Normally Open Reed Switch[2]

Updated: 2026-09-15

Overview

The Normally Open Reed Switch is a passive magnetic sensor consisting of two ferromagnetic reeds sealed in a glass tube with inert gas. In its default state, the contacts remain separated, interrupting current flow. When exposed to a magnetic field exceeding its activation threshold (measured in Ampere-Turns), the reeds magnetically attract each other, completing the circuit. First developed by Bell Laboratories in the 1930s, modern reed switches maintain the same fundamental design while achieving higher reliability through advanced materials and manufacturing. Their hermetically sealed construction makes them resistant to environmental contaminants, distinguishing them from mechanical switches in dirty or humid conditions.

Structure and Working Principle

The core component is a pair of overlapping nickel-iron alloy reeds (typically 0.1-0.3mm thick) with gold or ruthenium-plated contact points. These are precisely aligned within a borosilicate glass tube filled with nitrogen or other inert gas to prevent oxidation. The reeds act as both magnetic flux conductors and movable contacts. When a magnet approaches, the resulting magnetic field causes the reeds to polarize and attract each other. The contact force (usually 2-20mN) ensures low contact resistance (typically <100mΩ). Release occurs when the field drops below 20-80% of the activation value (hysteresis effect), with the reeds' spring tension returning them to the open position.

Key Features

1) Zero power operation: Requires no external power for sensing, making it ideal for battery-powered devices. 2) High reliability: Hermetic sealing prevents contact corrosion, achieving >100 million operations in quality models. 3) Fast response: Typical operate/release times under 2 milliseconds enable rapid state detection. Additional advantages include small form factors (as compact as 5mm length), wide temperature tolerance (-40°C to +150°C for standard models), and compatibility with AC/DC circuits up to 10kV breakdown voltage. Modern variants feature mercury-wetted contacts for reduced bounce or epoxy coatings for vibration resistance.

Application Areas

Industrial automation: Position sensing in CNC machines, robotic end-effectors, and flow meters. Security systems: Door/window sensors in alarm systems (over 80% of burglar alarms use reed switches). Automotive: Gear position sensors, brake pedal switches, and EV battery disconnect systems. Consumer applications include laptop lid sensors, white goods controls (washing machine drum position), and IoT devices. Medical equipment utilizes them for sterile environment switching where traditional contacts would contaminate. Emerging uses include smart building automation and renewable energy systems for fault detection.

Maintenance and Precautions

Avoid mechanical shock: The glass envelope is fragile - mounting should allow for 1-2mm free movement. Current limitations: Exceeding rated current (typically 0.5-2A) causes contact welding. For inductive loads, use snubber circuits. Environmental factors: Strong external magnetic fields (>100mT) may cause false triggering. High-vibration applications require shock-mounted installation or mercury-wetted models. Storage should be in non-magnetic containers at <85% relative humidity. Periodic testing with a gaussmeter verifies consistent operate/release values after long-term use.

B2B Procurement Guide

Technical specifications to specify: 1) Operate AT (10-50AT for standard models) 2) Contact rating (resistive/inductive load distinction) 3) Insulation resistance (>10^8Ω) 4) Maximum switching frequency (up to 1kHz for fast models). Quality indicators include MIL-PRF-83446 certification for military-grade reliability or EN 50010 for explosion-proof versions. Lead time considerations: Standard configurations have 2-4 week delivery, while custom AT values or lead configurations may require 8-12 weeks. Bulk pricing breaks typically occur at 1k, 10k, and 100k unit quantities with 30-50% cost reduction at highest volumes.