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Reed Switch: Understanding Specifications

Written By: 

Preeti Jain


· Pull-In (PI)

Consider a permanent magnet operated normally open Reed Switch. If we are bringing the magnet close to the Reed Switch, when it reaches a certain distance from the Reed Switch, the switch suddenly closes. This is called Pull-In and this distance is called Pull-In distance and is measured in mm or inches.

Pull In


Now let us consider a Coil operated normally open Reed Switch. If we are increasing the voltage across the coil, at certain voltage level, the switch closes suddenly. This voltage is called Pull-In voltage. It is measured in Volts (applied voltage across the coil at PI), Mille-amperes (current flowing through the coil at PI) or Ampere-Turns (product of current flowing at PI and the number of turns in the coil).

Reed Switch Pull In


·Drop-Out (DO)

It is the distance from the magnet and the Reed Switch or the voltage applied across the coil, at which the previously closed switch opens again.

Reed Switch Drop OutReed Switch Drop Out


It can be observed that, the Pull-In happens at a particular distance or voltage. But once the PI has occurred, means the metals have made contact and if we slightly increase the distance or slightly reduce the voltage metals remain in contact, although it is not possible to make a contact at these points in normal cases.

Simply once the PI occurred, the DO will occur after a certain increase in distance or certain decrease in voltage only and vice versa. This effect is called Hysteresis.


Reed Switch Hysterisis

Hysterisis Graph


 Hysteresis Graph

The above plot shows that that variation in hysteresis is for low ampere turns (AT) is very small and increases with higher AT. For a low hysteresis operation of the Reed Switch the magnetic influence on the switch should be kept low.

·Contact Resistance


Contact resistance is the resistance introduced by the contact metals of the Reed Switch in a circuit, in its closed (switch on) condition.

Contact Resistance

This contact resistance is introduced by the resistance of metals, usually greater than resistance of copper.

There are basically two kind of contact resistance.

-          Dynamic contact resistance

-         Static contact resistance

Dynamic contact resistance

Dynamic contact resistance is the resistance experienced within the period of de-bouncing.

De-bouncing is an effect that exists in all kind of metallic switches which can make or break a contact. We normally expect that when we turn on a switch, it simply make contact instantly. But actually it is not like that. The switch continuously make and break contact for a few milliseconds, as if the metal pieces bouncing off from each other after hitting together, like a ball dropped on the floor does.

After the de-bouncing time only the switch makes permanent contact.

Dynamic Resistance

Since the contact is making and breaking continuously in a de-bouncing period, the resistance also increases and decreases accordingly; hence it is called dynamic resistance.

Before taking a decision about the state of the switch whether closed or open, one should wait until it finishes de-bouncing.


Dynamic contact resistance pattern

Dynamic contact resistance pattern

The above plot shows that the dynamic resistance falls significantly after the de- bounce period.

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