REED SWITCH BASICS PART III

Introduction
Table of Contents› Dynamic Contact Resistance› Reed Switch as a Reed Relay› Reed Switch as a Reed Sensor

What causes DCR disruptions?

››
›
›

›

›

Over-stressed reed switch
A small crack on the reed switch seal
A broken reed switch
Plating or sputtering peeling or flaking off thecontact area
Improper air mixture (moisture) inside the glass
capsuleParticles on the contacts

Why test for DCR?

›››

DCR testing is a great way to qualify a new sensor orrelay to make sure that all tools involved are notadversely affecting the fragile reed switch.
This is particularly true in any operation involving
bending or forming the reed, along with any overmolding of the reed.
DCR testing will eliminate early failures and improve
long term reliability in the customer’s equipment
and/or technical systems

DCR Parameters›››

Reed switch size and the subsequent inductance of its coil canhave a major influence in the dynamic switching
characteristics.
When the reed contacts come together, they do so with acertain momentum. That momentum makes the reeds vibrate
in a simple critically damped harmonic motion.
Critically damped harmonic motion is an important concept
in our DCR testing.

DCR Parameters›

Larger reed switches have more inertia and the reed blades
are stiffer. This in effect will create three things:
1. The need for a magnetically stronger more inductive coil is required.
2. It will increase the initial reed closure time.
3. It will increase the effects of the critically damped harmonic motion.

››

Conversely, smaller reed switches have less inertia and are
not as stiff. Therefore, they will behave in an opposite
manner compared to larger switches.
Taking the size of the switch into consideration, therefore, isan important step in determining the parameters of the DCRtesting.

DCR Parameters››››

When the reeds undergo the critically damped harmonic
motion they are moving microscopically inside the glasscapsule.
This movement is occurring in the magnetic field generated
by the coil.
When a metal is in motion in a magnetic field a current will
be induced in the metal.
This current is a critical part of the measurement of ourDynamic Contact Resistance.

DCR Parameters›››

The overdrive of the coil is also a critical parameter in making
the DCR measurement. Simply defined: it is the voltage (orcurrent) above the actual pull-in (or closure) point where theDCR measurement is made.
If the reeds close with 3.0 volts applied, adding an increased
voltage above 3.0 Volts and testing at that point would
represent the overdrive level.
A reasonable overdrive number is 40%. Here for 3.0 Volts thisrepresents a voltage increase of 1.2 volts or a test level of 4.2
volts applied to the coil.

DCR Parameters measured at Max Pull-in››››Max pull-in as the overdrive level is another approach.
Here if the max Pull-in spec is 3.75 volts that is where the DCR
measurement is taken.
In this case, however, if some of the population are pulling-in
at or near 3.2 volt this will represent only an overdrive of lessthan 15%.
Again, this may represent an unfair testing approach and one
may reject perfectly good reed relays (unfair in this casemeans throwing away perfectly good products).

DCR Parameters measured at Nominal Voltage›››››Using the DCR measured at 5 volts also easy to set up by thetest engineer may disguise potential problems with too muchoverdrive.
It is not uncommon to have a lot of relays made that typicallyhave a distribution of Pull-in over an expansive range.
For a 5 volt relay it would not be uncommon to have a Pull-inof 2.0 volts. In this case, testing the DCR at 5 volts wouldrepresent 150% overdrive.
150% overdrive may cover up potential reed switch problems.
Testing the DCR at 2.8 volts (40% overdrive) would be a bettertest

DCR Parameters measured at Nominal Voltage››››

Larger reed switches take longer to close as alreadydescribed.
Because of this, starting the DCR too early will mean
throwing away perfectly good productStarting the DCR later with a smaller reed may create
the opposite situation allowing too much time forsettling
1.5 ms usually is an appropriate amount of timeafter the coil has been energized to perform the DCR
measurement

Dynamic Contact ResistanceThe Reed Switch as a Reed Relay›››››

Placing a coil around the Reed Switch and passing a current thru the coilproduces a magnetic field equivalent to a permanent magnet.
Placing a coaxial shield around the switch allows signals to be switched up
to 20 GHz.
Because the Reed Switch has no wearing parts the contacts can switch
low level signals well into the billions of operations.
The Reed Relay is used extensively throughout the test and measurement
field.
Reed Relays are used in test systems, matrices, RF, modems, alarms, ideal
for high cycle count, ideal for high voltage applications, ideal for low
current and low voltage switching, etc.

The Reed Switch as a Reed Sensor›››››››As a Sensor the Reed Switch may sense all kinds of movement.
To do this a permanent magnet is used in conjunction with the ReedSwitch.
The Reed Switch in the open state draws zero current.
The magnet’s field can be effective even when separated by air, plastics,and metals.
This feature opens up a plethora of applications, where the sensing
environment does not allow the movement of the magnet and switch tophysically come together.
Usually the magnet and Reed Switch are divided or separated by physicalhousings or other obstacles.
Reed sensors are used for sensing movement, counting, detecting fluidlevels, measuring fluid levels, switching in harsh environments,implantables, etc.

Reed Switch Basics Part III - END
Content of part IV:› How a Reed Switch Sensing application works› How a Reed Switch is used with a Permanent
MagnetFOR MORE INFORMATION ON OUR CAPABILITIES,AND HOW WE CAN PARTNER, SOLVE, AND DELIVER®
TO YOUR NEEDS, PLEASE VISIT US AT
STANDEXELECTRONICS.COM