Manufacturer’s of electromechanical relays always supply a specification sheet for each of their relays. The specification sheet contains voltage and current ratings for both the relay coil and its switch contacts. The spec sheet also includes information regarding the location of the relay coil and switching contact terminals. In addition to that the spec sheet will also list if the relay can be energized by an AC or a DC source. The list below is an explanation of a relays most important ratings.
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Pickup voltage- The minimum amount of relay coil voltage necessary to energize or operate the relay.
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Pickup current- The minimum amount of relay coil current necessary to energize or operate the relay.
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Holding current- The minimum amount of current required to keep a relay energized or operating. Note (The holding current is less than the pickup current.)
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Dropout voltage- The maximum relay coil voltage at which the relay is no longer energized.
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Contact voltage rating- The maximum voltage the relay contacts can switch safely.
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Contact current rating- The maximum current the relay contacts can switch safely.
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Contact voltage drop- The voltage drop across the closed contacts of a relay when operating.
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Insulation resistance- The resistance measured across the relay contacts in the open position.