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Over size diode problem

  1. Oct 7, 2007 #1
    Over size diode problem
    There are two sensitive auxiliary relay in one portion of DC control circuit (relay no.1 & no.2) separated by one semiconductor diode. Two relays (R1, R2), their two run up contacts (C1, C2) and that mentioned diode bridge (D) is located in "H" form.
    When relay no.1 picks up via related N.O. dry contact, the other relay will pick up via the same contact and diode that is directly biased too.
    In opposite, the dry contact specified for relay no.2 only picks up one of the two relays because of diode inverse biased activity. In this case diode blocks applied voltage to relay no.2 .
    After some time the diode fails due to overheating and it must be replaced by a similar diode. But maintenance man can't find the diode with the same characteristics in market, and he decides to use an over size option. The replaced diode had the same rating of voltage but the current-carrying capability of it was more than the last diode.
    It seems all things are okay. But sometimes the circuit can not operate correctly. For example in some condition the auxiliary relay no.1 doesn't drop out after one temporary close-open command.
    Indeed when two dry contacts (C1, C2) run up related relay and only the contact C1 return to first position, the relay no.1 doesn't drop out and remains in holding position.

    How can you explain the reason of this problem?


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  3. Oct 7, 2007 #2


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    "...only the contact C1 return to first position, the relay no.1 doesn't drop out and remains in holding position."

    Bad relay no. 1?
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