How do I select a magnet to use with a reed switch rated in ampere-turns?
Reed switches are rated, in their as-manufactured geometry, for the value of the ampere-turn product required for “pull in” and “drop out” in “standard” test coils. These standard test coils are described in reed switch manufacturers’ literature; geometries and turn counts vary with manufacturer, but some use the EIA/NARM-421-A standard coils. The magnet selected must produce the same flux density at the position of the reed switch as the coil when the rated ampere-turn value is applied. If the switch leads are shortened or bent, the Amp-Turn values will increase, so a stronger magnet would be required. The reason for this is that the altered reed switch has a lower magnetic permeance.
