Reducing Magnetic Interaction in Reed Relay Applications

Magnetic Interactions

Reducing Magnetic Interaction in Reed Relay Applications

Reed Relays are susceptible to magnetic effects which may degrade performance under certain conditions. This report presents a practical approach to reducing magnetic effects among and between Reed Relays. The guidelines can be applied to many cases.

With the trend toward reducing the size of electronic equipment, Reed Relays are typically placed in proximity to one another. Magnetic coupling between relays can affect parameters such as pull-in and dropout voltage. In some circumstances, adjacent relays will be adversely affected by their neighbors.
Experimental data is provided for some basic Reed Relay arrays under worst-case conditions. An analysis of the data is presented with equations. The data was gathered on single inline package (SIL) Reed Relays, but applies to most Reed Relay packages because the basic physical principles are the same.
A checklist for designing a relay array or matrix covers the factors necessary to minimize the electromagnetic effects most likely to be encountered. Systematically progressing through the checklist will aid in reducing or eliminating many troublesome variables.

Factors Affecting Reed Relay Magnetic Interaction

A host of factors, internal and external, determine how a Reed Relay will perform when installed in a matrix assembly and subjected to electromagnetic interference (EMI).

Internal Factors. Early in the design phase, the user and the manufacturer must discuss the application and consider all the internal factors:

Coil wire gauge.
Coil resistance.
Coil ampere-turns (AT).
Coil winding direction.
Coil winding terminations.
Type of Reed Switch assembly.
Number of Reed Switches in the relay.
Internal magnetic shielding.

External Factors. Controlling external factors generally is accomplished by giving proper attention to the operating environment of the Reed Relay. How much effort is expended on these factors will depend on how strongly they adversely affect design performance. Consideration should be given to these factors:

Nearby magnetic fields.
Relay spacing in the relay matrix.
Magnetic polarity arrangement.
External magnetic shielding.

Magnetic Coupling Between Reed Relays

To better understand the magnetic coupling between adjacent Reed Relays, consider this example. The illustration on the left shows a portion of a relay matrix with two adjacent Reed Relays mounted on a PC board. The relays, K1 and K2, are identical in construction and the direction of current flow is the same in each.


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