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# Electrical riddle no.7 – Over size diode problem

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• #60
Hamid

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?

Viewing 5 replies - 1 through 5 (of 5 total)
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• #1180
Haajee

1. R1 picked-up & held by its self-holding contact. 2. R2 is picked-up via C1 & Fwd-biased D & held by its self-holding contact C2 3. If now C1 is opened only R1 will open but R2 will remain held by its self-holding contact C2. 4. Now D is reverse-biased so R1 cannot pick-up via C2 & D. 5. Now if D fails it is not the hi-current as no fwd-current is flowing thru it, but it will be the Reverse-Break-Down of D as it is not rated to block the suppliy voltage. 6. Instead of replacing it with a higher-current rated diode but with a higher-voltage PIV rated diode. 7. You should know that diodes for almost 50v to 1200v are made in the same package size, like 1N-4000 series comes 4001 <100v piv> to 4012 <1200v piv>

#1184
MDShunk

MDShunk Moderator Join Date: Jan 2007 Posts: 1,823 ——————————————————————————– I’d say you’ve welded the dry contacts in relay #1 by some means every once in a while. (possibly the same overcurrent situation that is taking out the diode). Have you checked the current draw on a recording basis, and compared it with the rating of the relay? Since relay #2 is unaffected, I think it’s safe to say that D1 isn’t the issue. Now, on the other hand, if relay #1 only failes to drop out when relay #2 is powered, then I’d guess that you have a “leaky” diode on your hands, passing enough holding current the wrong direction to keep relay #1 closed even after its contact is opened.

#1185
Hamid

——————————————————————————– Quote: ——————————————————————————————————- Originally Posted by MDShunk Since relay #2 is unaffected, I think it’s safe to say that D1 isn’t the issue. Now, on the other hand, if relay #1 only failes to drop out when relay #2 is powered, then I’d guess that you have a “leaky” diode on your hands, passing enough holding current the wrong direction to keep relay #1 closed even after its contact is opened. ———————————————————————————————————- Thank you very much. Your answer is correct completely. Indeed in semiconductor diode, the reverse leakage current is usually specified at a voltage and temperature. Also in general, the conclusion of higher the current carrying capability of the diode is the higher the leakage current. In this case the higher reverse leakage current can to hold auxiliary relay no.1 via contact C2.

#1186
Haajee

Quote: “I’d say you’ve welded the dry contacts in relay #1 by some means every once in a while. (possibly the same over-current situation that is taking out the diode). Have you checked the current draw on a recording basis, and compared it with the rating of the relay? ” You mean dry-contact by Dry-switch operated by its own coil & not the contact of R2? I took it as the NO contact of R@. Regards

#1187
Haajee

You call it “When relay no.1 picks up via related N.O. dry contact,” while it is called “Reed-Swith” which is operated by its own magnet/ Electro-magnet. by calling it “When relay no.1 picks up via related N.O. dry contact,” means it is aux-contact of relay #1, which means it a self-holding contact of relay #1, So once held it can only be de-energised by breaking the path between C1 & R1. Will you please use the standard Terms & Standard Drawing-formats

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