Appendix; Reed Switch Protection - ABB PP10 Operating Instructions Manual

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