Relay Contact Current Question

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3v0

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The ROV sub thread has me thinking about relays CONTACTS and how they are rated. When it comes to analog I do not get much past ohms law.

The contacts carry power so I think of that as P = IV

If that is correct a relay with contacts rated at 5A 24V could be used with 10A 12V.

If I am still on the right track what is one to think when no voltage is given for the contacts?

I know the voltage for the contacts are in regard to (also in regard to?) flash over between the poles. Or is it all to do with flash over and nothing about current? If so how can that be? Does P=IV not apply?
 
I think you need to think about the two ratings as defining a "safe operating area" If there is a fixed contact resistance and you double the current the power will be quadrupled.

I would think that a local AND applies here and that you need to be at or below both ratings.

Still I could be wrong, but that is how I've always interpreted those ratings.
 
Contacts specs (current and voltage) should be looked at as seperate specifications and not as a composite power spec. Also if you check the spec on most relays they have different max current specs for AC Vs DC current. This is because AC can be more easily switched off as the AC voltage zero crossings help keep contact arcing down, where as DC is much harder on contacts at the same ratings. Not a good idea to utilize relay contacts at or near their rated maximum as contact life will be shortened. I would not go above 50% of ratings if reliablity is important.

Lefty
 
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You need to do a search for "relay contact ratings", and read some of the hits. I found some very good info by doing this.
 
Please do not slam the door on this with "google for it". There is a lot of information on the web and not all of it is good. The hope is that someone here could explain it or point to a web page that does.

The following is a "Danger Will Robinson" sort of thing.


This is more specific.


It seems that the info provided for relays by the people selling them in not enough to work with. To be sure one needs to visit the makers site.
 
hi 3v0,

This pdf is produced by a relay manufacturer, so it should be reliable.

Hope it helps.
 
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I wasn't slamming any doors. You have apparently cited something you found with a search, proving my point. No?
I don't think you will find someone on this forum who knows more about this subject than the manufacturers of relays, and it is very likely you will get bad advice here.
 
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