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Frequently Asked Questions

Author : KJT Date : 2015-6-10 17:36:26
 

Frequently Asked Questions

 
How does a reed switch work?

A reed switch consists of two or three metal reed contacts (blades) that are hermetically sealed inside a glass tube containing an inert atmosphere. Reed switches come in various sizes, magnetic sensitivities, high power capabilities, high voltage capabilities, contact configurations and lead configurations.

The reed blades act as magnetic flux conductors or “magnetic antennae.” When exposed to an external magnetic field from a magnet or an electromagnetic coil, poles of opposite polarity are created across the open contact. When the magnetic force exceeds the spring force of the reed blades, the contacts will close (pull-in). When the magnetic force of the magnetic field is less than the spring force of the reed blades, the contacts will open (drop-out).

 

What are the advantages of reed switches?
  • No power is needed to operate
  • Hermetically sealed contacts
  • Not Electrostatic Discharge (ESD) sensitive
  • Switching and sensing functions in one package
  • Established and proven technology
What are the different reed switch types?

Form A - SPST (Single pole, single throw).  Normally-open reed switch contacts close in the presence of a magnetic field.

Form B - SPST (Single pole, single throw).  Normally-closed reed switch contacts open in the presence of a magnetic field.

Form C - SPDT (Single pole, double throw).  Break-before-make reed, the normally-closed contact opens, and then the normally-open contact closes in the presence of a magnetic field.

Latch Reed Switch contacts lock into either position until reset by the reversal of the magnetic field. Available in SPST and SPDT.

What is the expected lifetime of a reed switch?

Under normal operating conditions, a reed is mechanically capable of switching up to billions of cycles, depending on the internal contact material.

What does "AT" stand for?

Reed switches are tested in standard test coils. Being long and slender with contact leads at each end, a coil of magnetic wire spread over the glass length provides excellent magnetic coupling. The switch leads protrude from each end of the coil for connection during the test. The magnetic sensitivity is stated in Ampere-Turns, which is related to that specific test coil. The sensitivity rating reflects the amount of current (Ampere) that is flowing through the coil having a specific number of TURNS, when the switch contacts operate.

How does AT affect reed switch sensitivity?
  • High AT value = low sensitivity, thus a strong magnetic field is necessary to operate the reed switch (respectively the switching distance between magnet and switch will be smaller)
  • Low AT value = high sensitivity, allowing operation of the switch even with a weaker magnetic field (respectively the switching distance between magnet and switch can be increased)
How do you achieve the highest switching accuracy?
  • The magnetic field should be "spot focused" toward the contact area of the reed switch. Providing single-pole actuation, the result is the smallest possible switching difference.
  • Lowest hysteresis of the reed switch is recommended since the difference in switching distance between ON and OFF is very small for this type.
  • The less tolerances for the magnet and reed switch, the better.
What is the difference between switching current and carry current?

Switching current is the maximum electrical current that should be flowing when the switch contacts open, or the maximum amount that will flow soon after the contact closes. The maximum current rating is for pure resistive-type loads. Inductive, capacitive or lamp loads are more destructive and should be evaluated for life expectancy with a given switch model. 

Carry current is the maximum electrical current that can be passed through the switch without damaging the contact, or causing it to heat sufficiently enough to cause loss of magnetic conductance. This loss of magnetic conductance will allow the switch contact to momentarily open, then cool and re-close on a high-current load (higher than the maximum switching load) and further damage the contact. This rating is at 25 degrees C. Operation of the switch at temperatures above this should be evaluated and may require a slightly lower rating.