Technical Information

Reed Switch

The basic reed switch consists of two ferromagnetic nickel-iron wires and a glass capsule. The two wires are formed into "reeds" by flattening one end. The reed ends are carefully aligned with a small overlap and then permanently sealed inside the glass capsule. The overlap or contact area of the reeds is coated with special metals such as ruthenium. The nickel-iron leads are tin plated so that they can be soldered.

The two reeds act as magnetic flux conductors when exposed to an external magnetic field from either a permanent magnet or an electromagnetic coil. Poles of opposite polarity are created at the contact gap and the contacts close when the magnetic force exceeds the spring force of the reeds. The contacts open when the external magnetic field is reduced so that the magnetic attractive force between the reeds is less than the restoring spring force of the reeds.

The basic reed switch is a Single Pole Single Throw Normally Open switch, also known as a SPST-NO or Form A switch. By adding an additional nonmagnetic contact that is electrically closed with no magnetic field present, a Single Pole Double Throw switch can be made. This is also known as a changeover switch or a SPDT switch. This is a break-before-make switch in that the closed contact opens before the open contact closes.

The electrical contacts in a reed switch are hermetically sealed. That is, airtight glass-to-metal seals prevent the contacts from being exposed to the environment. Hamlin reed switches are sealed with an inert nitrogen atmosphere inside the switch. The Hamlin MDRR-6, MRPR-8, DRR-DTH, and DRT-DTH reed switches are sealed with a higher-than-atmospheric pressure. This increases the breakdown voltage and makes these switches particularly well suited to 240 VAC mains switching applications. The Hamlin MARR-5 reed switch has a vacuum inside the switch capsule. Since there is no gas to ionize and arc, this switch has very high voltage switching capability.

Reed switches can operate for billions of operations on low voltage logic loads. Life is typically in the millions of operations at higher loads such as 10 to 120 V AC/DC and 10 to 250 mA. However, for extreme loads, end of life may be reached in less than a million operations. See application notes below for further information on capacitive loads and incandescent lamp loads. Contact Hamlin with the load voltage, current, type (AC or DC, inductive or capacitive or resistive), required life, and application for recommendations on the best switch type to use.

Reed switches are the major component in reed sensors as well as reed relays. Since Hamlin manufactures both standard and custom reed sensors and relays, applications can very often benefit from Hamlin’s engineering expertise, production capabilites, and quality systems to provide parts in a cost-effective and timely manner. Options include standard surface mount reed switches, custom cropping & forming of leads, modifications to existing standard product, and full custom product. Contact Hamlin to discuss your application needs.

Application Notes:

Ampere*turn versus mT and Gauss

This application note explains the relationship between AT (Ampere*turn) and Tesla or Gauss and also activation distance.

Storage of Hamlin components

General guidelines for storage conditions of Hamlin components.

Capacitive Loads

This application note shows capacitance details for for Hamlin reed sensors.

Handling of Reed Sensors, Relays, & Switches

This application note shows handling guidelines for Reed Sensors, Reed Relays, & Reed Switches.

Reed Switch Selector Guide

This application note shows key Hamlin reed switch characteristics in a matrix format. There is also a short paragraph on the features of each reed switch.

Modifying Reed Switches

This application note explains how to cut and form reed switches without compromising the integrity of the glass to metal hermetic seals.

Extended Temperature Operation

Most Hamlin reed switches have a temperature rating of –40°C to 125°C. However, reed switches can operate from less than –270°C to over 400°C. This application note explains the issues to be aware of when using Hamlin reed switches beyond their rated temperatures.

Shock

This application note explains Hamlin’s shock test procedure, the effects of shock on a reed switch, and using Hamlin switches beyond their shock rating.

Vibration

This application note explains Hamlin’s vibration test procedure, the effects of vibration on a reed switch, and precautions for high vibration environments.

Measuring Reed Switches

Reed Switch sensitivity is normally measured by placing it inside a test coil and using a unit of measure called an Ampere-turn (also AT or AW or At). This application note explains the issues involved in accurately measuring reed switch sensitivity and contact resistance. Techniques to obtain repeatable and accurate values are provided.