Relay current rating

Hi,
I did look this up but I want to double check because I'm making something for other people, not just myself:

An article I read claimed that if a relay was rated for 10 amps at 24 volts DC, then it would be rated for 20 amps at 12 volts DC, so the wattage is what the relay is really rated for. If this is true I wish the relay would just show the wattage. This makes sense to me, I'm just hoping for someone to confirm this. Thank you!

No, what limits is the current. Voltage is important when opening or closing the contact as sparking may occur, and using different voltage may result in a different contact life.

Are you asking about the contact rating or the coil rating?

Sorry, it is a 5v DC relay model. Crud I meant to paste the link:
https://m.aliexpress.com/item/1886207068.html

The numbers on the top don't match my example in my original post, which was just for illustration. I assume there are 2 sets of numbers on the top of the relay because there is a slight difference for the normally open and normally closed contacts.

I am taking about the contact side. The coil side will be a small 5v signal from the Arduino. The contact side will be up to 15A at 12-14V.

I'm less concerned withcontact longevity and more concerned with the module burning or melting down. The 15A current will be of short duration when it occurs.

Here is the article I was referring to, which seems like a knowledgeable source:
https://relaypros.com/choosing_proper_amperage.htm

The limits to the current a relay can carry is the resistance of the contact materials and the internal wiring to the contacts, this is irrespective of voltage. Power, Watts, is equal to I^2 * R. In this case I is the current the relay is carrying, which has nothing to do with the voltage. R is the resistance of the contacts and the internal wiring of the relay. Double the current and the power increases by a factor of 4. This is not the power to the load, this is the power wasted inside the relay, which heats it up. Enough current and the insides of the relay melt, or the contacts get hot enough to weld together. A low supply voltage is not relevant.

Apart from that, the ratings of the relay contacts are maximum ratings. If it's rated at 10A 250VAC then the maximum current is 10A, there is no condition to this. Ideally you assume this is an optimistic rating and that in reality the relay is only good for, maybe, 7.5A.

DC ratings are lower because when the contacts open they draw an arc. As AC reverses 100 (or 120) times per second the arc is quickly extinguished as the current passed through zero. DC doesn't reverse, the current does not pass through zero, the arc does not extinguish so quickly and the contacts can be quickly burnt and damaged.

The answer to your original question is, no. The source is not correct. 10 Amps is 10 Amps regardless of the voltage. You can put 20 Amps at 1.5 Volts through the contacts and you will get smoke.

If the current rating of the relay is for AC current, then the DC rating is 0.707 X the AC rating.

In my experience, relays like the one in your photo are not rated for 100% duty cycle. In other words, they can handle 10 Amps for a few minutes, at best, before they melt down.

Thank you both! Perry, that p = i^2 * r was a very helpful reminder of something I learned and forgot long ago.

I'm generally surprised by all of this, it seemed very logical that power would be the important factor. I am familiar with all the water analogies, but I still don't grok the interplay of current and voltage. I will have to have a conversation with my EE friend.

I do have one question though - it makes sense that DC would sustain an arc more easily than AC, but it seems arcing is still a significant enough concern for household light switches to be of the spring-loaded variety to quickly connect/disconnect the contacts...? Veering off-topic / may not be worth getting into...

It seems arcing is still a significant enough concern for household light switches to be of the spring-loaded variety to quickly connect/disconnect the contacts...

I don't really know the answer to that. I do know that if you tried to switch 230VDC 1A with a household light switch the arc would probably destroy it; I am sure they don't break fast enough for DC at the same voltage. Maybe someone on here knows about switch design and can inform both of us.

If the current rating of the relay is for AC current, then the DC rating is 0.707 X the AC rating.

Steve, I have to disagree. I understand how you arrived at that, but it does not take into account the tendency for DC arcs to sustain themselves, especially at voltages of, maybe, 50VDC and above.

SteveMann:
In my experience, relays like the one in your photo are not rated for 100% duty cycle. In other words, they can handle 10 Amps for a few minutes, at best, before they melt down.

They also usually have a different rating for inductive loads. I can reliably kill them with my brand rollershutter motor. despite using much less than 10A at 230V. Went through 2 prototypes befor I exchanged the breaker relay with a "beefy" SSR and programmed the direction relay to strictly only operate when the SSR is off.

I’m generally surprised by all of this, it seemed very logical that power would be the important factor.

Don’t be. Here’s another way to think about it:
If a relay is rated at 10A 250VAC would you really expect it to be able to switch 100A at 25VAC or 1000A at 2.5VAC? Or be able to withstand 2500VAC as long as it was only switching 1A? Or 25kV at 100mA?

A relay does not see the power supplied to the load, it sees either a voltage across its open contacts or current through them.

PerryBebbington:
would you really expect it to be able to switch 100A at 25VAC

Ha, this is a technique I use frequently in analyzing systems/concepts - taking them to extremes and seeing if they hold true. I certainly wouldn't expect it to handle 100A at any voltage, but as I said, electricity is still a bit of a mystery to me.

I realized a while ago that when compared to hardware, (single-threaded, non-distributed) software is a bit like physics lab - you get to idealize a lot of things! "Weird" things like floating pins (requiring pull-down resistors) and using resistors that are "close enough" instead of exactly the value you need (in simple projects) are foreign to me, so I was not clear if the taking a concept to its extreme would work with electricity. It more or less works in software.

Thanks again!

Using resistors that are “close enough” instead of exactly the value you need.

My background is electronics, not software, some of the things you mention seem obvious to me. A very good friend taught me C, his background is software, not electronics. When I tell him that I am going to replace a faulty component with a different one of different value he can’t understand how I know it will work, even though the value is different. Complete mystery to him. Some things in software are a complete mystery to me! :confused:

SteveMann:
The answer to your original question is, no. The source is not correct. 10 Amps is 10 Amps regardless of the voltage. You can put 20 Amps at 1.5 Volts through the contacts and you will get smoke.

Doesn't follow, DC ratings can be to do with arc damage which is highly dependent on voltage. Excess contact current normally welds the contacts, rather than producing smoke. Welded contacts can take higher currents!!

If the current rating of the relay is for AC current, then the DC rating is 0.707 X the AC rating.

Again doesn't follow, if the rating is thermally related then the values would be the same, because 10Adc and 10Arms have the same dissipation. For relays the DC voltage is highly important due to the arcing dynamics. I have a 1kV DC supply that can sustain a 2cm fireball continuously at only 0.5A, for instance, 1kV of AC couldn't do anything like that.

Most relays quote AC at 250V and DC at 28V, as these are common uses (mains and vehicle), but datasheets may give more information about ratings at other voltages

In my experience, relays like the one in your photo are not rated for 100% duty cycle. In other words, they can handle 10 Amps for a few minutes, at best, before they melt down.

I'd read the datasheet - commonly relays are rated for 100%, solenoids and electromagnets for low-duty cycle. Heat dissipation requirements should also be given (packing relays side by side without air flow may be an issue). Of course cheap relays off eBay are not something I'd trust particularly!

And to further your relay related education. All relays have a rated lifetime number of operations. Good commercial relays will tell you this number which is the "mean time to failure". The cheap ones don't even know or care.

Another relay problem when using DC is how long to you run the relay closed with DC flowing through the contact points? If we are talking months, then you need to have a scheme to open the relay every few weeks. I used to manage communications equipment that were phone line connected and the normal function of several relays was to be closed for an extended time. Many times when I need the relay to open for remote diagnostics, the points were locked closed because of metal migration. Had to replace the unit and send to the factory for repair.

Another design point is open or sealed relay units. Open will eventually gather dust/dirt. etc and stop working. The little blue units from Ebay are sealed, but some of the units I have opened after they failed, showed they had crap on the contact points that welded them together. No idea what it was.

If your project is supposed to operate for an extended time, please use relays from a reputable distributor.

Paul

Contact physics is extremely complex. I originally was surprised to learn that on every closing under load, the parts of the contact surface that initially touches the mating contact is very small. (Microscopically the contacts look like sandpaper.) The initial contact area is so small the current through them quickly softens/melts the area where they touch, allowing the contact surfaces become closer together and adds more area to conduct the current.

I am not trying to throw a bunch of theory out there but simply communicate that contact ratings are not simple. I also used "contact" in too many places... sorry, I couldn't think of an alternate word.

The point being the current handling capability is not something you can easily convert from one relay to another, nor one current capability to another. Although as pointed out generally the more the current the sooner the relay will fail.

I don't remember the reference now, but I read somewhere that the superficial treatment for contacts operating DC is different from the treatment for contacts working in AC. If anyone knows a reference I would like to read about it again when I have the opportunity.