Can an Oversized Diode Cause Problems in a DC Control Circuit?

[m.s.j]

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?


REGARDS
MSJ

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[dlgoff]

"...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?

 

[ravenprp]

The over size diode problem can cause issues in the DC control circuit, specifically with the two sensitive auxiliary relays. When the diode fails and is replaced with an over size option, it may seem like everything is functioning correctly. However, the increased current-carrying capability of the new diode can cause unexpected issues in the circuit.

In this case, the problem arises when one of the two dry contacts (C1, C2) is closed and then opened, causing relay no.1 to pick up and relay no.2 to drop out. When only contact C1 returns to its first position, the over size diode may not allow enough current to flow through it to fully drop out relay no.1. This can result in the relay remaining in a holding position, causing malfunctions in the circuit.

One possible explanation for this is that the over size diode has a higher forward voltage drop than the original diode, which can cause a decrease in the current passing through it. This decrease in current may not be enough to fully drop out relay no.1, leading to the issue described.

In order to solve this problem, it is important to use diodes with the correct specifications for the circuit. If the original diode is not available, it is crucial to find a replacement with similar ratings to avoid issues like this. Additionally, regular maintenance and checking of the circuit can help identify any potential issues before they become major problems.