fried relays

Hi I have been working on another NAD2200- this one had no output, though the protection circuit worked- so diagnosed it was the relays, both of which were well and truly fried- they were not the originals, but replacements I had fitted a couple of years ago. The amp works perfectly, so my question is what could cause the relays to fry themselves (and in so doing protect the amp). My theory is that there was some spike external to the amp that caused the problem, however the speakers to which it was connected at the time were also not damaged. The relays have been replaced, the amp works fine, but I have not experienced this before- any thoughts? Peter
 

Flossie

Member
2016-10-10 11:43 pm
What part of the relay was "fried"? If the whole body of the relay was overheating, this suggests that the voltage rating of the coils was too low for the supply voltage being applied to hold the relays closed.


If the contacts have burned out, that would suggest that the contact current rating was too low, but I suspect that's unlikely.
 
1. Most likely the relays are not "fried" but just "rusted"/corroded. Be sure to replace them with gold plated contact relays. Many larger relays and switches depend on a little arcing to keep the oxide down so when they are used on low voltage they corrode and fail.
2. Way out there possibility is that you have some kind of oscillation that is burning your contacts, but this is very unlikely.
 
@flossie- The fried part was the audio connection, the coil voltage is correct as is the rating- these relays have been working flawlessly in over 20 of these units- so this is why I am wondering how they got fried. @steveu- definitely not corroded - and no oscillation (I did measure and check this) and it happened to both channels- apparently there was a loud "bang" when it happened- I was wondering about an electrical storm thanks for the responses
 
@flossie- The fried part was the audio connection, the coil voltage is correct as is the rating- these relays have been working flawlessly in over 20 of these units- so this is why I am wondering how they got fried.

There are generally two things that will cook relay contacts.

1) More than rated currents. The contacts in the relay will be rated for maximum current loading and beyond that heat at the contact point can cause problems. In that case, you need to find a relay with the same coil voltage but higher rated contacts.

2) Chatter and arcing. In some cases relays require a specific amount of current to lock the contacts in solidly. Below this, you will get contact but not good continuity so the contacts can chatter and arc, causing damage to the contacts that will eventually fail.

Also most relays are paired with a reverse biased diode that is intended to absorb the back EMF when the relay drops out. If this diode is shorted or open it is possible the relay is slowly degrading causing #2 above.

Hope that helps....
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Relay failures aren't uncommon with this model and IIRC, I've repaired 2 myself. However, I don't believe gold plating is appropriate for audio output relay contacts. It's quite common to have gold contacts in miniature signal relays, where there is little current or voltage but gold plating is soft, very thin and easily blown away with the fat sparks occasionally produced when switching output stages.

This guy covers the relay replacement well, though the comments are a little lengthy: YouTube
 
Protection relays need an appropriate DC rating or they will arc severely trying to break a high current DC fault. Contact material must be for high current switching applications (definitely not gold plating) - usually some silver alloy. I suspect the relay that failed took direct short-circuit current from one of the filter caps - that's 100's of amps and will make a very loud bang. Normally a transistor would be in the way and protect the relay from extreme current, perhaps it was busy vaporizing at the time.