by reversing the magnetic fieldKey Terms
Bias or Permanent Magnet - Steady magnetic field applied tothe magnetic circuit of a relay or sensor. Aids or impedes
operation of the switch contacts
Coil - Electromagnetic assembly made of copper insulated wire.
When current is applied to the coil, a magnetic field isgenerated. When energized, its magnetic field operates thecontacts
De-energize - The act of removing power from a relay coil
Energization - The application of power to a coil winding of arelay

Key Terms
Reed Switch or Reed Sensor - A bare or encased switch containing glass
sealed magnetic reeds as the contact members including mercury-wetted
and dry contact types
Reed Relay - A relay containing one or more internal reed switches
Latching Relay - A relay that maintains its contacts in the last assumed
position without needing to maintain coil energization. To change thestate of the contacts, the magnetic field must be reversed.
Normally Open (N.O.) Contacts (Form A) - The contact state before any
magnetic field is applied to them in which they exist in the open state
Normally Closed (N.C.) Contacts (Form B) - The contact state before any
magnetic field is applied to them in which they exist in the closed state.

FORM B NORMALLY CLOSED REED RELAYSOverview of Form B Normally Closed Reed Relays

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The normally open Form A reed relay’s internal reedswitches exist in the normally open state
Applying coil power closes the contacts
The relay contacts may remain closed for long
periods of time supplying continuous power such asin safety circuits
The Form B reed relay allows contact closure forextended periods without continuous power draw

Form B Normally Closed Reed Relays›››

Normally Closed (Form B) Reed Relays require theuse of a permanent magnet
This magnet is adjusted such that the reed switch
changes from its normally open state to a normallyclosed contact state
In this case, no current or external energy isexpended maintaining the reed switch in the closedposition

Form B Normally Closed Reed Relays››

A relay coil of equal strength and opposite polarity isneeded to open the Reed Switch in the reed relay
To open the contacts the relay coil is energized withthe relay’s nominal voltage

Form B Normally Closed Reed Relays
Permanent Magnet› A permanent magnet keeps the contacts closed› No power is requiredForm B Normally Closed Reed Relays
Applying power to the Coil› Apply the relay’s nominal coil power to open the contacts› The correct coil voltage polarity is importantForm B Normally Closed Reed Relays›

De-energizing the coil returns the contacts to theirnormally closed contact state provided by thepermanent magnet

LATCHING REED RELAYSOverview of Latching Reed Relays›

A latching reed relay has two bi-stable states
1.
2.

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Open contact state
Closed contact state

In either state it draws no coil power
Generally the latching relay will have two coils
1.
2.

A latching coil that closes the contacts
An unlatching coil which will open the contacts

Overview of Latching Reed Relays››

The latching relay also requires an internal magnetfor proper operation
The latching relay magnet is partially magnetizedsuch that the reed switch contacts do not fully close

Latching Reed Relays

Latching Reed Relays

Latching Reed Relays

Latching Reed Relays

FORM B NORMALLY CLOSED REEDSENSOR

Form B Normally Closed Reed Sensor

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Reed sensors also have a reed switch as their mainswitching element
These reed switches exist in their normally open
state
Reed sensors generally exist in the normally openstate
The reed sensor can consist of simply a bare reedswitch or a packaged device having one or more
reed switches and/or magnets

Form B Normally Closed Reed Sensor››

Form B reed sensors follow the same rules as withthe Form B reed relays
A magnet that has been magnetized to a suitablelevel closes the contacts

Form B Normally Closed Reed Sensor›››

A magnet of opposite polarity cancels out themagnetic field thereby opening the contacts
Too strong a magnet may reclose the contacts
Proper operation requires the correct use magneticlobe

Form B Normally Closed Reed Sensor›

Once the magnet is removed the contacts willreclose and stay in that state until a magnet isbrought into its influenceLATCHING REED SENSOR

Latching Reed Sensor• A partially magnetized permanent magnet keeps the reed switch
open• A second magnet with an aiding field will close the contacts• Removing the second magnet, the contacts remain in the closedstate• Applying the second magnet again with its field reversed opens thecontacts

Summary

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The reed relay can be configured as a normally closed reed
relay to conserve power when the requirement calls forextended use in the normally closed state
The reed sensor can be configured in the normally closed
state as well for requirements calling for extended use in thenormally closed state.
For applications calling for extremely small power
requirements, a latching reed relay can be the best selection,
particularly in battery powered devices.
Reed sensors can be used in a latching manner for specificdesign requirements

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