Relay Contact Current Question - Electronic Circuits Projects Diagrams Free

**3v0** · 26th April 2008, 09:29 PM

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?

Papabravo · 26th April 2008, 09:42 PM

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.

Leftyretro · 27th April 2008, 12:25 AM

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

Roff · 27th April 2008, 01:52 AM

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.

**3v0** · 27th April 2008, 08:06 AM

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.

Quote:

Originally Posted by http://www.leachintl2.com/english/english2/vol6/properties/how1.htm

Relay contact ratings are the most misunderstood specifications and the most common cause of relay mis use. It is not enough to call out the currents and voltages that the relay will see. Relay contacts are designed, built, rated and must be specified in terms of the specific type of application for the relay. A 5-amp, 115 volt set of contacts in one circuit may or may not be a 5 amp, 115 volt set of contacts in another application. The designer should specify the type of load, duty and environment to which the relay will be exposed. Incomplete specifications from the user often result in misapplications. The consequences can be tremendous for such a "simple" device.

This is more specific.

Quote:

Originally Posted by http://www.exman.com/relay.html

The contact rating must be large enough to handle the load current and it must be rated for the voltage.

The same contact ratings will be different for a Resistive Load than for an Inductive load.

A Resistive Load will have the same rating for Make, Break and Continuous amps.

An Inductive Load will have a different rating for each Make, Break and Continuous amps.

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.

ericgibbs · 27th April 2008, 08:15 AM

hi 3v0,

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

Hope it helps.

**3v0** · 27th April 2008, 01:28 PM

Thanks. It should be helpful.

Roff · 28th April 2008, 12:23 AM

Quote:

Originally Posted by 3v0

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.

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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26th April 2008, 09:29 PM	(permalink)
3v0	Relay Contact Current Question 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? __________________ search engine for electronic parts Junebug USB PIC programmer kit., USB Bit Wacker, 3v0's Homepage The 15 Minute Printed Circuit Board! (+drill time)

26th April 2008, 09:42 PM	(permalink)
Papabravo	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. __________________ We never have time to do it right; but we always have time to do it over.

27th April 2008, 12:25 AM	(permalink)
Leftyretro	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 __________________ Measurement changes behavior Last edited by Leftyretro; 27th April 2008 at 12:29 AM.

27th April 2008, 01:52 AM	(permalink)
Roff	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. __________________ Ron

27th April 2008, 08:15 AM	(permalink)
ericgibbs	hi 3v0, This pdf is produced by a relay manufacturer, so it should be reliable. Hope it helps. __________________ Eric "Good enough is Perfect" PIC tutorials: Gramo's: www.digital-diy.net/ Bill's: www.blueroomelectronics.com/ Last edited by ericgibbs; 7th July 2008 at 11:23 AM.

27th April 2008, 01:28 PM	(permalink)
3v0	Thanks. It should be helpful. __________________ search engine for electronic parts Junebug USB PIC programmer kit., USB Bit Wacker, 3v0's Homepage The 15 Minute Printed Circuit Board! (+drill time)