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Coil Suppression: Number one cause for Relay Misapplication
According to a recent Kilovac survey, the number one cause for misapplication of power relays is the use of improper coil suppression techniques. In spite of the fact that this has been widely known in the relay industry for years, it continues to be a major problem for power relay users. The problem is easy to avoid; simply don't put anything across the relay coil that will slow down the release time of the relay. If this cannot be done, you should contact the relay manufacturer for the best suppression technique before designing your circuit.
The reason improper coil suppression causes a problem is simple. The relay manufacturer life-tests the relay and establishes its rating without coil suppression. Improper coil suppression normally extends the release time, and the life of the relay is thus significantly reduced because the arc time is extended and the contact materials erode faster. This can also cause high contact resistance or loss of dielectric, especially with highly inductive loads. Worse yet, relays can often fail on the first operation when switching high inrush capacitive, motor, lamp, and overloads. This is because the contacts will usually "tack weld" on the make, and improper coil suppression reduces the coil energy which is needed to break the weld.
Additional information can be found on page 69 of our "Application and Product Guide" catalog, or if you have questions about your specific circuit, just drop us a fax at (805) 684-9679 or call our toll free application hot-line at (800) 253-4560.