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5.0 Appendix - Reed Switch Protection

Many of ABB's switch products (MS30, MS30/EX, MS50) are based on magnetically operated reed switches. Since

reed switches have the inherent characteristic of very closely spaced switch contacts, it is extremely important to

protect these contacts from high voltage transients caused by inductive loads. When an inductive load is deener-

gized, the collapsing magnetic field induces a high voltage of opposite polarity into itself and thus the switch. Two

basic methods exist to clamp this voltage and thus protect the switch contacts.

D.C. Applications

For D.C. applications, a diode is placed in parallel with the inductive load (note the polarity of the diode and power

supply). A 1N4001 general purpose diode is normally sufficient to clamp the induced voltage of the inductive load

to a safe level.

A.C. Applications

For A.C. applications, a Metal Oxide Varistor (MOV transient surge suppressor) is placed either in parallel with the

switch or the inductive load. The MOV changes from a high impedance when the voltage across the MOV exceeds

its rated voltage (the MOV rating must correspond with the power supply voltage). For 120 VAC control systems a

typical MOV would be the GE (General Electric Co.) part number V130LA10A. In either case shown, the result is

the limiting of the voltage across the switch contacts to a safe level.

8 IR10/PP10 Field Indicator and Controllers | Operating instruction manual

D.C. Contact Protection

A.C. Contact Protection