Machine Design

Switch Tips: Reed switch basics

A reed switch gets its name from the use of two or three thin metal pieces, called reeds, with plated contacts at their tips and spaced a small distance apart. The reeds are typically encapsulated in a sealed glass tube filled with inert gas. A field from a magnet or an electromagnet deflects the reeds, making or breaking switch contact.

Two-reed devices have normally open contacts which close when actuated. Three-reed versions have a pair of normally open and normally closed contacts. Operation of the switch causes these parts to change to the opposite state. An applied field makes the reeds magnetic so their ends attract. Removal of the field lets the springy metal reeds return to their original positions.

The movement of the magnet relative to the reed switch determines how the switch toggles. Moving the magnet perpendicular to the side of the switch causes one switch closure per pass. Moving the magnet parallel to the switch provides as many as three closures with the maximum magnet travel. Another option is to spin the magnet near the switch. When the pole axis and the switch axis are parallel, the switch closes. When the axes are perpendicular, the switch opens. There are two or more closures with each revolution depending on the number of poles.

Another typical scenario for a reed switch employed in position sensing is to either move the magnet near the switch or slide ferrous metal between the switch and the magnet, thereby toggling the contacts. Alternatively, the reed can be biased closed with one magnet sitting just inside the actuation distance. A second magnet can then open the switch by canceling the field of the first.

Typical commercial-grade reed switches handle currents in the milliamp range on up to about 1 A of either dc or ac current. However, special designs can get to around 10 A or more by incorporating separate magnetic and contact units, thereby allowing optimization of the contact area.

Reed switches frequently get incorporated into sensors and into relays. One of their advantages is a built-in hysteresis with regard to magnet position. For example, a magnet coming within range forces the switch contacts closed. They will stay closed as the magnet draws nearer and will remain closed until the magnet is out of range.

One important quality of the switch is its sensitivity, the amount of magnetic energy necessary to actuate it. Sensitivity is measured in units of Ampere-turns, corresponding to the current in a coil multiplied by the number of turns. Typical pull-in sensitivities for commercial devices are in the 10 to 60 AT range.

Reed devices can be obtained bare or packaged in plastic and aluminum housings ready to install. Typical applications are in proximity sensing, liquid level sensing, security systems, blood processing machines, baby incubators, IV drips, and in current-sensing relays. A point to note is that reed switches have a resonant frequency that relates to the length of the reeds. Typical resonances are in the range of 2 to 3 kHz but can depend on a variety of factors.

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