AN103C

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Specifications	AN103C
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Application Note AN103C www.hamlin.com © 2007 Hamlin Electronics L. P. p 1/4 Measuring Reed Switch Sensitivity and Conta

Specifications	AN103C
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Content	Application Note AN103C www.hamlin.com © 2007 Hamlin Electronics L. P. p 1/4 Measuring Reed Switch Sensitivity and Contact Resistance The purpose of this application note is to document the preferred method of reading the sensitivity and contact resistance of Hamlin reed switches. Terminology: • Contact resistance or CR is the resistance of the reed switch when the contacts are closed. It is measured in the units of Ohm (Ω) or mOhm (mΩ) where 0.1 Ohm = 100 mOhm. • Pull-In or PI is the magnetization level at which the SPST (Form A) reed switch closes. Other terms sometimes used are Pick-Up (PU) or Operate Point. Pull-In is measured in the units of AT (Ampere*turn). This is also sometimes written as At or NI, the latter representing number of turns (N) and current (I). • Drop-Out or DO is the magnetization level at which the SPST (Form A) reed switch opens. It is sometimes referred to as the Release Point. Like Pull-In, this is measured in units of AT. Standards related to reed switches: • ANSI/EIA/NARM RS-421 Standard for Dry Reed Switches • IEC 62246 Reed Contact Units • MIL-S-55433 Military Specification - General Specification for Switch Capsules, Dry Reed Type Test Coils Reed switches are activated by magnetic fields. The magnetic field can be produced by a magnet or a coil. For reasons of history, accuracy and practicality, reed switches are tested by placing the reed switch inside a standard test coil. Hamlin specifies the test coil used in each reed switch data sheet. If a different coil is used, the readings may be different. The more the coil length or coil diameter differs, the more the readings are likely to differ. It is common practice to correlate one coil with another by measuring a set of reed switches with a range of sensitivities in both coils. Hamlin standard test coils have the following dimensions for the winding: Test Coil Winding Length Winding Inside Diameter L4991 10.39 mm (0.409 inch) 3.96 mm (0.156 inch) L4989 18.54 mm (0.730 inch) 5.08 mm (0.200 inch) L4988 49.53 mm (1.950 inch) 6.99 mm (0.275 inch) Note that the specified winding inside diameter equals the bobbin outside diameter, and that the winding length equals the bobbin length minus twice the flange width. Sources of Error There are several sources of variation or error that are important to consider in obtaining accurate, stable Pull-In and Drop-Out readings. 1. Since the reed switch responds to a magnetic field and a coil’s magnetic field is directly proportional to the current through it, the coil’s current needs to be carefully controlled or measured. Measuring the coil’s voltage does not work well because of temperature effects. Copper has a temperature coefficient of resistivity of 0.393%/°C, so ambient temperature or coil self-heating can easily affect measurements made using the coil’s voltage. 2. The position of the reed switch inside the coil should be controlled. It is standard practice to center the reed switch contact gap in the center of the coil. If the center of the contact
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Following Datasheets	AN104 (2 pages) AN105A-Tech-Extended_Temperature_Operation (6 pages) AN107-Capacitive_Loads (1 pages) AN110A-Tech-Reed_Switch_Selector_Guide (3 pages) AN111A-Tech-Shock (1 pages) AN113A-Tech-Vibration (1 pages) AN115-Tech-Soldering_Temperatures (1 pages) AN121-Storage (1 pages) an605cn (4 pages) an608-cn (6 pages)
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