Learn More About Magnetic Sensitivity Testing on Reed Switches

Reed switches, invented in the 1930s, have always required a way to accurately and efficiently measure their magnetic sensitivity. That’s why standard test coils were developed. These coils are available in various sizes and can be operated rapidly by computer-controlled test systems. For efficient operation, the coils have either 5,000 or 10,000 turns, and the coil lengths are designed to deliver an efficient magnetic field shape which is typically over the glass length of the reed switch. With precise, computer-controlled, incremental steps of current flowing in the coil, the switch is tested into specific sensitivity windows of ampere turns (i.e. the number of turns in the coil multiplied by the current flow required to activate the switch).

For some of our world’s smallest switches, and/or our tiny surface-mount sensors, we employ equipment utilizing a Helmholtz coil to generate a uniform magnetic field to surround the component. The computerized test system then takes the sensor through several functional activation and deactivation cycles depending on the requirements of the application.

In magnetic sensitivity testing, we generally employ all four standard test coils defined in EIA/ NARM RS-421A and MIL-S-55433. Standard and specific test coils are used to test the operating characteristics of reed switches. However, it is essential to note that different coils provide different measured values. Make sure to ensure that you are using the proper coil for each reed switch.  The following factors are important to consider in order to obtain accurate, stable, and consistent pull-in and drop-out readings:

  • The current in the test coil must be carefully controlled and measured with a calibrated instrument.
  • The position of the reed switch inside the coil should be controlled. Changing the position of the switch inside the coil will change the AT measurement of the reed switch.
  • External magnetic fields need to be considered. The earth’s magnetic field can cause reading errors of over 1 AT. Magnetic fields can be produced by nearby fans, motors, moving magnets, or other field generating devices and sources.
  • Ferrous material near a test coil can cause errors by altering the magnetic field. Possible ferrous items include screws, brackets, connectors and tabletops.
  • Stresses applied on the reed switch glass or leads by test equipment can affect the sensitivity readings of the switch by slightly changing the reed switch contact gap.
  • During the manufacturing process, HSI Sensing tests all of our reed switches at ambient temp (approximately 25 °C). Reed switches are normally designed to operate in a temperature range of – 40°C to 125 °C. Sensitivity increases as temperature increases.
  • The reed switch blades form a part of the magnetic circuit. Cutting and or bending the leads will result in changed pull-in and drop-out values.

Need help? HSI Sensing is proud to provide custom solutions to customers facing challenging reed switch design applications. We work with you every step of the way to make sure all of your design goals and applications are met. Learn more about our custom solutions here.