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relay wear and tear


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5/3/2000 10:02 PM
steveR
relay wear and tear
Which will reduce the life of a relay faster?  
 
Is it more the total number of switches or is it the overall time in the energized state?  
 
The reason I'm curious about this is that I want to be able to manually (via a chassis panel switch) set the "energized" relay state. I can do this easily by having the chassis switch in parallel with the footswitch so that either one will give me the hot channel.  
 
Or I can actually duplicate the dpdt switch so that the panel switch activates the hot channel or switches to whatever the relay (and hence whichever state it's in). I don't really want to do this method though because it's a lot of extra signal wiring that could pick up noise. The bonus of this method is that if the relay ever completely fails, I can still switch between states because the failed relay would permanently be "relaxed".  
 
My concern is in totally relying on the relay for one of the switched states, and if the energized state was used more than not would this create excesive thermal wear on the relay possibly causing it to fail prematurely?  
 
I know this is probably pretty anal retentive stuff, but if there is an overwhelming consensus as to one way being more right than the other, I'd like to know.  
 
Thanks.  
 
steveR
 
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5/4/2000 2:50 PM
Graywater

Steve,  
 
Relay failure is usually due to oxidized, worn, and pitted contacts caused by arcing during the make/break. A relay switching signal level voltages/currents should last indefinitely - especially if a vacuum sealed relay is used (hard to oxidize with no O2). Butttttt...the cheap miniature relays used in current production amps often fail due to sheer cheapness - very fine coil wire that doesn't take much stress, plastic bodies that flex when the amp is subject to normal musician wear and tear (ever seen a drunk roadie sling an amp into a van after the show?), and contacts subject to misalignment due to flexing. There are lots of surplus 6.3vac vacuum sealed octal based relays around - they take up more space but if you are worried about reliability these are what you want - you can even keep a spare and field replace it.  
 
GW
 
5/4/2000 3:49 PM
Don Symes

Sone of the coil wearout and stress issues can be dealt with by paying attention to the coil current - the relay usually specs 'pull-in' current (higher) and 'hold' current (lower).  
 
Someone smart could fix up a series R bypassed with an RC to create a guaranteed-safe profile that minimizes coil power during hold.
 
5/4/2000 5:40 PM
steveR

GW,  
 
Thanks for the info.  
 
Other than Mouser for new parts, can you recommend anywhere to look for these surplus socket relays. I have a couple of rectangular sockets relays, but I don't want to have to cut the chassis. It's a gutted Bandmaster chassis which already has a couple extra sockets holes punched. An octal based relay would be perfect. I've never seen relays at AES.  
 
I could also incorporate a high quality sealed relay permanently into the board layout.  
 
I'll probably go with making the footswitch and panel switch in parallel.  
 
thanks again.  
 
steveR
 
5/5/2000 11:05 AM
Stephen Conner

Small signal relays are not worn out by arcing because there is not enough signal current to cause an arc. Instead, the contacts get oxidised by pollution in the air. This is a time thing and is not affected by how often the relay switches or how long it is left powered.  
 
You can avoid this by using sealed relays with gold contacts.  
 
Steve C.
 

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