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Old 26-07-2013, 13:35   #16
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Re: 12V Relay Amp Ratings?

OK when carrying a current through the NO contact and the relay RELEASES, there is arcing at the NO contact.

When carrying a current through the NC contact and the relay OPERATES there is just as much arcing.

There is no difference in the amount of arcing for either contact so no justification based on arcing for any difference in rating.

If you look back an an earlier post I explained why the NO contact can handle heavier loads which is related to mechanical contact pressure.
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Old 26-07-2013, 14:30   #17
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Re: 12V Relay Amp Ratings?

Please explain why in a relay with a 50A/40A rating the NO contact can handle 100% or the current max rating and the NC contacts can only handle 80% of the current do only to mechanical pressure. I could understand a 1 or 2 percent difference -- but a 20 percent difference is a very big difference for two identical mechanical structures.

Thanks.
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Old 26-07-2013, 14:58   #18
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Re: 12V Relay Amp Ratings?

The magnetic force on the armature when it first turns on has to be at least twice the return spring pressure to get the armature moving reliably.

The magnetic force at end of stroke is more than 50 times the spring force because the magnetic field decreases as a function of the square of the air gap distance. As the distance drops to zero the force increases astronomically . That armature force is sufficient to bend the spring arm between the armature and the contact. As that arm flexes the moving contact slides a small distance over the fixed contact cleaning off any debris or oxide from the last arcing. The final pressure will also increase the contact area by displacing microscopic imperfections on the contact surface and increasing the area of contact..

These forces and actions are only less than 10% as strong when the spring causes the moving contact to land back on the NC contact material.
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Old 26-07-2013, 15:54   #19
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Re: 12V Relay Amp Ratings?

Arcing damage to the contacts is one of those things that is specification dependent. If you want to rate a relay for N full current open/close operations then you can have a certain current rating. If you want to rate it for 10N operations then the current rating will be significantly less.

However, the DC current rating can be further limited due to heating. So the current rating is not a cliff where 1A more and everything explodes. Rather it is a multidimensional problem where number of cycles, contact resistance (due to magnetic or spring pressure), temperature, allowed temperature rise and internal wire sizing all combine to arrive at a current rating.

Also, arcing can be effectively eliminated by the judicial usage of rectifiers across the contacts to limit inductively induced arcing.
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Old 01-08-2013, 21:47   #20
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Re: 12V Relay Amp Ratings?

I haven't inspected the insides of the particular relays involved here, so I will just make a few comments about mechanical relays in general.

The difference in opening & closing force between the electromagnet & the spring has already been covered & correctly described. That is one piece of the puzzle.

Another piece is the wiping effect on the contact surfaces. The opening & closing of the contacts causes a slight bending of the contact arms, or a compression of the arm retainer springs. This causes one contact to slide sideways across the other, wiping away the byproducts of arcing. The wiping geometry is different when opening compared to closing. Siemens made great inroads with the robotic welding equipment in UAW plants in the 1970's with improvements in this aspect of relay design.

Although it is not always labeled on the outside of the relay case, the contacts also have different current ratings for different types of current & load conditions. AC ratings are different from DC ratings. Inductive loads have different ratings compared to resistive loads. The worst possible combination for most mechanical relays is high voltage DC on an inductive load.

There can also be duty cycle ratings, especially with definite purpose relays.
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Old 01-08-2013, 22:30   #21
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Re: 12V Relay Amp Ratings?

I suppose, that like most dogs (what was it the old New Yorker cartoon said, no one knows if you're a dog on the internet?) I just find it upsetting when my food has been changed.

All those years of seeing relays simply labelled "5A" or "10A" or, in this application, the universal Bosche, etc. relays all labelled "30A" with no difference for NO/NC...what happened, truth in advertising? Someone bought a vowel?
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Old 01-08-2013, 22:35   #22
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Re: 12V Relay Amp Ratings?

Maybe they are just giving you too much information now. It's probably still the same old relay. They are probably just providing one new piece of information about it. If you use it like you always did, you will probably see the same old results that you always saw.

It's probably the same food, just in new bag with a different label.
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Old 09-08-2013, 13:16   #23
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Re: 12V Relay Amp Ratings?

I don't think it has anything to do with arcing. Bosch asumes the NC contact has current flowing for some long duration, the NO has current flowing intermittent.
The relay will carry 30 amps for days. The other contact can carry more for a shorter time.
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Old 09-08-2013, 16:35   #24
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Re: 12V Relay Amp Ratings?

Quote:
Originally Posted by Zil View Post
I don't think it has anything to do with arcing. Bosch asumes the NC contact has current flowing for some long duration, the NO has current flowing intermittent.
The relay will carry 30 amps for days. The other contact can carry more for a shorter time.
That is an interesting assumption that I have not encountered before. What do you base that on?
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Old 22-09-2013, 08:18   #25
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Re: 12V Relay Amp Ratings?

My understanding of Relays is the heart and soul of them is the coil..... Some manufactures build high quality (the coil) like Tyco/Bosh and others (no names mentioned) build cheap ones, again the coil. Relays are multi-listed 20/30 30/50 etc predicated on the life cycle. Example: The Tyco/Bosh Relay 0 332 019 150 has stamped on the cover 30 amps that is based on switching 30 amps for over 250,000 cycles. However, these relays can also switch 40 amps for over 100,000 cycles. Thus it it can be used as a 40 amp relay where needed.
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Old 22-09-2013, 09:35   #26
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Quote:
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My understanding of Relays is the heart and soul of them is the coil.....
Probably the contacts are the reason most relays wear out. That's the part that wears a little bit each time it makes or breaks an active circuit. The bit of arcing that happens each cycle chews up the contacts. The more contact material the longer they will last but the more costly to build.
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Old 22-09-2013, 15:07   #27
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Re: 12V Relay Amp Ratings?

cn-
The coil has no connection, no relationship, to the power being carried by the contacts. So whether a relay is carrying 1/10th amp of 100 amps across the contacts, the coil is seeing a completely different picture based only on the circuit that is energizing the coil. Totally separate, although I'm sure someone can find a way to cross them up. Typically the coil resistance itself limits the power the coil will be carrying and the voltage it sees is a design choice.
Besides, these ratings were based on "NC/NO" indicating the arm contacts themselves were different for each function. Or as some suggest, that the contacts are the same but the function causes them to be loaded differently, i.e. from arcing.
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Old 09-04-2014, 10:52   #28
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Re: 12V Relay Amp Ratings?

@hellosailor, I was wondering the same thing but I thought to myself back then that this has to be regulated somehow by the government standards and they can’t be let loose to make us believe in things that just don’t exist. These actually are two different types of power banks which behave in a certain way when connected to different circuits.
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Old 09-04-2014, 11:11   #29
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Re: 12V Relay Amp Ratings?

can't you just install a automotive capacitor? to stop the arcing
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Old 09-04-2014, 13:08   #30
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Re: 12V Relay Amp Ratings?

Arcing is a function of the voltage and inductance of the load. Either the NO or NC contact could be subject to arcing depending on what is in the rest of the circuit.

NC contacts will arc when the relay operates if they have a load going through the NC contacts.

NO contacts will arc when the relay releases if the NO circuit has a load.

Both contacts can make and break circuits and deterioration due to arcing is a function of the circuit arrangement rather than which contact it occurred on.
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