Reed Switch: Understanding Specifications

Written By:

Preeti Jain

A reed switch consists of two ferromagnetic nickel-iron wires and specially shaped contact blades (reeds) positioned in a hermetically sealed glass capsule with a gap between them and in a protective atmosphere. The glass capsule is filled with inert gas to prevent activation of the contacts. The ruthenium or rhodium plated contact surfaces are isolated from the outside environment, which protects the contacts from contamination.

Reed Switch

Reed switches can be operated using a magnetic field generated by either a permanent magnet or current-carrying coil. They induce north (N) and south(S) poles on the reeds. The magnetic attractive force leads to closure of the reed contacts. Upon removal of the magnetic field, contact open again due to the elasticity of the reeds. There are Reed Switches which operate with permanent magnet, while there are Reed Switches operate with coils which can be magnetized by sending current through them.

Reed Switch Operation Reed Switch Working Reed switches offer advantages of compact size, low weight, quick response time, long life and low cost. Common applications of reed switches are in reed relays, automotive sensors, security systems, robotics, toys and games.

In this article, we will be discussing about the basic electrical parameters of the Reed Switch. There are lots of technical terms associated with Reed Switch, which can be found in the datasheets.

SPECIFICATIONS-GENERAL

{C}{C}{C}{C}{C}· Reed Switch Type

Type of reed switch specifies whether the reed switch is a dry reed switch or a mercury wetted reed switch.

Dry reed switch consists of ferromagnetic contact blades housed in a hermetically sealed glass container filled with an inert gas.

In mercury wetted reed switch, mercury is used as the contact material for electrical circuit; the reed contacts are covered with a film of mercury by capillary action. With each operation, the mercury film contact is renewed. Mercury wetted reed switches should be mounted in a specific orientation to avoid bridging the contacts due to mercury droplets.

Sensing Distance

Sensing Distance specifies the maximum distance between the magnet and the reed switch at which the reed switch functions satisfactorily.

Input Requirements

Input requirements specifies the magnitude of external magnetic field which to operate the reed switch.

Contact Position

Reed switches are available with gap in two positions: one at center and other off-center. In Center gap reed switches, gap or contacts are at the center of the reed switch. In Off Center gap reed switches, gap or contacts are not at the center of the reed switch.

Pole and Throw Specification

Pole and Throw specification refers to the switch contact configurations. The available options are

§ Single Pole Single Throw (SPST)

SPST switch has two terminals and it simply makes and breaks the contact between the two terminals

§ Single Pole Double Throw –SPDT

Also, known as changeover switch, it has three terminals labeled COM, A and B. COM can be connected to A in one state and to B in other.

§ Double Pole Single Throw(DPST)

DPST can be considered as two SPST switches with one actuator. A pair of ON-OFF Switch operates simultaneously.

§ Double Pole Double Throw(DPDT)

Similarly, two SPDT in parallel are operated by one actuator forms DPDT.

Based upon the Pole and Throw specifications, Reed switches are categorized as

·Form A ( also known as SPST-NO)

·Form B (also known as SPST-NC)

·Form C (also known as SPDT; changeover type)

·Form E (Bistable, latching type)

Configuration

Configuration specifies whether a switch is Normally Open or Normally Closed

Normally OPEN (NO) switches are those which remain in OFF state. On activation, the switch gets closed (or becomes ON). Exactly opposite Normally CLOSED (NC) switches are those which remain in ON state. On activating the switch, it gets opened (or becomes OFF).